Saturday, August 24th, 2019


from: Foundations of Earth Science (Third Edition) by Frederick K. Lutgens and Edward J. Tarbuck
















Minerals: Building Blocks of Rocks



A mineral is a naturally occurring inorganic solid that possesses a definite chemical structure, which gives it a unique set of physical properties. Most rocks are aggregates composed of two or more minerals.

The building blocks of minerals are elements. An atom is the smallest particle of matter that still retains the characteristics of an element. Each atom has a nucleus which contains protons and neutrons. Orbiting the nucleus of an atom are electrons. The number of protons in an atom's nucleus determines its atomic number and the name of the element. Atoms bond together to form a compound by either gaining, losing, or sharing electrons with another atom.

Isotopes are variants of the same element, but with a different mass number (the total number of neutrons plus protons found in an atom's nucleus). Some isotopes are unstable and disintegrate naturally through a process called radioactive decay.

The properties of minerals include crystal form, luster, color, streak, hardness, cleavage, fracture, and specific gravity. In addition, a number of special physical and chemical properties (taste, smell, elasticity, malleability, feel, magnetism, double refraction, and chemical reaction to hydrochloric acid) are useful in identifying certain minerals. Each mineral has a unique set of properties which can be used for identification.

The eight most abundant elements found in Earth's continental crust (oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium) also compose the majority of minerals.

The most common mineral group is the silicates. All silicate minerals have the silicon-oxygen tetrahedron as their fundamental building block. In some silicate minerals, the tetrahedra are joined in chains; in others, the tetrahedra are arranged into sheets, or three-dimensional networks. Each silicate mineral has a structure and a chemical composition that indicates the conditions under which it was formed.

The nonsilicate mineral groups include the oxides (e.g., magnetite, mined for iron), sulfides (e.g., sphalerite, mined for zinc), sulfates (e.g., gypsum, used in plaster and frequently found in sedimentary rocks), native elements (e.g., graphite, a dry lubricant), halides (e.g., halite, common salt and frequently found in sedimentary rocks), and carbonates (e.g., calcite, used in portland cement and a major constituent in two well-known rocks: limestone and marble).

The term ore is used to denote useful metallic minerals, like hematite (mined for iron) and galena (mined for lead), that can be mined for a profit, as well as some nonmetallic minerals, such as fluorite and sulfur, that contain useful substances.


Rocks: Materials of the Lithosphere



Igneous rock forms from magma that cools and solidifies in a process called crystallization. Sedimentary rock forms from the lithification of sediment. Metamorphic rock forms from rock that has been subjected to great pressure and heat in a process called metamorphism.

The rate of cooling of magma greatly influences the size of mineral crystals in igneous rock. The four basic igneous rock textures are 1) fine-grained, 2) coarse-grained, 3) porphyritic, and 4) glassy.

The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized. N.L. Bowen showed that as magma cools, minerals crystallize in an orderly fashion. Crystal settling can change the composition of magma and cause more than one rock type to form from a common parent magma.

Igneous rocks are classified by their texture and mineral composition.

Weathering is the response of surface materials to a changing environment. Mechanical weathering, the physical disintegration of material into smaller fragments, is accomplished by frost wedging, expansion resulting from unloading, and biological activity. Chemical weathering involves processes by which the internal structures of minerals are altered by the removal and/or addition of elements. It occurs when materials are oxidized or react with acid, such as carbonic acid.

Detrital sediments are materials that originate and are transported as solid particles derived from weathering. Chemical sediments are soluble materials produced largely by chemical weathering that are precipitated by either inorganic or organic processes. Detrital sedimentary rocks, which are classified by particle size, contain a variety of mineral and rock fragments, with clay minerals and quartz the chief constituents. Chemical sedimentary rocks often contain the products of biological processes such as shells or mineral crystals that form as water evaporates and minerals precipitate. Lithification refers to the processes by which sediments are transformed into solid sedimentary rocks.

Common detrital sedimentary rocks include shale (the most common sedimentary rock), sandstone, and conglomerate. The most abundant chemical sedimentary rock is limestone, composed chiefly of the mineral calcite. Rock gypsum and rock salt are chemical rocks that form as water evaporates and triggers the deposition of chemical precipitates.

Some of the features of sedimentary rocks that are often used in the interpretation of Earth history and past environments include strata, or beds (the single most characteristic feature), bedding planes, and fossils.

Two types of metamorphism are 1) regional metamorphism and 2) contact metamorphism. The agents of metamorphism include heat, pressure, and chemically active fluids. Heat is perhaps the most important because it provides the energy to drive the reactions that result in the recrystallization of minerals. Metamorphic processes cause many changes in rocks, including increased density, growth of larger mineral crystals, reorientation of the mineral grains into a layered or banded appearance known as foliation, and the formation of new minerals.

Some common metamorphic rocks with a foliated texture include slate, schist, and gneiss. Metamorphic rocks with a nonfoliated texture include marble and quartzite.


Landscapes Fashioned by Water



Weathering, mass wasting, and erosion are responsible for transforming solid rock into sediment. They are called external processes because they occur at or near Earth's surface and are powered by energy from the Sun. By contrast, internal processes, such as volcanism and mountain building, derive their energy from Earth's interior.

Mass wasting is the downslope movement of rock and soil under the direct influence of gravity. Although gravity is the controlling force of mass wasting, water influences mass wasting by saturating the pore spaces and destroying the cohesion between particles. Oversteepening is one factor that triggers mass wasting.

The water cycle describes the continuous interchange of water among the oceans, atmosphere, and continents. Powered by energy from the sun, it is a global system in which the atmosphere provides the link between the oceans and continents. The processes involved in the water cycle include precipitation, evaporation, infiltration (the movement of water into rocks or soil through cracks and pore spaces), runoff (water that flows over the land, rather than infiltrating into the ground), and transpiration (the release of water vapor to the atmosphere by plants).

The factors that determine a stream's velocity are gradient (slope of the stream channel), shape, size and roughness of the channel, and the stream's discharge (amount of water passing a given point per unit of time, frequently measured in cubic feet per second). Most often, the gradient and roughness of a stream decrease downstream while width, depth, discharge, and velocity increase.

The two general types of base level (the lowest point to which a stream may erode its channel) are 1) ultimate base level, and 2) temporary, or local base level. Any change in base level will cause a stream to adjust and establish a new balance. Lowering base level will cause a stream to erode, while raising base level results in deposition of material in the channel.

The work of a stream includes erosion (the incorporation of material), transportation (as dissolved load, suspended load, and bed load), and, whenever a stream's velocity decreases, deposition.

Although many gradations exist, the two general types of stream valleys are 1) narrow V-shaped valleys and 2) wide valleys with flat floors. Because the dominant activity is downcutting toward base level, narrow valleys often contain waterfalls and rapids. When a stream has cut its channel closer to base level, its energy is directed from side to side, and erosion produces a flat valley floor, or floodplain. Streams that flow upon floodplains often move in sweeping bends called meanders. Widespread meandering may result in shorter channel segments, called cutoffs, and/or abandoned bends, called oxbow lakes.

Floods are triggered by heavy rains and/or snowmelt. Sometimes human interference can worsen or even cause floods. Flood-control measures include the building of artificial levees and dams, as well as channelization, which could involve creating artificial cutoffs. Many scientists and engineers advocate a nonstructural approach to flood control that involves more appropriate land use.

Common drainage patterns produced by streams include 1) dendritic, 2) radial, 3) rectangular, and 4) trellis.

As a resource, groundwater represents the largest reservoir of freshwater that is readily available to humans. Geologically, the dissolving action of groundwater produces caves and sinkholes. Groundwater is also an equalizer of stream flow.

Groundwater is that water which occupies the pore spaces in sediment and rock in a zone beneath the surface called the zone of saturation. The upper limit of this zone is the water table. The zone of aeration is above the water table where the soil, sediment, and rock are not saturated. Groundwater generally moves within the zone of saturation. The quantity of water that can be stored depends on the porosity (the volume of open spaces) of the material. However, the permeability (the ability to transmit a fluid through interconnected pore spaces) of a material is the primary factor controlling the movement of groundwater.

Springs occur whenever the water table intersects the land surface and a natural flow of groundwater results. Wells, openings bored into the zone of saturation, withdraw groundwater and create roughly conical depressions in the water table known as cones of depression. Artesian wells occur when water rises above the level at which it was initially encountered. Most caverns form in limestone at or below the water table when acidic groundwater dissolves rock along lines of weakness, such as joints and bedding planes. Karst topography exhibits an irregular terrain punctuated with many depressions, called sinkholes.

Some of the current environmental problems involving groundwater include 1) overuse by intense irrigation, 2) land subsidence caused by groundwater withdrawal, and 3) contamination.


Glacial and Arid Landscapes



A glacier is a thick mass of ice originating on the land as a result of the compaction and recrystallization of snow, and shows evidence of past or present flow. Today, valley or alpine glaciers are found in mountain areas where they usually follow valleys that were originally occupied by streams. Ice sheets exist on a much larger scale, covering most of Greenland and Antarctica.

On the surface of a glacier, ice is brittle. However, below about 50 meters, pressure is great, causing ice to flow like a plastic material. A second important mechanism of glacial movement consists of the whole ice mass slipping along the ground.

Glaciers erode land by plucking (lifting pieces of bedrock out of place) and abrasion (grinding and scraping of a rock surface). Erosional features produced by valley glaciers include glacial troughs, hanging valleys, cirques, arêtes, horns, and fiords.

Any sediment of glacial origin is called drift. The two distinct types of glacial drift are 1) till, which is material deposited directly by the ice; and 2) stratified drift, which is sediment laid down by meltwater from a glacier.

The most widespread features created by glacial deposition are layers or ridges of till, called moraines. Associated with valley glaciers are lateral moraines, formed along the sides of the valley, and medial moraines, formed between two valley glaciers that have joined. End moraines, which mark the former position of the front of a glacier, and ground moraine, an undulating layer of till deposited as the ice front retreats, are common to both valley glaciers and ice sheets.

Perhaps the most convincing evidence for the occurrence of several glacial advances during the Ice Age is the widespread existence of multiple layers of drift and an uninterrupted record of climate cycles preserved in sea- floor sediments. In addition to massive erosional and depositional work, other effects of Ice Age glaciers included the forced migration of animals, changes in stream and river courses, adjustment of the crust by rebounding after the removal of the immense load of ice, and climate changes caused by the existence of the glaciers themselves. In the sea, the most far-reaching effect of the Ice Age was the worldwide change in sea level that accompanied each advance and retreat of the ice sheets.

Deserts in the lower latitudes coincide with zones of high air pressure known as subtropical highs. Middle latitude deserts exist because of their positions in the deep interiors of large continents, far removed from oceans. Mountains also act to shield these regions from marine air masses.

Practically all desert streams are dry most of the time and are said to be ephemeral. Nevertheless, running water is responsible for most of the erosional work in a desert. Although wind erosion is more significant in dry areas than elsewhere, the main role of wind in a desert is in the transportation and deposition of sediment.

Many of the landscapes of the Basin and Range region of the western and southwestern United States are the result of interior drainage with streams eroding uplifted mountain blocks and depositing the sediment in interior basins. Alluvial fans, playas, playa lakes, and inselbergs are features often associated with these landscapes.

In order for wind erosion to be effective, dryness and scant vegetation are essential. Deflation, the lifting and removal of loose material, often produces shallow depressions called blowouts and can also lower the surface by removing sand and silt, leaving behind a stony veneer called desert pavement. Abrasion, the "sandblasting" effect of wind, is often given too much credit for producing desert features. However, abrasion does cut and polish rock near the surface.

Wind deposits are of two distinct types: 1) extensive blankets of silt, called loess, that is carried by wind in suspension; and 2) mounds and ridges of sand, called dunes, which are formed from sediment that is carried as part of the wind's bed load.


Plate Tectonics: A Unifying Theory



In the early 1900s, Alfred Wegener set forth his continental drift hypothesis. One of its major tenets was that a supercontinent called Pangaea began breaking apart into smaller continents about 200 million years ago. The smaller continental fragments then "drifted" to their present positions. To support the claim that the now-separate continents were once joined, Wegener and others used the fit of South America and Africa, distribution of ancient climates, fossil evidence, and rock structures.

One of the main objections to the continental drift hypothesis was its inability to provide an acceptable mechanism for the movement of continents.

The theory of plate tectonics, a far more encompassing theory than continental drift, holds that Earth's rigid outer shell, called the lithosphere, consists of seven large and numerous smaller segments called plates that are in motion relative to each other. Most of Earth's seismic activity, volcanism, and mountain building occur along the dynamic margins of these plates.

A major departure of the plate tectonics theory from the continental drift hypothesis is that large plates contain both continental and ocean crust and the entire plate moves. By contrast, in continental drift, Wegener proposed that the sturdier continents "drifted" by breaking through the oceanic crust, much like ice breakers cut through ice.

The three distinct types of plate boundaries are 1) divergent boundaries–where plates move apart, 2) convergent boundaries–where plates move together as in oceanic-continental convergence, oceanic-oceanic convergence, or continental-continental convergence, and 3) transform boundaries–where plates slide past each other.

The theory of plate tectonics is supported by 1) paleomagnetism, the direction and intensity of Earth's magnetism in the geologic past; 2) the global distribution of earthquakes and their close association with plate boundaries; 3) the ages of sediments from the floors of the deep-ocean basins; and 4) the existence of island groups that formed over hot spots and provide a frame of reference for tracing the direction of plate motion.

Several models for the driving mechanism of plates have been proposed. One model involves large convection cells within the mantle carrying the overlying plates. Another model called slab-pull proposes that dense oceanic material descends and pulls the lithosphere along. A third model suggests that hot, buoyant plumes of rock are the upward flowing arms, while the downward limbs of these convective cells are the cold, dense subducting plates. No single driving mechanism can account for all of the major facets of plate motion.


Restless Earth: Earthquakes, Geologic Structures, and Mountain Building



Earthquakes are vibrations of Earth produced by the rapid release of energy from rocks that rupture because they have been subjected to stresses beyond their limit. This energy, which takes the form of waves, radiates in all directions from the earthquake's source, called the focus. The movements that produce most earthquakes occur along large fractures, called faults, that are associated with plate boundaries.

Two main groups of seismic waves are generated during an earthquake: 1) surface waves, which travel along the outer layer of Earth; and 2) body waves, which travel through Earth's interior. Body waves are further divided into primary, or P, waves, which push (compress) and pull (dilate) rocks in the direction the wave is traveling, and secondary, or S, waves, which "shake" the particles in rock at right angles to their direction of travel. P waves can travel through solids, liquids, and gases. Fluids (gases and liquids) will not transmit S waves. In any solid material, P waves travel about 1.7 times faster than S waves.

The location on Earth's surface directly above the focus of an earthquake is the epicenter. An epicenter is determined using the difference in velocities of P and S waves.

There is a close correlation between earthquake epicenters and plate boundaries. The principal earthquake epicenter zones are along the outer margin of the Pacific Ocean, known as the circum-Pacific belt, and through the world's oceans along the oceanic ridge system.

Earthquake intensity depends not only on the strength of the earthquake, but on other factors, such as distance from the epicenter, the nature of surface materials, and building design. The Mercalli intensity scale assesses the damage from a quake at a specific location. Using the Richter scale, the magnitude (a measure of the total amount of energy released) of an earthquake is determined by measuring the amplitude (maximum displacement) of the largest seismic wave recorded, with adjustments of the amplitude made for the weakening of seismic waves as they move from the focus, as well as for the sensitivity of the recording instrument. A logarithmic scale is used to express magnitude, in which a tenfold increase in recorded wave amplitude corresponds to an increase of one on the magnitude scale. Each unit of Richter magnitude equates to roughly a 30-fold energy increase.

The most obvious factors that determine the amount of destruction accompanying an earthquake are the magnitude of the earthquake and the proximity of the quake to a populated area. Structural damage attributable to earthquake vibrations depends on several factors, including 1) intensity, 2) duration of the vibrations, 3) nature of the material upon which the structure rests, and 4) the design of the structure. Secondary effects of earthquakes include tsunamis, landslides, ground subsidence, and fire.

Substantial research to predict earthquakes is underway in Japan, the United States, China, and Russia– countries where earthquake risk is high. No consistent method of short-range prediction has yet been devised. Long-range forecasts are based on the premise that earthquakes are repetitive or cyclical. Seismologists study the history of earthquakes for patterns, so their occurrences might be predicted.

As indicated by the behavior of P and S waves as they travel through Earth, the four major zones of Earth's interior are the 1) crust (the very thin outer layer), 2) mantle (a rocky layer located below the crust with a thickness of 2885 kilometers), 3) outer core (a layer about 2270 kilometers thick, which exhibits the characteristics of a mobile liquid), and 4) inner core (a solid metallic sphere with a radius of about 1216 kilometers).

The continental crust is primarily made of granitic rocks, while the oceanic crust is of basaltic composition. Ultramafic rocks such as peridotite are thought to make up the mantle. The core is composed mainly of iron and nickel.

The most basic geologic structures associated with rock deformation are folds (flat-lying sedimentary and volcanic rocks bent into a series of wavelike undulations) and faults (fractures in the crust along which appreciable displacement has occurred). The two most common types of folds are anticlines, formed by the upfolding, or arching, of rock layers, and synclines, which are downfolds, or troughs. Faults in which the movement is primarily vertical are called dip-slip faults. Dip-slip faults include both normal and reverse faults. In strike-slip faults, horizontal movement causes displacement along the trend, or strike, of the fault.

Major mountain systems form along convergent plate boundaries. Andean-type mountain building along continental margins involves the convergence of an oceanic plate and a plate whose leading edge contains continental crust. At some point in the formation of Andean-type mountains a subduction zone forms. Continental collisions, in which both plates may be carrying continental crust, have resulted in the formation of the Himalaya Mountains and Tibetan Highlands. Recent investigations indicate that accretion, a third mechanism of orogenesis (the processes that collectively result in the formation of mountains), takes place where smaller crustal fragments collide and merge with continental margins along some plate boundaries. Many of the mountainous regions rimming the Pacific have formed in this manner.


Fires Within: Igneous Activity



The primary factors that determine the nature of volcanic eruptions include the magma's temperature, its composition, and the amount of dissolved gases it contains. As lava cools, it begins to congeal, and as viscosity increases, its mobility decreases. The viscosity of magma is directly related to its silica content. Granitic lava, with its high silica content, is very viscous and forms short, thick flows. Basaltic lava, with a lower silica content, is more fluid and may travel a long distance before congealing. Dissolved gases provide the force which propels molten rock from the vent of a volcano.

The materials associated with a volcanic eruption include lava flows, (pahoehoe and aa flows for basaltic lavas), gases (primarily in the form of water vapor), and pyroclastic material (pulverized rock and lava fragments blown from the volcano's vent, which include ash, pumice, lapilli, cinders, blocks, and bombs).

Shield cones are broad, slightly domed volcanoes built primarily of fluid, basaltic lava. Cinder cones have very steep slopes composed of pyroclastic material. Composite cones, or stratovolcanoes, are large, nearly symmetrical structures built of interbedded lavas and pyroclastic deposits. Composite cones represent the most violent type of volcanic activity.

Other than volcanoes, regions of volcanic activity may contain volcanic necks (rocks that once occupied the vents of volcanoes but are now exposed because of erosion), craters (steep walled depressions at the summit of most volcanoes), calderas (craters that exceed one kilometer in diameter), fissure eruptions (volcanic material extruded from fractures in the crust), and pyroclastic flows.

Igneous intrusive bodies are classified according to their shape and by their orientation with respect to the host rock, generally sedimentary rock. The two general shapes are tabular (sheetlike) and massive. Intrusive igneous bodies that cut across existing sedimentary beds are said to be discordant, whereas those that form parallel to existing sedimentary beds are concordant.

Dikes are tabular, discordant igneous bodies produced when magma is injected into fractures that cut across rock layers. Tabular, concordant bodies called sills form when magma is injected along the bedding surfaces of sedimentary rocks. Laccoliths are similar to sills but form from less-fluid magma that collects as a lens-shaped mass that arches the overlying strata upward. Batholiths, the largest intrusive igneous bodies with surface exposures of more than 100 square kilometers (40 square miles), frequently compose the cores of mountains.

Active areas of volcanism are found along the oceanic ridges, adjacent to ocean trenches, as well as the interiors of plates themselves. Most active volcanoes are associated with plate boundaries.


Geologic Time



The doctrine of uniformitarianism, one of the fundamental principles of modern geology put forth by James Hutton in the late 1700s, states that the physical, chemical, and biological laws that operate today have also operated in the geologic past. The idea is often summarized as, "the present is the key to the past." Hutton argued that processes that appear to be slow-acting could, over long spans of time, produce effects that were just as great as those resulting from sudden catastrophic events. Catastrophism, on the other hand, states that Earth's landscapes have been developed primarily by great catastrophes.

The two types of dates used by geologists to interpret Earth history are 1) relative dates, which put events in their proper sequence of formation, and 2) numerical dates, which pinpoint the time in years when an event took place.

Relative dates can be established using the law of superposition, principle of original horizontality, principle of cross-cutting relationships, inclusions, and unconformities.

Correlation, the matching up of two or more geologic phenomena in different areas, is used to develop a geologic time scale that applies to the whole Earth.

Fossils are the remains or traces of prehistoric life. The special conditions that favor preservation are rapid burial and the possession of hard parts such as shells, bones, or teeth.

Fossils are used to correlate sedimentary rocks that are from different regions by using the rocks' distinctive fossil content and applying the principle of fossil succession. The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.

Radioactivity is the spontaneous breaking apart (decay) of certain unstable atomic nuclei. Three common forms of radioactive decay are 1) emission of alpha particles from the nucleus, 2) emission of a beta particle (or electron) from the nucleus, and 3) capture of an electron by the nucleus.

An unstable radioactive isotope, called the parent, will decay and form daughter products. The length of time for one-half of the nuclei of a radioactive isotope to decay is called the half-life of the isotope. If the half-life of the isotope is known, and the parent/daughter ratio can be measured, the age of a sample can be calculated.

The geologic time scale divides Earth's history into units of varying magnitude. It is commonly presented in chart form, with the oldest time and event at the bottom and the youngest at the top. The principal subdivisions of the geologic time scale, called eons, include the Hadean, Archean, Proterozoic (together, these three eons are commonly referred to as the Precambrian), and, beginning about 540 million years ago, the Phanerozoic. The Phanerozoic (meaning "visible life") eon is divided into the following eras: Paleozoic ("ancient life"), Mesozoic ("middle life"), and Cenozoic ("recent life").

The primary problem in assigning numerical dates to units of time is that not all rocks can be dated radiometrically. A sedimentary rock may contain particles of many ages that have been weathered from different rocks that formed at various times. One way geologists assign numerical dates to sedimentary rocks is to relate them to datable igneous masses, such as volcanic ash beds.


Oceans: The Last Frontier



Oceanography is a composite science that draws on the methods and knowledge of biology, chemistry, physics, and geology to study all aspects of the world ocean.

Earth is a planet dominated by oceans. Seventy-one percent of Earth's area consists of oceans and marginal seas. In the Southern Hemisphere, often called the water hemisphere, about 81% of the surface is water. Of the three major oceans, Pacific, Atlantic, and Indian, the Pacific Ocean is the largest, contains slightly more than half of the water in the world ocean, and has the greatest average depth–3940 meters (12,900 feet).

Salinity is the proportion of dissolved salts to pure water, usually expressed in parts per thousand (‰). The average salinity in the open ocean ranges from 35‰ to 37‰. The principal elements that contribute to the ocean's salinity are chlorine (55%) and sodium (31%). The primary sources for the salts in the ocean are chemical weathering of rocks on the continents and outgassing through volcanism. Outgassing is also considered to be the principal source of water in the oceans as well as in the atmosphere.

In most regions, open oceans exhibit a three-layered temperature and salinity structure. Ocean water temperatures are warmest at the surface because of solar energy. The mixing of waves as well as the turbulence from currents can distribute this heat to a depth of about 450 meters or more. Beneath the sun-warmed zone of mixing, a layer of rapid temperature change, called the thermocline, occurs. Below the thermocline, in the deep zone, temperatures fall only a few more degrees. The changes in salinity with increasing depth correspond to the general three-layered temperature structure. In the low and middle latitudes, a surface zone of higher salinity is underlain by a layer of rapidly decreasing salinity, called the halocline. Below the halocline, salinity changes are small.

The zones that collectively make up a passive continental margin include the continental shelf (a gently sloping, submerged surface extending from the shoreline toward the deep-ocean basin), continental slope (the true edge of the continent, which has a steep slope that leads from the continental shelf into deep water), and in regions where trenches do not exist, the steep continental slope merges into a gradual incline known as the continental rise. The continental rise consists of sediments that have moved downslope from the continental shelf to the deep-ocean floor.

Active continental margins are located primarily around the Pacific Ocean in areas where the leading edge of a continent is overrunning oceanic lithosphere. Here sediment scraped from the descending oceanic plate is plastered against the continent to form a collection of sediments called an accretionary wedge. An active continental margin generally has a narrow continental shelf, which grades into a deep-ocean trench.

Submarine canyons are deep, steep-sided valleys that originate on the continental slope and may extend to depths of three kilometers. Many submarine canyons have been excavated by turbidity currents (downslope movements of dense, sediment-laden water).

The deep-ocean basin lies between the continental margin and the mid-oceanic ridge system. The features of the deep-ocean basin include deep-ocean trenches (the deepest parts of the ocean, where moving crustal plates descend into the mantle), abyssal plains (the most level places on Earth, consisting of thick accumulations of sediments that were deposited atop the low, rough portions of the ocean floor by turbidity currents), and seamounts (isolated volcanic peaks on the ocean floor that originate near oceanic ridges or in association with volcanic hot spots).

Mid-ocean ridges, the sites of sea-floor spreading, are found in all major oceans and represent more than 20 percent of Earth's surface. These broad features are characterized by an elevated position, extensive faulting, and volcanic structures that have developed on newly formed oceanic crust. Most of the geologic activity associated with ridges occurs along a narrow region on the ridge crest, called the rift zone, where magma from the asthenosphere moves upward to create new slivers of oceanic crust.

Coral reefs, which are confined largely to the warm, sunlit waters of the Pacific and Indian Oceans, are constructed over thousands of years primarily from the skeletal remains and secretions of corals and certain algae. Coral islands, called atolls, consist of a continuous or broken ring of coral reef surrounding a central lagoon. Atolls form from corals that grow on the flanks of sinking volcanic islands, where the corals continue to build the reef complex upward as the island sinks.

There are three broad categories of sea floor sediments. Terrigenous sediment consists primarily of mineral grains that were weathered from continental rocks and transported to the ocean. Biogenous sediment consists of shells and skeletons of marine animals and plants. Hydrogenous sediment includes minerals that crystallize directly from seawater through various chemical reactions. Sea floor sediments are helpful when studying worldwide climate changes because they often contain the remains of organisms that once lived near the sea surface. The numbers and types of these organisms change as the climate changes, and their remains in the sediments record these changes.


The Restless Ocean



Surface ocean currents are parts of huge, slowly moving, circular whirls, or gyres, that begin near the equator in each ocean. Wind is the driving force for the ocean's surface currents. Where wind is in contact with the ocean, it passes energy to the water through friction and causes the surface layer to move. The most significant factor other than wind that influences the movement of surface ocean currents is the Coriolis effect, the deflective force of Earth's rotation which causes free-moving objects to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Because of the Coriolis effect, surface currents form clockwise gyres in the Northern Hemisphere and counterclockwise gyres in the Southern Hemisphere.

Ocean currents are important in navigation and travel and for the effect that they have on climates. The moderating effect of poleward-moving warm ocean currents during the winter in middle latitudes is well known. Cold currents depress air temperatures and cause increased fog and reduced rainfall totals.

In contrast to surface currents, deep-ocean circulation is governed by gravity and driven by density differences. The two factors that are most significant in creating a dense mass of water are temperature and salinity.

Tides, the daily rise and fall in the elevation of the ocean surface at a specific location, are caused by the gravitational attraction of the Moon and, to a lesser extent, by the Sun. Near the times of new and full Moons, the Sun and Moon are aligned and their gravitational forces are added together to produce especially high and low tides. These are called the spring tides. Conversely, at about the times of the first and third quarters of the Moon, when the gravitational forces of the Moon and Sun are at right angles, the daily tidal range is less. These are called neap tides.

The three factors that influence the height, wavelength, and period of a wave are 1) wind speed, 2) length of time the wind has blown, and 3) fetch, the distance that the wind has traveled across the open water.

The two types of wind-generated waves are 1) waves of oscillation, which are waves in the open sea in which the wave form advances as the water particles move in circular orbits, and 2) waves of translation, the turbulent advance of water formed near the shore as waves of oscillation collapse, or break, and form surf.

Features produced by shoreline erosion include wave-cut cliffs (which originate from the cutting action of the surf against the base of coastal land), wave-cut platforms (relatively flat, benchlike surfaces left behind by receding cliffs), sea arches (formed when a headland is eroded and two caves from opposite sides unite), and sea stacks (formed when the roof of a sea arch collapses).

Some of the features formed when sediment is moved by beach drift and longshore currents are spits (elongated ridges of sand that project from the land into the mouth of an adjacent bay), baymouth bars (sand bars that completely cross a bay), and tombolos (ridges of sand that connect an island to the mainland or to another island).

Local factors that influence shoreline erosion are 1) the proximity of a coast to sediment-laden rivers, 2) the degree of tectonic activity, 3) the topography and composition of the land, 4) prevailing winds and weather patterns, and 5) the configuration of the coastline and nearshore areas.

Three basic responses to shoreline erosion problems are 1) building structures such as groins (short walls built at a right angle to the shore to trap moving sand) and seawalls (barriers constructed to prevent waves from reaching the area behind the wall) to hold the shoreline in place, 2) beach nourishment, which involves the addition of sand to replenish eroding beaches, and 3) relocate buildings away from the beach.

One frequently used classification of coasts is based upon changes that have occurred with respect to sea level. Emergent coasts, often with wave-cut cliffs and wave-cut platforms above sea level, develop either because an area experiences uplift or as a result of a drop in sea level. Conversely, submergent coasts, with their drowned river mouths, called estuaries, are created when sea level rises or the land adjacent to the sea subsides.


Heating the Atmosphere



Weather is the state of the atmosphere at a particular place for a short period of time. Climate, on the other hand, is a generalization of the weather conditions of a place over a long period of time.

The most important elements, those quantities or properties that are measured regularly, of weather and climate are 1) air temperature, 2) humidity, 3) type and amount of cloudiness, 4) type and amount of precipitation, 5) air pressure, and 6) the speed and direction of the wind.

If water vapor, dust, and other variable components of the atmosphere were removed, clean, dry air would be composed almost entirely of nitrogen (N), about 78% of the atmosphere by volume, and oxygen (O2), about 21%. Carbon dioxide (CO2), although present only in minute amounts (0.036%), is important because it has the ability to absorb heat radiated by Earth and thus helps keep the atmosphere warm. Among the variable components of air, water vapor is very important because it is the source of all clouds and precipitation and, like carbon dioxide, it is also a heat absorber.

Ozone (O3), the triatomic form of oxygen, is concentrated in the 10- to 50-kilometer altitude range of the atmosphere, and is important to life because of its ability to absorb potentially harmful ultraviolet radiation from the Sun.

Because the atmosphere gradually thins with increasing altitude, it has no sharp upper boundary but simply blends into outer space. Based on temperature, the atmosphere is divided vertically into four layers. The troposphere is the lowermost layer. In the troposphere, temperature usually decreases with increasing altitude. This environmental lapse rate is variable, but averages about 6.5°C per kilometer (3.5°F per 1000 feet). Essentially all important weather phenomena occur in the troposphere. Above the troposphere is the stratosphere, which exhibits warming because of absorption of ultraviolet radiation by ozone. In the mesosphere, temperatures again decrease. Upward from the mesosphere is the thermosphere, a layer with only a minute fraction of the atmosphere's mass and no well-defined upper limit.

The two principal motions of Earth are 1) rotation, the spinning of Earth about its axis, which produces the daily cycle of daylight and darkness, and 2) revolution, the movement of Earth in its orbit around the Sun.

Several factors act together to cause the seasons. Earth's axis is inclined 23° degrees from the perpendicular to the plane of its orbit around the Sun and remains pointed in the same direction (toward the North Star) as Earth journeys around the Sun. As a consequence, Earth's orientation to the Sun continually changes. The yearly fluctuations in the angle of the Sun and length of daylight brought about by Earth's changing orientation to the Sun cause seasons.

The three mechanisms of heat transfer are 1) conduction, the transfer of heat through matter by molecular activity, 2) convection, the transfer of heat by the movement of a mass or substance from one place to another, and 3) radiation, the transfer of heat by electromagnetic waves.

Electromagnetic radiation is energy emitted in the form of rays, or waves, called electromagnetic waves. All radiation is capable of transmitting energy through the vacuum of space. One of the most important differences between electromagnetic waves are their wavelengths, which range from very long radio waves to very short gamma rays. Visible light is the only portion of the electromagnetic spectrum we can see. Some of the basic laws that govern radiation as it heats the atmosphere are 1) all objects with temperatures above -273 degrees Celsius (absolute zero) emit radiant energy, 2) hotter objects radiate more total energy than do colder objects, 3) the hotter the radiating body, the shorter the wavelengths of maximum radiation, and 4) objects that are good absorbers of radiation are good emitters as well.

The general drop in temperature with increasing altitude in the troposphere supports the fact that the atmosphere is heated from the ground up. Approximately 50% of the solar energy, primarily in the form of the shorter wavelengths, that strikes the top of the atmosphere is ultimately absorbed at Earth's surface. Earth releases the absorbed radiation in the form of long-wave radiation. The atmospheric absorption of this long-wave terrestrial radiation, primarily by water vapor and carbon dioxide, is responsible for heating the atmosphere.

Carbon dioxide, an important heat absorber in the atmosphere, is one of several gases that influence global warming. Some consequences of global warming could be 1) shifts in temperature and rainfall patterns, 2) a gradual rise in sea level, 3) changing storm tracks and both the higher frequency and greater intensity of hurricanes, and 4) an increase in the frequency and intensity of heat waves and droughts.

The factors that cause temperature to vary from place to place, also called the controls of temperature, are 1) differences in the receipt of solar radiation—the greatest single cause, 2) the unequal heating and cooling of land and water, in which land heats more rapidly and to higher temperatures than water and cools more rapidly and to lower temperatures than water, 3) altitude, 4) geographic position, 5) cloud cover and albedo, and 6) ocean currents.

Temperature distribution is shown on a map by using isotherms, which are lines that connect equal temperatures.


Clouds and Precipitation



Water vapor, an odorless, colorless gas, changes from one state of matter (solid, liquid, or gas) to another at the temperatures and pressures experienced near Earth's surface. The processes involved in changing the state of matter of water are evaporation, condensation, melting, freezing, sublimation, and deposition.

Humidity is the general term used to describe the amount of water vapor in the air. Relative humidity, the ratio (expressed as a percent) of the air's water vapor content to its water vapor capacity at a given temperature, is the most familiar term used to describe humidity. The water vapor capacity of air is temperature dependent, with warm air having a much greater capacity than cold air.

Relative humidity can be changed in two ways. One is by adding or subtracting water vapor. The second is by changing the air's temperature. When air is cooled, its relative humidity increases. Air is said to be saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure. Dew point is the temperature to which air would have to be cooled in order to reach saturation.

The cooling of air as it rises and expands due to successively lower pressure is the basic cloud-forming process. Temperature changes in air brought about by compressing or expanding the air are called adiabatic temperature changes. Unsaturated air warms by compression and cools by expansion at the rather constant rate of 10°C per 1000 meters of altitude change, a figure called the dry adiabatic rate. If air rises high enough, it will cool sufficiently to cause condensation and form a cloud. From this point on, air that continues to rise will cool at the wet adiabatic rate which varies from 5°C to 9°C per 1000 meters of ascent. The difference in the wet and dry adiabatic rates is caused by the condensing water vapor releasing latent heat, thereby reducing the rate at which the air cools.

Three mechanisms that can initiate the vertical movement of air are 1) orographic lifting, which occurs when elevated terrains, such as mountains, act as barriers to the flow of air; 2) frontal wedging, when cool air acts as a barrier over which warmer, less dense air rises; 3) convergence, which happens when air flows together and a general upward movement of air occurs; and 4) localized convective lifting, when unequal surface heating causes localized pockets of air to rise.

The stability of air is determined by examining the temperature of the atmosphere at various altitudes. Air is said to be unstable when the environmental lapse rate (the rate of temperature decrease with increasing altitude in the troposphere) is greater than the dry adiabatic rate. Stated differently, a column of air is unstable when the air near the bottom is significantly warmer (less dense) than the air aloft. When stable air is forced aloft, precipitation, if any, is light, whereas unstable air generates towering clouds and stormy conditions.

For condensation to occur, air must be saturated. Saturation takes place either when air is cooled to its dew point, which most commonly happens, or when water vapor is added to the air. There must also be a surface on which the water vapor may condense. In cloud and fog formation, tiny particles called condensation nuclei serve this purpose.

Clouds are classified on the basis of their appearance and height. The three basic forms are cirrus (high, white, thin, wispy fibers), cumulus (globular, individual cloud masses), and stratus (sheets or layers that cover much or all of the sky). The four categories based on height are high clouds (bases normally above 6000 meters), middle clouds (from 2000 to 6000 meters), low clouds (below 2000 meters), and clouds of vertical development.

Fog is defined as a cloud with its base at or very near the ground. Fogs form when air is cooled below its dew point or when enough water vapor is added to the air to bring about saturation. Various types of fog include advection fog, radiation fog, upslope fog, steam fog, and frontal (or precipitation), fog.

For precipitation to form, millions of cloud droplets must somehow join together into large drops. Two mechanisms for the formation of precipitation have been proposed. One, in clouds where the temperatures are below freezing, ice crystals form and fall as snowflakes. At lower altitudes the snowflakes melt and become raindrops before they reach the ground. Two, large droplets form in warm clouds that contain large hygroscopic ("water seeking") nuclei, such as salt particles. As these big droplets descend, they collide and join with smaller water droplets. After many collisions the droplets are large enough to fall to the ground as rain.

The forms of precipitation include rain, snow, sleet, hail, and rime.


The Atmosphere in Motion



Air has weight: at sea level it exerts a pressure of 1 kilogram per square centimeter (14.7 pounds per square inch). Air pressure is the force exerted by the weight of air above. With increasing altitude, there is less air above to exert a force, and thus air pressure decreases with altitude, rapidly at first, then much more slowly. The unit used by meteorologists to measure atmospheric pressure is the millibar. Standard sea level pressure is expressed as 1013.2 millibars. Isobars are lines on a weather map that connect places of equal air pressure.

A mercury barometer measures air pressure using a column of mercury in a glass tube that is sealed at one end and inverted in a dish of mercury. As air pressure increases, the mercury in the tube rises; conversely, when air pressure decreases, so does the height of the column of mercury. A mercury barometer measures atmospheric pressure in "inches of mercury;" the height of the column of mercury in the barometer. Standard atmospheric pressure at sea level equals 29.92 inches of mercury. Aneroid ("without liquid") barometers consist of partially- evacuated metal chambers that compress as air pressure increases and expand as pressure decreases.

Wind is the horizontal flow of air from areas of higher pressure toward areas of lower pressure. Winds are controlled by the following combination of forces: 1) the pressure gradient force (amount of pressure change over a given distance), 2) Coriolis effect (deflective effect of Earth's rotation–to the right in the Northern Hemisphere and to the left in the Southern Hemisphere), 3) friction with Earth's surface (slows the movement of air and alters wind direction), and 4) the tendency of a moving object to continue moving in a straight line.

The two types of pressure centers are 1) cyclones, or lows (centers of low pressure), and 2) anticyclones, or highs (high-pressure centers). In the Northern Hemisphere, winds around a low (cyclone) are counterclockwise and inward. Around a high (anticyclone), they are clockwise and outward. In the Southern Hemisphere, the Coriolis effect causes winds to be clockwise around a low and counterclockwise around a high. Since air rises and cools adiabatically in a low pressure center, cloudy conditions and precipitation are often associated with their passage. In a high pressure center, descending air is compressed and warmed; therefore, cloud formation and precipitation are unlikely in an anticyclone, and "fair" weather is usually expected. Earth's global pressure zones include the equatorial low, subtropical high, subpolar low, and polar high. The global surface winds associated with these pressure zones are the trade winds, westerlies, and polar easterlies.

Particularly in the Northern Hemisphere, large seasonal temperature differences over continents disrupt the idealized, or zonal, global patterns of pressure and wind. In winter, large, cold landmasses develop a seasonal high-pressure system from which surface air flow is directed off the land. In summer, landmasses are heated and a low-pressure system develops over them, which permits air to flow onto the land. These seasonal changes in wind direction are known as monsoons.

In the middle latitudes, between 30 and 60 degrees latitude, the general west-to- east flow of the westerlies is interrupted by the migration of cyclones and anticyclones. The paths taken by these cyclonic and anticyclonic systems is closely correlated to upper-level air flow and the polar jet stream. The average position of the polar jet stream, and hence the paths of cyclonic systems, migrates equatorward with the approach of winter and poleward as summer nears.

Local winds are small-scale winds produced by a locally generated pressure gradient. Local winds include sea and land breezes (formed along a coast because of daily pressure differences over land and water), valley and mountain breezes (daily wind similar to sea and land breezes except in a mountainous area where the air along slopes heats differently than the air at the same elevation over the valley floor), chinook and Santa Ana winds (warm, dry winds created when air descends the leeward side of a mountain and warms by compression).

The two basic wind measurements are direction and speed. Winds are always labeled by the direction from which they blow. Wind direction is measured with a wind vane and wind speed is measured using a cup anemometer.


Weather Patterns and Severe Weather



An air mass is a large body of air, usually 1600 kilometers (1000 miles) or more across, which is characterized by a sameness of temperature and moisture at any given altitude. When this air moves out of its region of origin, called the source region, it will carry these temperatures and moisture conditions elsewhere, perhaps eventually affecting a large portion of a continent.

Air masses are classified according to 1) the nature of the surface in the source region and 2) the latitude of the source region. Continental (c) designates an air mass of land origin, with the air likely to be dry; whereas a maritime (m) air mass originates over water, and therefore will be relatively humid. Polar (P) air masses originate in high latitudes and are cold. Tropical (T) air masses form in low latitudes and are warm. According to this classification scheme, the four basic types of air masses are continental polar (cP), continental tropical (cT), maritime polar (mP), and maritime tropical (mT). Continental polar (cP) and maritime tropical (mT) air masses influence the weather of North America most, especially east of the Rocky Mountains. Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.

Fronts are boundaries that separate air masses of different densities, one warmer and often higher in moisture content than the other. A warm front occurs when the surface position of the front moves so that warm air occupies territory formerly covered by cooler air. Along a warm front, a warm air mass overrides a retreating mass of cooler air. As the warm air ascends, it cools adiabatically to produce clouds and frequently, light-to-moderate precipitation over a large area. A cold front forms where cold air is actively advancing into a region occupied by warmer air. Cold fronts are about twice as steep and move more rapidly than warm fronts. Because of these two differences, precipitation along a cold front is more intense and of shorter duration than precipitation associated with a warm front.

The primary weather producers in the middle latitudes are large centers of low pressure that generally travel from west to east, called middle-latitude cyclones. These bearers of stormy weather, which last from a few days to a week, have a counterclockwise circulation pattern in the Northern Hemisphere, with an inward flow of air toward their centers. Most middle-latitude cyclones have a cold front and frequently a warm front extending from the central areas of low pressure. Convergence and forceful lifting along the fronts initiate cloud development and frequently cause precipitation. As a middle-latitude cyclone with its associated fronts passes over a region, it often brings with it abrupt changes in the weather. The particular weather experienced by an area depends on the path of the cyclone.

Thunderstorms are caused by the upward movement of warm, moist, unstable air, triggered by a number of different processes. They are associated with cumulonimbus clouds that generate heavy rainfall, lightning, thunder, and occasionally hail and tornadoes.

Tornadoes, destructive, local storms of short duration, are violent windstorms associated with severe thunderstorms that take the form of a rotating column of air that extends downward from a cumulonimbus cloud. Tornadoes are most often spawned along the cold front of a middle-latitude cyclone, most frequently during the spring months.

Hurricanes, the greatest storms on Earth, are tropical cyclones with wind speeds in excess of 119 kilometers (74 miles) per hour. These complex tropical disturbances develop over tropical ocean waters and are fueled by the latent heat liberated when huge quantities of water vapor condense. Hurricanes form most often in late summer when ocean-surface temperatures reach 27°C (80°F) or higher and thus are able to provide the necessary heat and moisture to the air. Hurricanes diminish in intensity whenever they 1) move over cool ocean water that cannot supply adequate heat and moisture, 2) move onto land, or 3) reach a location where large-scale flow aloft is unfavorable. Hurricane damage is of three types: 1) storm surge, 2) wind damage, and 3) inland flooding.


The Nature of the Solar System



Early Greeks held the geocentric ("Earth-centered") view of the universe, believing that Earth was a sphere that stayed motionless at the center of the universe. Orbiting Earth were the Moon, Sun, and the known planets—Mercury, Venus, Mars, Jupiter, and Saturn. To the early Greeks, the stars traveled daily around Earth on a transparent, hollow sphere called the celestial sphere. In A.D. 141, Claudius Ptolemy presented the geocentric outlook of the Greeks in its finest form which became known as the Ptolemaic system.

Modern astronomy evolved through the work of many dedicated individuals during the 1500s and 1600s. Nicolaus Copernicus (1473-1543) reconstructed the solar system with the Sun at the center and the planets orbiting around it, but erroneously continued to use circles to represent the orbits of planets. Tycho Brahe's (1546-1601) observations were far more precise than any made previously and are his legacy to astronomy. Johannes Kepler (1571-1630) ushered in the new astronomy with his three laws of planetary motion. After constructing his own telescope, Galileo Galilei (1564-1642) made many important discoveries that supported the Copernican view of a Sun-centered solar system. Sir Isaac Newton (1643-1727), developed laws of motion and proved that the force of gravity, combined with the tendency of an object to move in a straight line, results in elliptical orbits for planets.

The planets can be arranged into two groups: the terrestrial (Earthlike) planets (Mercury, Venus, Earth, and Mars) and the Jovian (Jupiterlike) planets (Jupiter, Saturn, Uranus, and Neptune). Pluto is not included in either group. When compared to the Jovian planets, the terrestrial planets are smaller, more dense, contain proportionally more rocky material, and have slower rates of rotation.

The nebular hypothesis describes the formation of the solar system. The planets and Sun began forming about 5 billion years ago from a large cloud of dust and gases called a nebula. As the nebular cloud contracted, it began to rotate and assume a disk shape. Material that was gravitationally pulled toward the center became the protosun. Within the rotating disk, small centers, called protoplanets, swept up more and more of the nebular debris. Due to their high temperatures and weak gravitational fields, the inner planets (Mercury, Venus, Earth, and Mars) were unable to accumulate many of the lighter components (hydrogen, ammonia, methane, and water) of the nebula. However, because of the very cold temperatures existing far from the Sun, the fragments from which the Jovian planets formed contained a high percentage of ices—water, carbon dioxide, ammonia, and methane.

The lunar surface exhibits several types of features. Most craters were produced by the impact of rapidly moving debris (meteoroids). Bright, densely cratered highlands make up most of the lunar surface. Dark, fairly smooth lowlands are called maria. Maria basins are enormous impact craters that have been flooded with layer upon layer of very fluid basaltic lava. All lunar terrains are mantled with a soil-like layer of gray, unconsolidated debris, called lunar regolith, which has been derived from a few billion years of meteoric bombardment.

Mercury is a small, dense planet that has no atmosphere and exhibits the greatest temperature extremes of any planet. Venus, the brightest planet in the sky, has a thick, heavy atmosphere composed primarily of carbon dioxide, a surface of relatively subdued plains and inactive volcanic features, a surface atmospheric pressure ninety times that of Earth's, and surface temperatures of 475°C (900°F). Mars, the red planet, has a carbon dioxide atmosphere only 1 percent as dense as Earth's, extensive dust storms, numerous inactive volcanoes, many large canyons, and several valleys of debatable origin exhibiting drainage patterns similar to stream valleys on Earth. Jupiter, the largest planet, rotates rapidly, has a banded appearance, a Great Red Spot that varies in size, a ring system, and at least sixteen moons (one of the moons, Io, is a volcanically active body). Saturn is best known for its system of rings. It also has a dynamic atmosphere with winds up to 930 miles per hour and "storms" similar to Jupiter's Great Red Spot. Uranus and Neptune are often called "the twins" because of similar structure and composition. A unique feature of Uranus is the fact that it rotates "on its side." Neptune has white, cirruslike clouds above its main cloud deck and an Earth-sized Great Dark Spot, assumed to be a large rotating storm similar to Jupiter's Great Red Spot. Pluto, a small frozen world with one moon, has an elongated orbit that causes it to occasionally travel inside the orbit of Neptune, but with no chance of collision.

The minor members of the solar system include the asteroids, comets, and meteoroids. No conclusive evidence has been found to explain the origin of the asteroids. Comets are made of frozen gases with small pieces of rocky and metallic material. Many travel in very elongated orbits that carry them beyond Pluto. Meteoroids, small solid particles that travel through interplanetary space, become meteors when they enter Earth's atmosphere and vaporize with a flash of light. Meteor showers appear to occur when Earth encounters a swarm of meteoroids, probably lost by a comet. Meteorites are the remains of meteoroids found on Earth. The three types of meteorites are 1) iron, 2) stony, and 3) stony-iron.


Beyond the Solar System



One method for determining the distance to a star is to use a measurement called stellar parallax, the extremely slight back-and-forth shifting in a nearby star's position due to the orbital motion of Earth. The farther away a star is, the less its parallax. A unit used to express stellar distance is the light-year, which is the distance light travels in a year–about 9.5 trillion kilometers (5.8 trillion miles).

The intrinsic properties of stars include brightness, color, temperature, mass, and size. Three factors control the brightness of a star as seen from Earth: how big it is, how hot it is, and how far away it is. Magnitude is the measure of a star's brightness. Apparent magnitude is how bright a star appears when viewed from Earth. Absolute magnitude is the "true" brightness if a star were at a standard distance of about 32.6 light-years. The difference between the two magnitudes is directly related to a star's distance. Color is a manifestation of a star's temperature. Very hot stars (surface temperatures above 30,000 K) appear blue; red stars are much cooler (surface temperatures generally less than 3000 K). Stars with surface temperatures between 5000 and 6000 K appear yellow, like the sun. The center of mass of orbiting binary stars (two stars revolving around a common center of mass under their mutual gravitational attraction) is used to determine the mass of the individual stars in a binary system.

A Hertzsprung-Russell diagram is constructed by plotting the absolute magnitudes and temperatures of stars on a graph. A great deal about the sizes of stars can be learned from H-R diagrams. Stars located in the upper-right position of an H-R diagram are called giants, luminous stars of large radius. Supergiants are very large. Very small white dwarf stars are located in the lower-central portion of an H-R diagram. Ninety percent of all stars, called main-sequence stars, are in a band that runs from the upper-left corner to the lower-right corner of an H-R diagram.

New stars are born out of enormous accumulations of dust and gases, called nebula, that are scattered between existing stars. A bright nebula glows because the matter is close to a very hot (blue) star. The two main types of bright nebulae are emission nebulae (which derive their visible light from the fluorescence of the ultraviolet light from a star in or near the nebula) and reflection nebulae (relatively dense dust clouds in interstellar space that are illuminated by reflecting the light of nearby stars). When a nebula is not close enough to a bright star to be illuminated, it is referred to as a dark nebula.

Stars are born when their nuclear furnaces are ignited by the unimaginable pressures and temperatures in collapsing nebulae. New stars not yet hot enough for nuclear fusion are called protostars. When collapse causes the core of a protostar to reach a temperature of at least 10 million K, the fusion of hydrogen nuclei into helium nuclei begins in a process called hydrogen burning. The opposing forces acting on a star are gravity trying to contract it and gas pressure (thermal nuclear energy) trying to expand it. When the two forces are balanced, the star becomes a stable main-sequence star. When the hydrogen in a star's core is consumed, its outer envelope expands enormously and a red giant star, hundreds to thousands of times larger than its main-sequence size, forms. When all the usable nuclear fuel in these giants is exhausted and gravity takes over, the stellar remnant collapses into a small dense body.

The final fate of a star is determined by its mass. Stars with less than one-half the mass of the sun collapse into hot, dense white dwarf stars. Medium mass stars (between 0.5 and 3.0 times the mass of the sun) become red giants, collapse, and end up as white dwarf stars, often surrounded by expanding spherical clouds of glowing gas called planetary nebulae. Stars more than three times the mass of the sun terminate in a brilliant explosion called a supernova. Supernovae events can produce small, extremely dense neutron stars, composed entirely of subatomic particles called neutrons; or even smaller and more dense black holes, objects that have such immense gravity that light cannot escape their surface.

The Milky Way galaxy is a large, disk-shaped, spiral galaxy about 100,000 light-years wide and about 10,000 light-years thick at the center. There are three distinct spiral arms of stars, with some showing splintering. The sun is positioned in one of these arms about two-thirds of the way from the galactic center, at a distance of about 30,000 light-years. Surrounding the galactic disk is a nearly spherical halo made of very tenuous gas and numerous globular clusters (nearly spherically shaped groups of densely packed stars).

The various types of galaxies include 1) irregular galaxies, which lack symmetry and account for only 10 percent of the known galaxies; 2) spiral galaxies, which are typically disk-shaped with a somewhat greater concentration of stars near their centers, often containing arms of stars extending from their central nucleus; and 3) elliptical galaxies, the most abundant type, which have an ellipsoidal shape that ranges to nearly spherical, and lack spiral arms.

By applying the Doppler effect (the apparent change in wavelength of radiation caused by the motions of the source and the observer) to the light of galaxies, galactic motion can be determined. Most galaxies have Doppler shifts toward the red end of the spectrum, indicating increasing distance. The amount of Doppler shift is dependent on the velocity at which the object is moving. Because the most distant galaxies have the greatest red shifts, Edwin Hubble concluded in the early 1900s that they were retreating from us with greater recessional velocities than more nearby galaxies. It was soon realized that an expanding universe can adequately account for the observed red shifts.

The belief in the expanding universe led to the widely accepted Big Bang theory. According to this theory, the entire universe was at one time confined in a dense, hot, supermassive concentration. About 20 billion years ago, a cataclysmic explosion hurled this material in all directions, creating all matter and space. Eventually the ejected masses of gas cooled and condensed, forming the stellar systems we now observe fleeing from their place of origin.