Thursday, November 23rd, 2017

* Nadir-The point on the celestial sphere directly opposite the zenith and underneath your feet.
* Natural gas-A mixture of naturally occurring light hydrocarbons composed mainly of methane, CH4.
* Natural levee-A ridge or embankment of flood-deposited sediment along both banks of a stream channel.
* Neap tide- a tide of small range occurring at the quarter phases of the moon. The gravity of the sun and of the moon pull at right angles to each other. High tides are lowest. Low tides are highest.
* Neap tide-A relatively small tide that forms when the Moon is 90 degrees out of alignment with the Sun and the Earth.
* Nebula -A diffuse mass of interstellar dust and gas.
* Nebula-An interstellar cloud of gas and dust. A planetary nebula is created when a star the size of our Sun explodes.
* Neutrino -A fundamental particle supposedly produced in massive numbers by the nuclear reactions in stars; they are very hard to detect because the vast majority of them pass completely through the Earth without interacting.
* Neutron star-A small, extremely dense star composed almost entirely of neutrons. (See also "pulsar.")
* Neutron-A subatomic particle with the mass of a proton but no electrical charge.
* Newton's law of gravitation-The force of gravity between any two objects in the universe is equal to the mass of the first object (m1), multiplied by the mass of the second object (m2), multiplied by a gravitational constant (G), all divided by the square of the distance between the two objects.
* Newton's laws of motion-Three laws of motion that give a more fundamental understanding of the universe than Kepler's laws. Law one- All objects at rest stay at rest. All objects in motion stay in motion, in a straight line and at a constant speed, unless acted upon by a force. Law two- Force equals mass times acceleration, or F = m x a3 Law three- For every force one body exerts on a second body, the second exerts an equal and opposite force on the first.
* Nimbo-A prefix or suffix added to cloud types to indicate precipitation.
* Nimbostratus cloud-A stratus cloud from which precipitation is falling.
* Nimbus- The Latin word for "rain" used to describe a cloud or group of clouds from which rain is falling, e.g., nimbostratus.
* Nonconformity-A type of unconformity in which layered sedimentary rocks lie on an erosion surface cut into igneous or metamorphic rocks.
* Nonfoliated-The lack of layering in metamorphic rock.
* Nonpoint source pollution-Pollution generated over a broad area, such as fertilizers and pesticides spread over agricultural fields.
* Nonrenewable resource-A resource in which formation of new deposits occurs much more slowly than consumption.
* Nor'easter- Common contraction for northeaster, a northeast wind often blowing at gale or storm speeds. Also refers to a type of storm moving up the North American Atlantic coast which is first felt as a northeast wind and it moves northward.
* Normal fault-A fault in which the hanging wall has moved downward relative to the footwall.
* Normal lapse rate-The decrease in temperature with elevation in air that is neither rising nor falling.
* Normal polarity-A magnetic orientation the same as that of the Earth's modern magnetic field.
* Nuclear fission-The breakdown of an atomic nucleus of an element or relatively high atomic number into two or more nuclei of lower atomic number, with conversion of part of its mass into energy.
* Nuclear fuel-Radioactive isotopes, such as those of uranium, used to generate electricity in nuclear reactors.
* Nuclear fusion -A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence E=mc2. This is the source of the Sun's energy and, ultimately, of (almost) all energy on Earth.
* Nucleus-The small, dense, central portion of an atom composed of protons and neutrons. Nearly all of the mass of an atom is concentrated in the nucleus.
* Nuee ardente-A swiftly flowing, often red-hot cloud of gas, volcanic ash, and other pyroclastics formed by an explosive volcanic eruption. (syn: ash flow)

* Obsidian- A black or dark-colored volcanic glass, usually composed of rhyolite.
* Obsidian-A black or dark-colored glassy volcanic rock usually of rhyolitic composition.
* Occluded front- a front formed when a cold front overtakes a warm front.
* Occluded front-A front that forms when a faster moving cold mass traps a warm air mass against a second mass of cold air. The faster moving cold air mass slides beneath the warm air, lifting it off the ground. Precipitation occurs along both frontal boundaries, resulting in a large zone of inclement weather.
* Ocean current- any continuous flow of water along a definite path in the oceans. Surface currents are caused by prevailing winds.
* Oceanic Crust- The earth's crust where it underlies oceans; thin, dense, basaltic, and young.
* Oceanic crust-The 7 to 10 kilometer thick layer of sediment and basalt that underlies the ocean basins.
* Oceanic island-A volcanic island formed at a hot spot above a mantle plume.
* Oceanic trench-A long, narrow, steep-sided depression of the sea floor formed where a subducting oceanic plate sinks into the mantle.
* Ogallala aquifer-The aquifer that extends for almost 1000 kilometers from the Rocky Mountains eastward beneath portions of the Great Plains.
* Oil shale-A kerogen-bearing sedimentary rock that yields liquid or gaseous hydrocarbons when heated.
* Oil trap-Any rock barrier that accumulates oil or gas by preventing its upward movement.
* Oil-A naturally occurring liquid or gas composed of a complex mixture of hydrocarbons. (syn: petroleum)
* Old -A planetary surface that has been modified little since its formation typically featuring large numbers of impact craters; (compare to young).
* Oligotrophic lake-A lake characterized by nearly pure water with low concentrations of nitrates, phosphates and other plant nutrients. Oligotrophic lakes have low productivities, meaning that they sustain relatively few living organisms, although lakes of this type typically contain a few huge trout or similar game fish, and are commonly deep.
* Olivine-A common rock-forming mineral in mafic and ultramafic rocks with a composition that varies between Mg2SiO4 and Fe2SiO4.
* Orbit- A specific path followed by a planet, satellite, etc.
* Ore-A natural material that is sufficiently enriched in one or more minerals to be mined profitably.
* Organic rocks- sedimentary rocks formed from the remains of plants or animals. Examples: coal, limestone.
* Organic sedimentary rock-Sedimentary rocks such as coal, that consist of the lithified organic remains of plants or animals.
* Original horizontality - A fundamental principle of geology, which states that sediments settle and accumulate horizontally on the sea floor.
* Original horizontality (principle of)-See "principle of original horizontality."
* Orogeny-The process of mountain building; all tectonic processes associated with mountain building.
* Orographic Clouds- Clouds formed when air forced to rise when flowing over mountains or large hills reaches the condensation level.
* Orographic lifting-Lifting of air that occurs when air flows over a mountain.
* Orthoclase-A common rock-forming mineral; a variety of potassium feldspar, (kalsi3o8).
* Outer planets- the five farthest planets (beyond the asteroid belt).
* Outwash plain-A broad level surface formed where glacial sediment is deposited in front of or beyond a glacier.
* Outwash- sediments deposited in front of a glacier by streams of meltwater. Often deposited to form outwash plains: broad, layered, gently-sloping
* Oxbow lake - the crescent-shaped lake formed when a river meander gets completely cut off from the river.
* Oxbow lake-A crescent-shaped lake formed where a meander is cut off from a stream and the ends of the cut off meander become plugged with sediment.
* Oxidation- the chemical reaction of oxygen with other substances. (commonly, rusting)
* Oxidation-The loss of electrons from a compound or element during a chemical reaction. In the weathering of common minerals, oxidation usually occurs when a mineral reacts with molecular oxygen.
* Ozone- A molecule made up of three oxygen atoms. It absorbs both ultraviolet-B and heat radiation. It is formed in the upper atmosphere through the dissociation of oxygen molecules by solar radiation. In the lower troposphere, it is a pollutant which characterizes photochemical or Los Angeles-type smog. It is formed by the photochemical reactions of nitrogen oxides and hydrocarbon gases.
* Ozone Layer- About 90% of the ozone in the Earth's atmosphere is found in a layer between 22 and 30 km above the surface. This layer is within a region of the atmosphere called the stratosphere. The stratosphere is a stable layer of the atmosphere where exchanges with the lower atmosphere or troposphere are minimal. Ozone is naturally formed here through the action of sunlight on molecular oxygen (O2). The introduction of ozone-depleting chemicals into the ozone layer destroy ozone at a faster rate than the natural rate of production.