Geology Terms

Abyssal Zone
A region of greatest ocean depths, generally greater than 1,000 meters, including the deep-sea trenches. Biological activity is rare in the abyssal zone; light does not penetrate the water, as the depth and pressure are tremendous. The region represents about 250 million square kilometers of Earth's surface.

Age
An interval of geological time that indicates when a body of rock was formed in the surface of Earth. A group of ages forms an epoch.

Alluvium
The sediment carried by rivers, including deposits from estuaries, lakes, and other freshwater bodies draining into a river. The particles of sediment are generally smaller than 0.02 millimeter, depending on such factors as valleyside slopes in the watershed, the distance carried downstream, and progressive wear on the particles as they move downstream.

Barrier Beach
An accumulation of sand, rock, and other material lying parallel to the coast but separated from it by a channel; a barrier beach measures from a few meters to a few kilometers in width. Large barrier beaches may be identified as barrier islands. They are formed by the action of waves but are usually vulnerable to overwashing or breaching during severe storms.

Bathyal Zone
A zone of ocean water ranging from about 200 meters to 1,000 meters in depth, generally located along continental slopes. Unlike the abyssal zone, light reaches the upper layer of the bathyal zone, and there is abundant biological activity in the water. The bathyal zone of the world covers a total of about 40 million square kilometers.

Bed
The smallest division of stratified sedimentary rock, usually occurring as a relatively thin sheet of sedimentary material separating distinctively different layers above and below it. A bed often marks a particular event in geologic history, such as a volcanic eruption, and it may contain fossils that help identify its age.

Cambrian
The earliest period of the Paleozoic era, about 600 million years ago. Rocks formed at this period contain the earliest fossil remains of invertebrate animals.

Continental Drift
The shifting of continental landmasses from one location to another on the face of earth, owing to seafloor spreading.

Coriolis Effect
A force produced on objects moving on a north-south line on the surface of Earth because of the angular velocity of Earth as it rotates from west to east. Thus, a projectile fired directly southward from the North Pole would be to the west. The Coriolis force affects mainly the flow of air in the atmosphere.

Creep
The slow movement of rocks and soil down slopes of hills, owing to the pull of gravity. It is believed the movement involves a sliding of the entire Earth mantle over the underlying bedrock rather than changes within the mantle itself. The effect can be observed in the tendency of telephone poles and other objects to alter positions on gentle slopes over a period of years.

Diagenesis
The process whereby sedimentary rock is formed from sediment because of compaction, reduced pore space between particles, and chemical reactions between molecules of the compressed particles and dissolved substances in moisture between the particles.

Era
An interval of geological time composed of a group of periods.

Estuary
The portion of a river that is affected by ocean tides above the mouth, with a resulting mixture of salt water and fresh water. Most estuaries are former valleys that were flooded by rising ocean levels after the last glacial event. The Hudson River is an example of an estuary.

Fjord
A narrow sea inlet between mountain slopes. Most fjords were once glaciated valleys that became flooded by rising sea water after the last ice age. In some cases, the bottom of the fjord may be lower than the bottom of the sea at its opening into the fjord.

Geology
The science of the structure and composition of Earth.

Glacier
An accumulation of land ice that develops in the colder regions and higher latitudes of the Earth. It is formed by compaction of accumulated snow moving downslope from a source area because of the force of gravity. A glacier is usually confined within the limited space of a valley or basin. It may be gaining ice at the source but losing ice a point where it melts while moving into warmer temperatures or a body of water.

Induration
The hardening of porous rocks or soils owing to weather conditions and the chemical actions of dissolved minerals, which form a cement. He concrete-like rock formed by induration usually consists of combinations of calcium, silicon, or iron with carbon and oxygen.

Lava
See magna.

Magma
Hot, molten material from deep underground, usually associated with volcanic eruptions. Magma that reaches the surface is called lava.

Mercator Projection
A map in which the spherical Earth is projected as a cylinder onto a flat surface, resulting in straight-line bearings that are correct. Such a map is most commonly used for navigation charts, although the projection distorts the areas toward the North and South poles.

Meridian
A line of longitude. It is formed by creating an imaginary line that approximates a semicircle around the earth through both poles and at a right angle to the equator.

Mesa
An isolated, flat-topped plateau with steep sides. Composed of limestone or hard sandstone, the mesa's top rock is usually more resistant to erosion that the underlying rock. Mesas eventually erode into buttes. Mesas are common in arid parts of the southwestern United States and Mexico.

Metal
Any elementary substance, such as gold, copper, or silver that is crystalline when solid and typically displays opacity, ductility, conductivity, and luster. Metals may be found in their natural state or in combination with other minerals, commonly called ores.

Mid-Ocean Ridge
A ridge of volcanic mountains on the ocean floor, usually associated with seafloor spreading of crustal plates. These ridges occasionally rise above the surface and form volcanic island arcs, as at the Hawaiian Islands or Japan.

Mineral
An inorganic compound naturally occurring in the Earth's crust and having a precise chemical formula and usually a crystalline structure. Minerals vary greatly in size, shape, color, and economic value. With the exception of natural glasses such as obsidian, they are the basic building blocks of rocks.

Moraine
A mound or ridge of unstratified rock and dirt deposited by a glacier. Moraines may dam up melting glacier water, forming circular mountain lakes called tarns or long, narrow lakes such as those found in New York State's Finger Lakes region.

Mountain
A naturally formed elevation that rises above the surrounding landmass and is higher than a hill, usually 2,000 feet (610 meters) or more. Mountains are formed by subduction (when a lithospheric plate dives under another plate) or by a collision between continental landmasses. The latter process produced the Andes and the Himalayas.

Parallel
A line of latitude. It is formed by creating an imaginary line that runs parallel to the equator and connects places with the same latitude.

Permafrost
A deep layer of soil that remains frozen during summer, despite the thawing of the ground above it. The result is in the poorly drained landscape typical of the arctic regions of Canada and northern Europe, especially the former Soviet Union.

Plateau
A broad, flat land area raised sharply above the surrounding landscape on at least one side. They form where erosion-resistant rock rests on weaker rocks or soil.

Prime Meridian
The line of zero degrees longitude that runs through Greenwich, England, and from which all other lines of longitude (meridians) are measured.

Projection
In cartography, a systematic construction of intersecting coordinate lines on a flat surface, representing the meridians and parallels of the curved surface of the earth. Each method of projection results in some distortion of the planet's features; See Mercator projection for an example.

Relief
The variations in elevation and slope between the higher and lower parts of a given landscape. A map displaying these contour changes is called a relief map.

Rock
A mass of naturally formed mineral water.

Sand
Small, loose, granular substance formed by the disintegration of rock due to erosion. Consisting mostly of silicates, sand has a number of industrial uses, especially in abrasives and glassmaking.

Scale
The ratio of the actual size of a place or region and its representation on a map.

Seamount
An isolated submarine mountain that rises from the abyssal plain of the ocean floor but does not reach the surface of the water. Seamounts are volcanic in origin and may develop at points where the oceanic crustal plate passes over hot spots. Their existence may be an indirect proof of the theory of plate tectonics.

Seismology
The study of the seismic waves generated by earthquakes or artificially produced vibrations of the Earth. Seismologists use these waves measured on a seismograph, to locate petroleum reserves or to estimate the size and location of the Earth's plates.

Soil
The layer of unconsolidated, fragmented, weathered rock mixed with organic material that makes up the topmost surface of the Earth.

Stalactite
A columnar deposit, usually of calcium carbonate, hanging from the ceiling of a cave. It is formed by the precipitation of mineral-rich water and is often shaped like an icicle.

Stalagmite
A columnar deposit, usually of calcium carbonate, that forms on a cavern floor. It is caused by the precipitation of mineral-rich water dripping from the ceiling.

Stone
A correction of mineral matter of indeterminate size and shape.

Trench
A long, deep valley found on the ocean floor and bordering a subduction zone. Formed by the downward movement of one oceanic plate as it is consumed by another, a trench is associated with the creation of new oceanic crust. The Marianas Trench is the deepest in the world, measuring 36,201 feet (11,034 meters).

Tundra
A vast, level, treeless plain characteristic of arctic and subarctic regions, especially in northern Europe, Asia, and North America. The top layer of the soil thaws each spring, while the base remains frozen, resulting in boggy areas. The dominant vegetation consists of mosses, lichens, and dwarf shrubs.

Vent
An opening in the Earth's crust through which volcanic materials are violently expelled. Hydrothermal vents on ocean floors emit mineral rich solutions that support a fantastic array of life, including tubeworms.

Volcano
A vent in a mountain or the Earth's crust through which gases, rock fragments, and hot, molten lava are expelled from the Earth's interior. Volcanic eruptions usually occur along subduction zones or above hot sports, places where magma from the Earth's mantle upwells and melts through the crust.

Water table
The irregular upper surface of underground water. It is usually highest beneath hills (though still farther below the surface) and about the same level as river channels in valleys.

Weathering
The alteration or decomposition of rocks or soil by heat, cold, wind, precipitation, or chemical reactions, such as leaching, brought on by contact with the atmosphere.