Occurs when more glacier
ice is lost by melting and evaporation each year than is added by snowfall.
The approximate age of a
geologic event, feature, fossil, or rock in years. 'Absolute' ages are
determined by using natural radioactive 'clocks'. The preferred term
is radiometric age.
A process that adds part of one tectonic plate to a larger plate along a convergent (collisional) plate boundary.
A bright to gray-green member of the amphibole mineral family. In addition to silica, it contains calcium, magnesium, and iron. Actinolite is a non-hazardous relative of asbestos and is a common mineral in metamorphic rocks.
A volcano that has erupted within historical time and is likely to do so again in the future.
The top layer of soil. Plant and other organic debris builds up in this layer. This is the part of the soil generally referred to as 'top soil'.
A fan-shaped pile of sediment
that forms where a rapidly flowing mountain stream enters a relatively
flat valley. As water slows down, it deposits sediment (alluvium) that
gradually builds a fan.
Sand, gravel, and silt deposited by rivers and streams in a valley bottom.
A family of silicate minerals
forming prism or needlelike crystals. Amphibole minerals generally contain
iron, magnesium, calcium and aluminum in varying amounts, along with
water. Hornblende always has aluminum and is a most common dark green
to black variety of amphibole; it, forms forming in many igneous and
metamorphic rocks. Actinolite has no aluminum; it and is needle-shaped
and light green. Blue amphibole contains sodium and, of course, is bluish
A rock made up mostly amphibole and plagioclase feldspar. Although the name amphibolite usually refers to a type of metamorphic rock, an igneous rock composed dominantly of amphibole can be called an amphibolite too.
Fine-grained, generally dark colored, igneous volcanic rock with more silica than basalt. Commonly with visible crystals of plagioclase feldspar. Generally occurs in lava flows, but also as dikes. The most common rock in volcanic arcs.
Literally, "without water". Refers to minerals or other materials which do not have water as an primary constituent.
A upward-curving (convex) fold in rock that resembles an arch. The central part contains the oldest section of rock.
A term used by glaciologists
(scientists who study glaciers) for the boundary where the amount of
snow loss from melting equals the amount of snow accumulation from snowfall
(also called firn limit).
An igneous rock texture in which individual mineral grains are too small to be distinguished with the naked eye.
A light-colored igneous rock with the same mineral composition as granite: quartz, plagioclase feldspar, and potassium feldspar, but with a fine-grained, almost sugary texture.
The time interval between
3800-2500 million years ago. The Archean is one of the Precambrian time
The science that focuses on the study of past human cultures.
Volcanic arc rocks.
A term used to describe clay-rich rocks.
A clay-rich layer of soil. Clay often forms in overlying soil layers from the decomposition of feldspars and other minerals. The extremely fine clay particles are gradually carried down by water to accumulate into the argillic horizon.
Name used for unusually hard, fine-grained sedimentary rocks, such as shale, mudstone, siltstone, and claystone. Commonly black.
A region without earthquakes (seismic activity).
Fine particles of volcanic
rock and glass blown into the atmosphere by a volcanic eruption.
The uppermost layer of the
mantle, located below the lithosphere. This zone of soft, easily deformed
rock exists at depths of 100 kilometers to as deep as 700 kilometers.
Augen are relatively large, eye-shaped mineral grains in certain types of metamorphic rocks, especially schist and gneiss. (Augen = eyes in German)
Masses of rock or ice that fall or slide suddenly under the force of gravity.
A dark, fine-grained, extrusive (volcanic) igneous rock
with a low silica content (40% to 50%), but rich in iron, magnesium and
calcium. Generally occurs in lava flows, but also as dikes. Basalt makes
up most of the ocean floor and is the most abundant volcanic rock in the
The level (elevation) at which a stream or river can erode no more, usually sea level.
A depression in the Earth’s surface that collects sediment.
Basin and Range province
This province extends from eastern California to central
Utah, and from southern Idaho into the state of Sonora in Mexico. Within
the Basin and Range province the Earth’s crust and uppermost mantle
have been stretched, creating large faults. Along these faults linear
mountain ranges were uplifted and flat valleys down-dropped, producing
the distinctive topography of the Basin and Range province.
Very large mass of intrusive (plutonic) igneous rock that forms when magma solidifies at depth. A batholith must have greater than 100 square kilometers (40 square miles) of exposed area. See pluton, stock.
A layer of sediment or sedimentary rock.
Parallel layers of sediment or sedimentary rock (beds) that can be distinguished from each other by characteristics such as grain size and chemical composition.
Sedimentary layers in a rock. The beds are distinguished from each other by grain size and composition, such as in shale and sandstone. Subtle changes, such as beds richer in iron-oxide, help distinguish bedding. Most beds are deposited essentially horizontally.
The solid rock that lies beneath soil and other loose surface materials.
In North America, 1,000,000,000
A common rock-forming mineral of the mica family. Biotite is a black or dark brown silicate rich in iron, magnesium, potassium, aluminum, and, of course, silica. Like other micas, it forms flat book-like crystals that peal apart into individual sheets on cleavage planes.
Metamorphic rock rich in blue amphibole.
A pit or excavation area used for gathering earth materials (borrow) such as sand or gravel.
Any loose rock (sediment) larger than 256 millimeters
Rock made up of angular fragments of other rocks held together by mineral cement or a fine-grained matrix. Volcanic breccia is made of volcanic rock fragments, generally blown from a volcano or eroded from it. Fault breccia is made by breaking and grinding rocks along a fault.