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Andesite: Volcanic rock that contains between 56 and 63 weight percent silica. Andesites are typically gray or black in color and contain visible crystals of plagioclase, augite, and hypersthene (Figure 5).

Arete: Narrow, commonly knife-edged ridge that has been undercut by glacial erosion on both sides.

Asthenosphere: Part of the Earth's mantle that lies below the lithosphere, at depths between about 100 and 350 kilometers. Rock here is relatively soft because its high temperature and relatively low confining pressure enable a small amount of melt to form and lubricate its movement.

Basalt: Volcanic rock that contains between 47 and 52 weight percent silica. Basalts are typically black and commonly contain visible crystals of olivine and plagioclase (Figure 5).

Basaltic andesite: Volcanic rock that contains between 52 and 57 weight percent silica. Basaltic andesites are typically black in color and contain visible crystals of plagioclase, olivine, and augite (Figure 5).

Cirque: Bowl-shaped depression or amphitheater carved at the head of a mountain valley by glacial erosion.

Dacite: Volcanic rock that contains between 63 and 72 weight percent silica. Dacites are typically gray to pink in color and contain visible crystals of plagioclase, hypersthene, and hornblende (Figure 5).

Debris avalanche: Dense, incoherent mixture of water, rock, and soil that flows downslope at speeds of 40 to 200 kilometers/hour (25 to 125 miles/hour).

Debris flow: Dense, incoherent mixture of water, rock, and soil that flows downslope at speed of 2 to 40 kilometers/hour (1 to 25 miles/hour).

Dome: Small steep-sided volcano that has been formed by pasty lava that has piled up atop its vent. Domes are typically no larger than 2 to 3 kilometers (1 to 2 miles) in diameter and are composed of silica-rich lavas.

Focus: Point within the Earth at which at which rock initially breaks to initiate an earthquake.

Fumarole: Small vent or opening from which volcanic gases are discharged.

Glacier: Mass of land ice that is large enough to flow down hill under its own weight.

Lava: Partially molten rock that has risen through Earth's crust and been erupted onto the surface.

Magma: Partially molten rock that consists of melt with or without suspended crystals and vapor bubbles.

Magnitude: Number that is scaled to the amount of energy released by an earthquake. An increase of one magnitude unit (for example, from 5 to 6) corresponds to a 10-fold increase in the amount of ground motion and an approximately 30-fold increase in the amount of energy produced by an earthquake.

Mineral: Naturally occurring solid that has a specific chemical composition and a unique internal arrangement of its atoms. For example, quartz consists of silicon and oxygen atoms in a 1:2 ratio (SiO2), and these atoms are bonded together in a hexagonal structure.

Moraine: Mass of poorly-consolidated rock debris deposited by a glacier. Moraines typically form elongate or curved ridges and contain rock fragments of a wide range of sizes, from fine silt and clay to coarse boulders.

Peridotite: Dense, coarse-grained rock that consists mostly of the magnesium and iron-silicate minerals olivine and pyroxene.

Phenocryst: A relatively large crystal set in the finer matrix of a volcanic rock. Such crystals grow slowly from the surrounding melt while the magma remains at depth and then are carried up as it rises to the surface and erupts.

Pleistocene Epoch: The period of time between 1.8 million and 10,000 years ago. On Earth this corresponds to the interval during which large continental glaciers repeatedly advanced and retreated across landmasses at high latitudes. Informally, this epoch is also called the "ice age".

Potassium-argon dating: A technique for determining the age of rock and mineral samples by measuring the amounts of radioactive potassium (40K) and its daughter element, a form of argon (40Ar) in a sample. Because 40K decays relatively slowly, this technique typically only yields reliable ages for samples that are more than about 100,000 years old.

Pumice: Porous volcanic glass that is formed by the rapid expansion of gas bubbles in melt that is quenched as it is erupted.

Pyroclastic material: Volcanic rock that has been fragmented by explosions during an eruption or by the collapse and disintegration of the flanks of domes or lava flows.

Radiocarbon dating: Technique for determining the age of a sample of organic material (charred wood, plant roots, cloth) by measuring the rate at which the radioactive carbon (14C) it contains is decaying. Because 14C decays relatively rapidly, this technique only yields accurate ages for samples that are less than about 60,000 years old.

Recurrence interval: Average period of time between two episodic events such as earthquakes, floods or volcanic eruptions.

Rockfall: Moving mass of rock fragments that has broken loose from an outcrop and cascaded down a slope.

Seismogram: Record of the movement of Earth's surface at a particular locality produced by an earthquake or other disturbance.

Shield volcano: Broad volcano built up from many thin, overlapping flows of basalt or basaltic andesite lavas. Shield volcanoes differ widely in size, from several kilometers to several hundreds of kilometers in diameter, and have very gently sloping flanks.

Stratovolcano: Volcano composed of alternating lava flows, layers of pyroclastic material, and debris flow deposits piled up around a central vent. Stratovolcanoes are typically 10 to 30 km (6 to 20 mi) in diameter, and have slopes that steepen gradually upwards towards their summits.

Subduction: Process in which a plate of dense oceanic lithosphere sinks back into Earth's interior along a dipping surface that separates it from the overriding lithosphere and asthenosphere.

Tephra: Fragmented volcanic rock that has been transported through the air. Some is formed from lavas that have been blown from a vent and solidified as they fell, whereas others are formed from solid rocks that have been shattered and thrown into the air by explosions.

Tephra cone: Small volcano composed of layers of tephra piled up around a central vent or crater. Tephra cones are typically 1 to 2 km (1 mile) in diameter and have steep (35 to 40) slopes determined by the angle at which the loose tephra begins to slide.

Tsunami: Wave produced in a large body of water (ocean or lake) by an earthquake, volcanic eruption, or underwater landslide. Tsunamis travel rapidly across deep ocean basins (about 480 mph!) and can grow to heights of tens of meters as they enter shallow water and approach shore.

Volatiles: Chemical compounds and elements (such as water, H2O, and nitrogen, N2) that occur as gases at high temperatures and atmospheric pressure.


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