Glossary

The following volcano terminology and landform features are pertinent to Crater Lake.

For a more detailed and complete list of volcano terminology, please see http://vulcan.wr.usgs.gov/Glossary/volcano_terminology.html.

Andesite:
A dark gray volcanic rock containing 53-63 percent silica of moderate viscosity when in a molten state. Its composition and eruptive character is between basalt and dacite.

Airfall (ashfall):
Volcanic ash falling from an eruption column or ashcloud.

Ash (volcanic):
Fragments of lava or rock smaller than 2 mm (fraction of an inch) in diameter that are blasted into the air by volcanic explosions.

Basalt:
Dark-colored, fine-grained volcanic rock. It is low viscosity (less than 53 percent SiO₂), relatively fluid when molten thus eruptions are generally nonexplosive.

Bathymetry:
Sea-floor mapping; the study of landform features beneath water surface.

Caldera:
A large, bowl-shaped volcanic depression formed by the collapse of a volcano due to rapid removal of voluminous magma from underneath the mountain. The caldera have a diameter many times greater than that of the original crater.

Cinder:
Fragment of lava or rock about 1 cm (fraction of an inch) in diameter.

Cinder cone:
A small cone-shaped volcano composed of cinders that formed around the vent at an angle of repose, approx. 32° (for example, the angle of which sand is piled).

Composite volcano (stratovolcano):
A steep-sided volcano composed of many layers of high-viscosity lava flows and fragmented debris such as lahar and pyroclastic deposits.

Crater:
A circular depression containing a volcanic vent.

Dacite:
Light-colored, silica-rich volcanic rock (63-68 percent SiO₂) of high viscosity when in a molten state; eruptions are commonly explosive and may produce voluminous tephra, pyroclastic flows, and lava domes.

Devil's Backbone:
Devil's Backbone is a dike, a vertical sheet of andesite magma that solidified in a fracture where it wedged apart some of the lava flows that made up Mount Mazama. This unusually thick dike once fed extensive lava flows on its flank of the volcano. It is more resistant to erosion than the adjacent lava flows of the caldera walls and thus stands out as a giant rib.

Dike:
A tabular intrusion of igneous magma that cuts across fractures or planar structures of the surrounding rocks.

Dormant volcano:
A volcano that is not currently erupting but is likely to erupt in the future.

Extinct volcano:
A volcano that is not currently erupting and is not likely to erupt in the future.

Fumarole:
A vent or opening in the ground that emits hot water vapor (steam) and (or) volcanic gases.

Hydrothermal:
Pertains to water heated by magma or in association with magma.

Igneous:
Igneous is a term used for solidified magma. It is also a term used for describing the processes related to the formation of igneous rocks.

Igneous rock:
Igneous rock are formed from magma (melted rock) that has cooled and solidified, either within the Earth's crust or on the Earth's surface. Magma that solidify within the Earth's crust cools slowly and result in coarse-grained (large crystal) rock; for example, granite. Magma that solidify at or near the Earth's surface cools rapidly and result in fine-grained (small or no crystal) rock; for example, basalt and obsidian. Igneous rock is composed of mostly silicate minerals

Lava:
Molten rock (magma) that has been erupted from a volcano onto the Earth's surface.

Llao Rock:
Llao Rock is composed of rhyodacite lava (silica-rich, highly viscous magma) that filled an explosion crater. The Llao Rock eruptions occurred about 200 years before the caldera-forming, climactic eruption of Mount Mazama.

Magma:
Molten rock consisting of liquids, with or without crystals or vapor bubbles, that forms within the upper part of the Earth's mantle and crust. When erupted onto the Earth's surface, it is called lava.

Phantom Ship:
Phantom Ship is made of andesite lava flows that are very resistant to weathering because they are bound together by minerals deposited by hot water inside Mount Mazama long ago. It is the oldest rock inside the caldera and was erupted about 400,000 years ago. Phantom Ship stands about 51 m (167 ft) above lake level (about 1,940 m or 6,345 ft above sea level).

Pumice:
A light-colored, frothy volcanic rock typically composed of dacite or rhyolite glass. It is formed by the expansion of gas in erupting lava. Because of the numerous gas bubbles trapped in the rock, pumice commonly floats on water.

Pumice Castle:
Pumice Castle is made of layers of dacite pumice fragments that fell during an explosive eruption about 70,000 years ago. Hot when it landed, some pumice welded together in particularly strong layers. The "castle" is an erosional remnant that is supported by a resistant dacite outcrop.

Pumice Desert:
Pumice Desert is an area of pumice deposited by pyroclastic flows during the climactic eruption of Mount Mazama.

Pyroclastic:
Fragmented (clastic) rock materials formed by a volcanic eruption.

Pyroclastic flow:
A hot, dry, fast-moving, and high-density mixture of of ash, pumice, rock fragments, and gas that formed during explosive eruptions or from the collapse of a lava dome.

Rhyodacite:
Extrusive igneous rock of crystalline or glassy texture, with an intermediate composition between dacite and rhyolite.

Scoria:
Dark volcanic rock containing cavities created by gas bubbles trapped in lava. Scoria is formed when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight.

Shield volcano:
Shield volcanoes are produced by multiple eruptions of low-viscosity basaltic or andesitic lava. They have long gentle slopes shaped like an inverted warrior's shield.

Tuff:
A collective term used for describing all consolidated pyroclastic materials.

U-shaped valley (glacial valley):
The U-shaped valleys (or beheaded glacial valleys) that form Sun Notch and Kerr Notch were carved by glaciers on Mount Mazama. The glaciers had retreated far up the volcano when the caldera collapsed, beheading these and other, less spectacular valleys at the caldera wall. Glacial valley is shaped like a U, whereas stream valley (V-shaped valley carved by water) is shaped like a V.

Vent:
An opening at the volcano's summit or on its slopes through which volcanic materials (lava and gas) erupt. Circular vent is known as crater and linear vent is known as fissure.

Viscosity:
Viscosity is a measure of the fluidity (or flow resistant) of a substance. Taffy and molasses have high viscousity; water has low viscosity. The more viscous the magma, the more explosive the volcano. Viscous lavas are high in silica.

Volcano:
A vent in the Earth's surface through which molten rock, ash, and gasses erupt. It is also a term for the landform feature that is constructed by the erupted material.

Wizard Island:
Wizard Island, like a volcanic "iceberg", is only the top of an andesite volcano that grew on the caldera floor soon after the climactic eruption of Mount Mazama and before the lake reached its present level. The Wizard Island volcano is thus about 7,500 years old. Wizard Island is a cinder cone composed of loose chunks of scoria and cinder that rest at the angle of repose, much like a pile of sand. Blocky flows of andesite lava emanate from the base of the cone. Wizard Island is more conical than most cinder cones because the caldera wall shelters it from wind distortion. Wizard Island stands about 232 m (760 ft) above lake level (about 2,115 m or 6,940 ft above sea level), with a 27 m (90 ft) deep and 91 m (300 ft) wide crater on its summit.