The following volcano terminology and landform features are pertinent
to Crater Lake.
For a more detailed and complete list of volcano terminology, please
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.
Volcanic ash falling from an eruption column or ashcloud.
Fragments of lava or rock smaller than 2 mm (fraction of an inch)
in diameter that are blasted into the air by volcanic explosions.
Dark-colored, fine-grained volcanic rock. It is low viscosity
(less than 53 percent SiO2), relatively fluid when
molten thus eruptions are generally nonexplosive.
Sea-floor mapping; the study of landform features beneath water
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.
Fragment of lava or rock about 1 cm (fraction of an inch) in
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
A circular depression containing a volcanic vent.
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 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.
A tabular intrusion of igneous magma that cuts across fractures
or planar structures of the surrounding rocks.
A volcano that is not currently erupting but is likely to erupt
in the future.
A volcano that is not currently erupting and is not likely to
erupt in the future.
A vent or opening in the ground that emits hot water vapor (steam)
and (or) volcanic gases.
Pertains to water heated by magma or in association with magma.
Igneous is a term used for solidified magma. It is also a term
used for describing the processes related to the formation of
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
Molten rock (magma) that has been erupted from a volcano onto
the Earth's surface.
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.
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
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).
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 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 is an area of pumice deposited by pyroclastic flows
during the climactic eruption of Mount Mazama.
Fragmented (clastic) rock materials formed by a volcanic eruption.
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.
Extrusive igneous rock of crystalline or glassy texture, with
an intermediate composition between dacite and rhyolite.
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 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.
A collective term used for describing all consolidated pyroclastic
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
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 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.
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, 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.