Volcanic and Plutonic
Vocabulary
Intrusive (Plutonic) Features:
Magma: Intrusive molten igneous rock.
Dike: A sheet of magma that cuts vertically through other layers of rock.
Sill: A sheet of magma that cuts horizontally between other layers of rock.
Laccolith: A magmatic body formed horizontally between layers of rock, exerting
pressure upwards so as to cause a bulge.
Lopolith: A magmatic body formed horizontally between layers of rock, trapped
from above by harder more resistant layer. The body then exerts a downward force
resulting in a depression in the layers below.
Batholith: A massive magmatic body that may feed dikes, sills, or volcanoes in an
area.
Pluton: A large igneous body that has cooled be beneath the Earth’s surface
Extrusive (Volcanic) features:
Effusive eruption: Low energy eruptions from Shield cone volcanoes involving
runny lava and lava flows.
Explosive eruption: High-energy eruptions, usually from composite volcanoes
that involve the ejection of large amounts of pyroclastic materials such as ash,
tephra, and hot gases.
Lava: Extrusive molten igneous rock.
Pillow lava: Lava that is ejected from submarine rifts and immediately cools
forming a crust on itself resembling a pillow
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AA: lava that is ejected from a shield volcano and cools to form a think bulging
pillowy flows.
Pahoehoe: Lava that is ejected from a Shield volcano and cools to form a ropey
braided shape.
Debris Avalanche: an extremely large movement of earth and debris from the side
of a volcano as a result of explosive force from beneath.
Pyroclastic flow (Nuee Ardente): A rush or surge of superheated gases, ash,
tephra, and pyroclasts that are ejected from the volcano and rush down its sides in
a cloud moving at up to 200 kms per hour.
Mud Flow/ Debris Flow/ Lahar: a flow of mud, ash, debris, and water surging
from the flanks of the volcano as the heat from the eruption melts its glaciers and
snow pack. The water produced mixes with the ash and debris from the explosion
and the initial debris avalanche to create an overwhelming flow.
Tephra: a general term for materials of varying sizes ejected from a volcano
during an explosive eruption. Tephra may range from fine ash to course pyroclasts
or bombs.
Volcanic Bombs: larger chucks of magma ejected during an eruption that cool
rapidly in the air and land in various shapes.
Volcanic Dome: a bulge of varying size that forms on the floor of a volcanic crater
after an eruption. This forms after an eruption has plugged the volcano’s neck with
cooled volcanic rock and upward pressure is applied to this plug by magma and
gases below.
Caldera: A large rimmed crater formed when a volcanic eruption empties the
magma chamber below resulting in what’s left of the mountain collapsing in on
itself.
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