E&E 151 S Introductory Geology: Chapter 4 (Volcanoes) Visualization Study

Volcano

A landform formed by the extrusion of lava or the ejection of rock fragments from an opening in the Earth's crust.

Vent

The opening through which a volcanic eruption takes place.

Crater

A basinlike depression located over a vent at the summit of a volcano's cone.

Pahoehoe

A basaltic lava flow characterized by a ropy or billowy surface formed by the rapid cooling and solidifying of the lava's outer skin.

A'a

A basaltic lava flow with a jagged, rubbly surface formed when cooling lava partially solidifies and moves slowly as a pasty mass.

Spatter Cone

A small, steep-sided cone (under 10 meters high) built from lava sputtering out of a vent on a solidifying lava flow.

Lava Tube

A tunnel-like conduit that develops after the outer roof and walls of a fluid pahoehoe flow solidify, insulating the flowing lava inside.

Flood Basalts

Vast outpourings of highly nonviscous mafic lava from fissures that cover wide areas and build thick lava plateaus.

Columnar Jointing

A distinctive feature in some basaltic flows forming parallel, mostly six-sided, vertical columns created as the rock contracts while cooling after solidifying.

Pillow Lava

Pillow-shaped, rounded masses of solid basalt formed when fluid lava erupts underwater and cools in seconds.

Dust

The finest pyroclastic particles, measuring under 1/8 millimeter in size.

Ash

Fine pyroclastic particles ranging from 1/8 to 2 millimeters in size.

Cinder (Lapilli)

Pyroclastic particles ranging from 2 to 64 millimeters in size (roughly between the size of a grain of rice and a peach).

Blocks

Pyroclasts larger than 64 millimeters consisting of angular pieces of blasted solid rock.

Bombs

Pyroclasts larger than 64 millimeters that begin as molten blobs and become streamlined in flight before solidifying.

Pyroclastic Flow

A dense, highly dangerous mixture of gas and pyroclastic debris that hugs the ground and flows rapidly into low areas at speeds up to 200 kilometers per hour.

Lahar

A dense, fast-moving volcanic mudflow created when loose pyroclastic material mixes with rainwater or snowmelt.

Shield Volcano

A broad, gently sloping volcano (2 to 10 degrees) constructed of solidified layers of low-viscosity, mostly basaltic lava flows.

Cinder Cone

A small, steep-sided volcano (up to 33 degrees) constructed exclusively of loose pyroclastic fragments ejected from a central vent.

Composite Volcano (Stratovolcano)

A large volcano with intermediate steepness constructed of alternating layers of pyroclastic fragments and solidified lava flows, primarily of andesitic composition.

Lava Dome

A steep-sided, dome-shaped mass of highly viscous lava (such as rhyolite or andesite) that solidifies in or immediately above a volcanic vent, trapping gases like a cork.

Caldera

A volcanic depression much larger than the original crater (at least 1 kilometer in diameter) created when a volcano blows off its summit or collapses into a partially emptied magma chamber.

Maria

Broad, dark-colored, smooth lava plains found on the Moon, formed by huge meteorite impact craters flooded with basaltic lava early in lunar history.