Geology Chapter 4

Lava

the magma that is on Earth's surface.

Pillow Structure

Rocks, usually basalt, rounded masses fitted together.

Pumice

A frothy glass with so much void space that it floats in water. When lava is cool quickly, this develops.

Scoria

A highly vesicular basalt, that actually contains more gas space than rock.

Volcano

A hill or mountain formed by the extrusion of lava or ejection of rock fragments from a vent.

Porphyritic Rock

A rock in which larger crystals are enclosed in a ground mass of much finer grained minerals or obsidian.

Volcanic Breccia

A rock that includes larger pieces of volcanic rock.

Caldera

A volcanic depression much larger than the original crater, having a diameter of at least 1 kilometer.

Cinder Cones

Also known as a pyroclastic cone, it is a volcano constructed of pyroclastic fragments ejected from a central vent.

Silicic Rocks

Also known as felsic, they are rocks that are silica rich with 65% of SiO2.

Viscosity

Also known as the resistance to flow. It determines how easily that gas escapes.

Volcanic Domes

Are steep sided, dome or spine shaped masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent.

Shield Volcanoes

Broad, gently sloping volcanoes constructed of solidified lava flows. During an eruption the lava spreads widely and thinly due to low viscosity.

Vesicles

Cavities in extrusive rocks resulting from gas bubbles that were in lava.

Crater

Is a basin like depression over a vent at the summit of the cone. Material is not always ejected from the central vent.

Pyroclastic Flow

Is a mixture of gas and pyroclastic debris that is so dense that is hugs the ground as it flows rapidly into low areas.

Composite Volcanoes

Is constructed of alternating layers of pyroclastic fragments and solidified lava flows.

Flank Eruption

Lava pours from a vent on the side of a volcano.

Mudflows

Mixture of water and volcanic debris.

Magma

Molten rock or liquid that is mostly silica. Melting of previously solid mantle rock that takes place at depth, just above the sub-ducting plate.

Extrusive Rock

Pyroclastic debris and rock formed by solidification of lava. Surface rock resulting from volcanic activity.

Vent

The opening through which an eruption takes place.

Obsidian

Volcanic glass that is usually silicic, one of the few rocks that is not composed of minerals. Usually have a very high silica content and is silicic.

Pyroclasts

When magma is solidified quickly as it is blasted explosively by gases into the air producing these rock fragments.