'a'a 'A'a (pronounced "ah-ah") is a Hawaiian term for a lava flow that has a rough, broken-up, spiny surface. It is composed of broken lava blocks called clinkers. The interior of 'a'a is very dense.

accretion Accretion is the accumulation of sediment (mud, sand, etc.) by deposition, often occurring along a shoreline or in a river delta.

acid rain Acid rain is polluted and harmful to the environment.

 

active volcano An active volcano is one that has erupted in recorded history or is currently erupting.

aerosol An aerosol consists of tiny particles of liquids or solids suspended in the air.

airfall Airfall (also called ashfall) is volcanic ash that falls from an ash cloud or a volcanic column.

 

andesite Andesite is a type of volcanic rock. This gray to black rock is composed of about 54 to 62 percent silica (SiO2), plus some iron and magnesium.

ash cloud An ash cloud (also called an eruption cloud) is the cloud of ash that forms after some volcanic eruptions.

ashfall Ashfall (also called airhfall) is volcanic ash that falls from an ashcloud or a volcanic column.

 

ash, volcanic Volcanic ash is a harsh abrasive type of ash (unlike ash from a wood fire) that is made up of small rock, mineral, and volcanic glass fragments (these fragments are less than 0.1 inch (2 mm) in diameter).

asthenosphere The asthenosphere (also spelled aesthenosphere) is a part of the Earth's upper mantle that exhibits plastic properties. It is located below the lithosphere (the crust and upper mantle), between about 100 and 250 kilometers deep.

 

atmospheric shock wave An atmospheric shock wave is compressed air that is formed by a volcanic eruption.

atom Everything is made up of tiny atoms. An atom is the smallest part of an element that has the properties of that element.

avalanche An avalanche is a a large mass of falling and/or sliding material. Avalanches can be composed of rock, snow, soil, or ice. Volcanic eruptions can cause avalanches.