'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 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.

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

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


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 (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).

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.


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

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.


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