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Amygdule
A
Aa lava
A type of lava with a broken, bouldery surface. Aa lava forms from a type of basalt lava that is relatively sticky. It tends to form a crust on the surface of flowing lava, which is then broken up by the moving lava below. (Compare with: Pahoehoe lava.)
Acceleration
The rate of change of the ground movement during an earthquake. The ground movement speeds up from its rest position as a seismic wave passes through. The faster the ground speeds up, the more damage the earthquake is likely to do.
Acid lava/acid rock
A type of lava or igneous
rock that is mainly made up
of light-colored minerals and is more than two-thirds silica (for example, rhyolite and granite). (See also: Viscous, viscosity.)
Active fault
A fault that is known to have moved in historic times.
(See also: Seismic gap.) Active volcano
(See: Volcanic activity.) Aftershock
An earthquake that happens after the main shock. Major earthquakes are always followed by a number of aftershocks that decrease in number over the following days and weeks. In general, the larger the main shock, the larger and more numerous the aftershocks, and the longer they continue.
 Aa lava—
Aa lava is thick and develops a crust that breaks up as the lava below moves.
Lava moves very slowly, and a thick crust forms on the surface.
Boulders of lava
Agglomerate
A rock made from the compacted particles thrown out by a volcano (for example, tephra).
Alkaline rock
A type of igneous rock containing less than half silica and normally dominated by dark-colored minerals. (See also: Gabbro.)
Amplification
The magnification of an earthquake when seismic waves travel through soft ground. Soft, wet materials such as bay and lake deposits amplify the waves greatly, causing the ground to wobble like
jelly. Places such as Mexico City and the bayfront (Marina) area
of San Francisco, California, are especially at risk from this effect.
Amplitude
A measurement of the strength
of an earthquake. Amplitude is measured on a seismograph. Large peaks on the trace, or seismogram, indicate a large amplitude and a strong quake.
Amygdule
A bubble in a volcanic rock filled with secondary minerals such as calcite or quartz. (See also: Vesicle.)
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