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San Francisco
San Andreas Fault
Caldera
Body wave
A seismic wave that moves through the interior of the Earth, as opposed to surface waves that travel along the Earth’s surface. P waves and
S waves are body waves, Love waves and Rayleigh waves are surface waves.
Bomb
Fragments of tephra larger than 64mm across that are thrown up into the air during a volcanic eruption. They usually occur in places where the magma is very sticky and the eruptions
violent. They are the result of exploding gas bubbles.
All lava contains gas bubbles. As lava moves
up the vent of a volcano,
the gas bubbles swell. When
the lava reaches the surface, the bubbles burst out just as when you shake a bottle of soda before opening it. Where the lava is runny, the excess gas bubbles out as the lava flows over the surface.
But where the lava is sticky, the gas cannot escape quietly; and
as the bubbles burst, they tear the lava apart.
The lava cools as it travels through the air. Large pieces of solidifying lava are called volcanic bombs. Explosions that produce
a very fine spray create ash. Cinders are pebble-sized pieces whose size is between that of bombs and ash. (See also: Strombolian- type eruption.)
Boss
An upward protrusion of a batholith. Some bosses may once have been the magma chambers of volcanoes. In England Land’s End and Dartmoor are bosses
that appear to form separate areas of rugged granite landscape. However, out of sight below ground they are connected by the main batholith.
C
Caldera
The collapsed cone of a volcano. It sometimes contains a crater lake. Caldera is the Spanish word for
kettle. The name derives from the shape of a kettle, having a wide, flat base and steep sides.
Calderas are roughly circular pits, usually caused by the collapse of the top of a volcanic cone during an explosion. The difference between a caldera and a crater is that calderas are big. To be named a caldera, the floor must be many kilometers across.
A caldera forms in a different way from a crater. A caldera
is caused by an explosion so violent that the top of the volcano, including the crater, simply collapses back in on itself. So, while a crater is simply the cone-shaped top of a vent, a caldera is the collapsed top of
an entire volcano.
Calderas form when the magma chamber that supplies the liquid rock for the eruption is quite close to the ground surface. Furthermore, the magma has to be under so much pressure that it causes the rocks above the magma chamber to crack and weaken. During an eruption some of the contents of the magma chamber erupt, sending ash and lava out over the surrounding
 Bomb—Volcanic bombs do
not travel very far and solidify as
they fall. Nevertheless, because they were partly molten when they were thrown from the vent, their shape is smoothed off by travel through the air.
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