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of sinking land that fill with sediment. When these trenches are finally crushed together, they form the bulk of a new mountain range.
Supervolcano
A volcano that produces an eruption on quite a different scale from the eruptions we normally experience. There are two kinds of supervolcanoes. One kind occurs when a large fissure opens up suddenly at a spreading plate boundary, providing immediate access from the mantle to the Earth’s surface (see: Fissure eruption). As a result basalt literally floods out of the ground, covering hundreds of thousands of square kilometers, possibly in a matter of days or weeks (see: Flood basalt). The trap, or basalt plateau, landscapes are examples of this. The amount of heat released during this kind of event could change the Earth’s climate.
The other kind of supereruption emerges from a central vent volcano that emits ash and gases far in excess of anything normally experienced. This kind of volcano can throw huge volumes of ash into the air and cover hundreds of
Subduction zone
thousands of square kilometers of land in ash, while at the same time sending fine ash into the
air and reducing the amount of sunlight that reaches the Earth’s surface. Such supervolcanoes may have been responsible for mass extinctions of life that have occurred from time to time during the Earth’s history.
Both kinds of supervolcano are extremely rare, and none has occurred within recorded history. However, the geological record makes it clear that they have occurred every few millions of years and will presumably happen in the future.
Surface wave
Any one of a number of seismic waves, such as Love waves or Rayleigh waves, that shake the ground surface just after an earthquake.
S wave, shear or secondary seismic wave This kind of body wave carries energy through the Earth like a rope being shaken. S waves cannot travel through the outer core of
the Earth because they cannot pass through fluids. (See also: P wave and Seismic waves.)
T
Tectonic plate
(See: Plate.)
Tephra
A general term for all fragmented volcanic material (for example, ash and bombs). Also called ejecta. (See also: Agglomerate; Lapilli; Pele’s hair; Pele’s tears; Scoria.)
Thrust fault
Another name for reversed fault, where one block pushes up and across another, shortening the crust. All thrust faults have a shallow angle to the horizontal.
Tongue, lava
A slow-moving flow of lava that streams down the side of a volcano.
Tsunami
A very large wave produced by an underwater earthquake or exploding volcanic island.
Tsunami
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