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Fissure eruption
Fault scarp
Fissure
A substantial crack in a rock; and also a long crack or opening on
a volcano. Lava fountains and lava flows commonly pour out of fissures during fissure eruptions. If the fissure remains underground, magma may flow into it and create a dike. (See also: Hot spot and Spreading boundary.)
Fault scarp
in many otherwise resistant rocks, breaking down into clay minerals and allowing the rest of the rock to fall apart.
Ferromagnesian minerals
A group of minerals containing a large proportion of iron and magnesium.
Examples include augite, hornblende, and olivine. In general, they are very difficult to tell apart because they are all green, dark-green, or black and often occur only as small crystals. Nevertheless, they form a major part of many igneous rocks and affect the overall color of the rock. For example, basalt appears black because it contains predominantly ferromagnesian minerals.
First motion
The direction of ground motion
of the first part of the P wave as
it arrives at the seismograph. If
it is upward, then the earthquake has occurred because the ground has pulled apart. If the first motion is downward, the earthquake has resulted from the ground being compressed.
A long, straight, steep slope in the landscape that has been produced by a fault.
Fault zone
A region in which there are many faults. Faults in a fault zone often run parallel to one another, but they may also cross. Not all faults in a fault zone need be the same age. One of the most well-known faults, the San Andreas Fault in California, is actually a fault zone.
Feldspars
The most common minerals found in the Earth’s crust. About half of all the Earth’s crustal rocks are made from feldspars (named after the German for “field crystals”). There are two kinds. One kind (orthoclase feldspar) is white or gray, and the other kind (plagioclase feldspar) is usually pink. You cannot see into a feldspar—it is opaque. But the surface shines (it has a luster like porcelain), and it breaks up into flat-faced blocks.
Feldspars are reasonably hard. They are, however, easily weathered and are the “weak link”
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Fissure eruption
An eruption that occurs when lava flows from a long crack, or fissure, in the Earth’s crust rather than from a single chimneylike vent. (See also: Oceanic spreading ridge.)
The material that comes from a fissure eruption is always very runny basalt lava. (See also: Dike and Dike swarm.)
A fissure occurs because the crust is splitting apart, and most fissures therefore occur at spreading boundaries between tectonic plates. The most spectacular example of this occurs in Iceland.
Fissure eruptions do not build
up volcanic cones because the
lava is far too runny. Instead, the lava spurts up out of the fissure
in a series of lava fountains
of orange and yellow-hot rock (incandescent) and then floods over the landscape, burying everything in its path. (See also: Hawaiian-type and Icelandic-type eruption.)
The floods of lava from fissure eruptions that have built up the whole of Iceland are small compared to the floods of lava from fissures in the more distant past.
The majority of fissure eruptions happen under the sea. Fissure eruptions are responsible for making all of the floor of the world’s oceans. However, they are not often observed or studied because the eruptions occur quietly