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There is obviously a lot of activity going on beneath the earth’s crust. Crustal plates are constantly moving and colliding with one another, creating earthquakes and pressure zones that lead to volcanic eruptions. Hot, melted rock known as magma is running beneath the earth’s surface, and sometimes they collect in areas of high pressure. When this pressure builds up and becomes too great, the magma sometimes explodes upward in a great event we know as a volcano.
A volcano by definition is any hole in the ground that emits hot gases, hot ashes, or molten lava, which escape through tunnels from huge cavities deep in the Earth’s crust. The hollow cavities are called magma chambers, because they contain the molten rock you know as magma. When the magma has reached the surface or the earth’s crust, it is called lava.
Volcanic gases are emitted during the eruption. The chief gas is water vapor or steam, although it can also contain carbon dioxide, nitrogen, sulphur, carbon monoxide, hydrogen, chlorine, and numerous other gases. The high-pressure gases also eject pyroclastics (volcanic solids like rock, cooled lava, etc.), red-hot fragments that range in size from huge blocks to fine dustlike particles.
Interestingly enough, there many types of pyroclastics:
The volcano will also emit lava (liquid, molten rock). Lava is composed mostly of silicon dioxide (SiO2), whose amount is the basis for separating lava into three categories:
- blocks - larger framents, made of crustal pieces or older lava
- volcanic bombs - masses of new lava blown from the crater and hardened during flight, becoming spindle-shaped as they fly through the air
- breadcrust bomb - a type of pyroclastic that resembles a loaf of French bread with large cracks in the crust
- lapilli - (Italian, meaning "little stones") smaller, broken fragments about the size of walnuts
- cinders - sand-size particles of crushed lava
- ash/dust - fine particles of lava
- pumice - produced by acidic lavas where there is so much gas content that the magma bubbles as it rises to the surface; pumice contains many air spaces that were formed by expanding gases, so it will float in water
- Pele’s hair - a material resembling spun glass created by lava fountains (where steam jets blow lava into the air); named after the Hawaiian goddess of volcanoes; very similar to rock wool, a man-made material widely used for insulation
- volcanic breccia - coarse, angular fragments which become cemented to from a rock
- tuff - cinders and ash that become consolidated by the constant trickle of ground water
After issuing forth from the crater, cracks, or fissures, lava will spread out like tongues or sheets of hot, melted rock. Sometimes sheets of lava can cover thousands of square miles, far away from their source. More liquid lava can flow farther than the thicker, more viscous lava types.
- acidic - lava that contains 66% or more of SiO2
- intermediate - lava that contains between 52% and 66% of SiO2
- basic - lava with less than 52% of SiO2
Lava will eventually cool and harden, usually into one of two types. The pahoehoe type (known as "corded" in Italy) has a smooth, billowy surface, and resembles huge coils of rope. These mainly develop from basic lava. The aa type of cooled lava consists of angular, jagged blocks, often with sharp edges and spiny crags.
Sometimes, the outer surface of a lava flow will harden when the inside core is still hot and flowing. If this middle part flows away, it will leave the outer cylinder, creating a lava tube. Lava tubes can continue for several miles. The longest tube is in Northern California, extending 13.8 miles in length. One such tube near Dubois, Idaho, has been made into a bomb and fall-out shelter that can hold the town’s entire population (about 2,500).
Another interesting creation is the tree mold or lava tree, and is formed when liquid lava encases a tree trunk. The tree will usually burn, leaving details of its bark or wood preserved in the cooled lava.
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