Page 14 - Curriculum Visions Dynamic Book. To close the book, close the tab.

Eruption
Eruption
An eruption occurs when a volcano expels solid or liquid materials. The liquid may be in the form of lava that flows from the volcano, or it may be in the form of fine ash thrown high into the air with gases to make tall clouds.
An eruption normally begins with a short amount of warning
in the form of earthquakes. The earthquakes signal that pressure
is building inside the magma chamber deep below the volcano, and that the pressure is starting to cause the rocks to fracture. Each earthquake is a place where rocks are cracking apart.
Once the pressure inside the magma chamber is great enough, the magma forces its way to the surface. Volcanoes may have a number of “false starts”; but when the main eruption begins, it is often with sudden and spectacular force.
The kind of eruption that occurs depends on the material making up the magma. There are several types of eruption shown here.
The eruption continues as long as the pressure in the magma chamber is high enough, but gradually the pressure is reduced, and the eruption slows down and finally stops.
Most eruptions last for between a day or two to a month or two.
Mount St. Helens is a good example of this process. The volcano had been thought dormant before the eruption of 1980. The first warning signs that something was going to happen occurred when ice started to melt off
the summit, and a small crater developed. Steam started to come from the small crater.
Earthquakes then started to happen close to the volcano, showing that the pressure
of magma was
causing the rocks in the volcano to crack. Then a bulge developed on one side, where the weakest rocks were located. Finally, the rocks of the bulge were weakened so much they fell away, allowing the eruption to begin out of the side of the cone, producing a pyroclastic flow.
The eruption then continued directly upward, sending gases and ash high into the sky. After some weeks a small amount of lava was seen in the new crater. Then, over
 Eruption—The main types of eruption.
the following months the volcano stopped sending up ash and steam, and the eruption stopped.
(For other erupted material see: Bomb; Glowing avalanche; Lahar; Lapilli; Pele’s hair; Pele’s tears; Pumice; Pyroclastic material; Scoria; Spatter; Tephra.)
(For eruption types see: Explosive; Hawaiian-type; Icelandic-type; Pelean-type; Phreatic; Plinian- type; Strombolian-type; Vulcanian-type.)
Pelean-type volcanic eruption
Shield-type volcanic eruption
Plinian-type volcanic eruption
14