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The Types of Volcano

The All About Volcanos Go Down Go Back
The three main earth rock types are igneous, sedimentary, and metamorphic. This study is about the first, igneous rock which is a word that comes from Latin ignis meaning fire. Igneous rock can also be called volcanic rock or magmatic rock.

Igneous Rock
Igneous rock is rock that was previously volcanic lava or magma but then forms into rock when it cools, crystallizes and solidifies. This solidification process can occur either below the surface of the earth or above the surface of the earth.
Igneous rock that is formed below the surface is called intrusive rocks.
Igneous rock that is formed above the surface is called extrusive rocks.

Igneous Intrusive Rock
Intrusive rock, which is also called plutonic rock, is formed when underground magma cools slowly, crystallizes and solidifies to create pockets of rocks called intrusions. These intrusions have different types including: batholiths, dikes, laccoliths, plutons, sills, and volcanic necks.
Most intrusive rocks have large, well-formed crystals. Some types of intrusive rock include: granite, gabbro, diorite and dunite.
Often, these underground rocks are exposed to the atmosphere from either continental uplifting or by natural erosions effects of the surrounding layers of softer earth and rock by the rain, sun, and sun.
Examples of igneous intrusive rock include:

Igneous Extrusive rock
Extrusive igneous rocks, also known as Volcanic rocks, are formed above the surface of the earth when after the magma within the mantle and crust of the earth is brought to the surface through fissures or volcanic eruptions.
After reaching the surface, the magma then forms the extrusive igneous rocks which cools and solidifies quicker than intrusive igneous rocks. Because the magma, which is brought to the surface, solidifies at a faster rate, these rocks are smooth, crystalline and fine-grained

Types of Volcanic Rock
Volcanic or igneous rocks are normally classified by their silicon dioxide content.
Komatiite and Picrite Basalt is volcanic rock with Less than 45% content
Basalt is volcanic rock with 45 - 52% content
Andesite is volcanic rock with 52 - 63% content
Dacite is volcanic rock with 63 - 69% content
Rhyolite is volcanic rock with more than 69%

Volcanic Explosivity Index
This index is a measure of the explosiveness of volcanic eruptions, on a scale of 0 to 8 where zero is gentle, non-explosive and has less than 10,000 cubic meters of ejected material (ejecta).
At the other end, a value of 8 is given to mega-colossal event, has an explosive eruption that, has more than 1000 cubic kilometers of ejecta and can have a cloud column height of over 12 miles (20 km).

0 has less than 10,000, cubic meters ejecta. (350,000 cubic feet)
1 has greater than 10,000 cubic meters ejecta. (350,000 cubic feet)
2 has greater than 1,000,000 cubic meters ejecta. (35,000,000 cubic feet)
3 has greater than 10,000,000 cubic meters ejecta. (350,000,000 cubic ft)
4 has greater than 0.1 cubic kilometer ejecta. (.024 cubic miles)
5 has greater than 1 cubic kilometer ejecta. (.24 cubic miles)
6 has greater than 10 cubic kilometers ejecta. (2.4 cubic miles)
7 has greater than 100 cubic kilometers ejecta. (24 cubic miles)
8 has greater than 1000 cubic kilometers ejecta. (240 cubic miles)

The above index is intended to classify volcanic eruptions during our historical time and does not even address the ancient volcanic eruptions. For instance, the Yellowstone caldera erupted 2059 mya with a volume of 2450 cubic kilometer and again 639,000 years ago with over 1000 cubic kilometers.
Too, there were at least another sixteen eruption in various places on the North American continent from 35 mya to 12 mya with the amount of ejecta ranging from over 1000 cubic kilometers to 5000 cubic kilometers.

List of Volcano Types
So far in my research, I have found seven types of volcanoes, that is those which eject magma. There are other non-magma volcanoes, such as the mud volcano found in Yellowstone, but these are not included in this list.

The Cinder Cones Go Down Go Up
1. Cinder Cones
Cinder Cones are usually created in conjunction with a shield volcano. These are smaller cone shaped volcanoes which occur in the outer areas around the central volcano.
Cinder cones may grow to become a shield volcano, but usually are only part of a larger shield volcano.

The Shield Volcanoes Go Down Go Up
2. Shield Volcanoes
Shield volcanoes are those that are less noticeable as a volcano, that is of course unless the volcano is active, because they are usually spread out over a large area and not shaped like the classical tall steep sided mountain like composite volcano. but are much flatter in elevation.
Instead, shield volcanoes are broad, flat volcanoes which cover a larger area that composite volcanoes.
Also, the magma has a lower viscosity than those of composite volcanoes and this low viscosity lava is much more prone to flowing and seldom ever explode. The shield volcano is typical of those that grow up from the ocean floors.
Examples of shield volcanoes include Haleakala and Kilauea, both in the Hawaiian islands and Medicine Lake volcano in northern California.

The Strato Volcanoes Go Down Go Up
3. Composite Volcanoes
Composite volcanoes are those with the classical steep-sided high-peaked mountain. Examples are Mount Rainier and Mount Baker, both in Washington state.
Composite volcanoes are those that have a tendency to blow off the top. The reason for this is that in most cases the lava from the previous eruption filled the core of the mountain and cooled to rock. When the lava wants to come out again, it must blow away the blocking hard rock. The best example of a composite volcano which has blown off it′s top is Mount Saint Hellens.

The Super Volcanoes Go Down Go Up
4. Caldera
A caldera is a large depression formed after the earth′s surface collapses into the magma chamber of a hot spot. This collapse of the earth occurs after a major eruption from the hot spot and usually leaves a large somewhat flat crater.
Often, a small cone will begin to grow inside of a caldera, which will continue to smoke and erupt lava, increasing the size of the cone for a long time after.
Examples of a caldera are Mount Aniakchak in Alaska, Mount Mazama in Oregon, and the Yellowstone caldera in Wyoming.

The Rift Volcanoes Go Down Go Up
5. Rift Volcano
A less common type of volcano is rift volcano which, like a fissure volcano, are both long linear cracks in the surface of the earth out of which lava exudes, usually in very large amounts.
A rift is usually larger than a fissure and in most cases, rifts are found on dry earth where a fissures are found at divergent plates in mid ocean ridges.
Rift volcanoes form when magma rises into the gap between diverging plates, thus, these volcanoes occur at or near actual tectonic plate boundaries.
The separation process of the plates is a continuous action that upon exceeding the strength of the surface rocks, causes intermittent fracturing in these rocks, which in turn, allows magma eruptions often accompanied with earthquakes.
The best example of a rift volcano is Craters of the Moon National Monument.

The Fissure Volcano Go Down Go Up
6. Fissure Volcano
A fissure volcano is also known as a fissure vent, volcanic fissure and eruption fissure, is a linear volcanic crack or vent through which lava erupts, usually without any explosive activity.
The fissure volcano is usually no more than ten feet wide but often is very long.
Fissure volcanoes can cause large flood basalts which run first in lava channels and later in lava tubes. However, after time the eruption will build spatter cones which often becomes the focal point of the eruption.
Fissure volcanoes are often found along rift zones and are often part of the structure of a shield volcano.
A good example of a fissure volcano is what has caused the island of Hawaii. Although most of Hawaii is now a shield volcano, is is likely that it all began with a fissure vent at the bottom of the ocean.

The Lava Dome Volcano Go Down Go Up
A lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano that will form when lava piles up over a volcano′s vent instead of moving away. Dome-building eruptions are common, particularly in convergent plate boundary settings. Some six percent of eruptions on Earth are lava dome forming. Often, a lava dome can form in the crater of a larger volcano.
The geochemistry of lava domes can vary from basalt to rhyolite although the majority are of intermediate composition such as Santiaguito, dacite-andesite. The characteristic dome shape is attributed to a high viscosity which prevents the lava from flowing far horizontally. This high viscosity can be obtained in two ways: by high levels of silica in the magma; or by degassing of fluid magma.
Since viscous basaltic and andesitic domes weather fast and easily break apart by further input of fluid lava, most of the preserved domes have high silica content and consist of rhyolite or dacite.

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