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 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
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 six 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.