Volcanic rock (volcanite) is formed as a result of volcanic eruptions. Depending on the type of explosion (a lava flow or explosive eruption), three types of rock are formed: intrusive, extrusive, and volcanogenic-detrital (pyroclastic) rock. They make up ninety percent of the Earth's crust and they have been building up from hundreds of millions of years ago, from the archean eon, the earliest phase of the Earth geological evolution.
Intrusive rock, also known as plutonic, is a volcanic rock formed in the process of injection and solidification of magmatic melt deep in the earth’s crust (intrusion, intrusive body, plutonic intrusion). Thus, this type of rock cooled off and solidified deep under the surface of the Earth. Granite, diorite, and granodiorite are the three types of intrusive rock and they are found in the shape of laccoliths and lopoliths, as well as batholiths, stocks, and dikes, all of which are magma formation by intruding deep under the surface of the planet. Intrusions are classified according to the depths at which the magmatic intrusion occurred as deep-seated intrusions (abyssal) and shallow-depth intrusions (hypabyssal). In this context the conditions of cooling of the magma and its effect on surrounding rock differ sharply. At shallow depths the cooling is rapid and fine crystalline or porphyritic rock is formed, and contact metamorphosis affects a small area of the country rock. At great depths medium-grain and large-grain rock and major changes in the surrounding rock are characteristic.
Extrusive rock (or effusive), on the other hand, is a magmatic rock formed with the cooling of lava on the earth’s surface or within the crust under near-surface conditions in contact with the atmosphere. Basalt is the hardest and the oldest type of extrusive rock. A significant proportion of the effusive rocks was formed during the eruption of underwater volcanoes. Typical characteristics of effusive rocks, associated with the rapid cooling of lava, are the presence of volcanic glass in their composition and, often, a unique porphyritic texture. The composition of effusive rocks varies widely. Basalts, andesites, and rocks intermediate between basalts and andesites are most common. Dacites and liparites are less common. Alkaline effusive rocks, such as phonolites and leucites, and ultrabasic effusive rocks, such as comateite, are even rarer. Effusive rocks were found on the moon. Effusive rocks are contrasted to intrusive rocks.
The volcanogenic-detrital (or pyroclastic) rock is divided into friable (volcanic ash, sand, bombs, and so on), compacted, and cemented rock (tuff, tuff breccia, and others). In addition, there are intermediate types of volcanic rocks: tuff lavas, which occur as a result of eruptions of gas-rich foaming lava flows, and ignimbrites, which occur as a result of violent eruptions when pieces of lava are carried into the air and fall on to the surface, forming masses of melted matter that occasionally occupy wide areas measuring hundreds and thousands of square kilometers.
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| Diadorite is a plutonic (intrusive) rock. It looks like granite and it is also extremely hard. |