Volcanic rocks are those that originate from the rapid or sudden cooling of lava or volcanic material that is expelled on the surface of the earth’s crust, from volcanic eruptions that can become very explosive, which is why they are also known as rocks. extrusive igneous.
Volcanic rocks are directly related to volcanic activity (volcanoes) and are opposite to intrusive igneous or plutonic rocks.
These types of rocks usually have several distinctive characteristics to the naked eye, especially in terms of mineral composition and texture, which differentiate them from intrusive igneous rocks.
How are volcanic rocks formed?
The rocks are formed when the lava or pyroclastic volcanic material that is expelled from the volcanoes cools or crystallizes to form consolidated volcanic materials in the form of rocks, and depending on the characteristics of the magmatic melt (lava) and its composition, different types will be formed. of volcanic rocks.
Types and Examples of Volcanic Rocks
| Volcanic rocks | Chemical composition |
| Rhyolite | Felsic (more than 63% SiO2) |
| Andesite | Intermediate (52 to 63% SiO2) |
| Basalt | Mafic (45 to 52% SiO2) |
| Pyroclastic volcanic rocks | |
| Volcanic tuff | Generally felsic (more than 63% SiO2), but can also be intermediate |
| Pumice (pumice) | Generally felsic (more than 63% SiO2) |
| Ignimbrite | Generally intermediate (52 to 63% SiO2) |
| Slag | Generally mafic (45 to 52% SiO2), but can also be intermediate |
| Obsidian | Felsic (more than 63% SiO2) |
The type of volcanic rocks will depend on the chemical composition of the magma from which they come, also, on the type of volcanic material expelled, below are some examples.
Thus, if they come from a mafic melt, it is almost certain that the volcanic rock is a basalt.
If it is a melt of intermediate composition, the volcanic rock is likely an andesite.
If it’s an acid melt, it’s probably a rhyolite.
If the volcanic material was ejected violently and its cooling was extremely abrupt, and it also came into contact with water, it is almost certain that obsidian will form.
There’s also volcanic rocks that are formed by the lithification of pyroclastic volcanic materials, are known as volcanic pyroclastic rocks.
Some examples are pyroclastic deposits, volcanic tuffs, volcanic scoria, and pumice, which have a large percentage of volcanic glass.
Distinctive features of volcanic rocks
| volcanic rocks | Color | Characteristic | Texture |
|---|---|---|---|
| rhyolite | Light (white, pink, greenish) | Rich in quartz and feldspars | Aphanitic and porphyritic with vitreous parts |
| Andesite | Medium gray (light greenish grey) | Rich in plagioclase and hornblende | porphyritic, aphanitic |
| Basalt | Dark (dark grey, black, dark greenish grey) | Rich in plagioclase, pyroxene and olivine. | Aphanitic, vesicular, with vitreous parts, sometimes porphyritic. |
| Pumice stone (pumice) | Light (white, light pink, creams) | Rich in volcanic ash and volcanic glass, it can float on water. | Vesicular and pyroclastic |
| volcanic tuff | Light (light grey, white, light to medium greenish) | Rich in volcanic ash and volcanic glass, it usually presents lamination. | pyroclastic |
| ignimbrite | Light to dark (greys, pinks, oranges, greens) | Shards of volcanic glass and pumice in a mass of ash. | pyroclastic flow |
| Human waste | Dark (dark gray) | Basaltic to andesitic composition | Vesicular |
| Obsidian | Dark (but has varied colors) | Felsic composition and high percentage of volcanic glass. conchoidal fracture. | vitreous |
The easiest way to distinguish this type of rock from plutonic rocks is thanks to its typical glassy, vesicular, aphanitic and sometimes porphyritic or porphyritic texture.
A volcanic rock that has undergone rapid cooling will have an aphanitic texture, that is to say that its mineral crystals cannot be seen with the naked eye without the help of a magnifying glass with a minimum of 10 magnification.
Sudden cooling is also responsible for the volcanic rock acquiring a glassy texture, the clearest example being obsidian.
Other examples that represent a glassy, vesicular and pyroclastic texture will be pumice, volcanic scoria, ignimbrites and volcanic breccias.
How to differentiate a volcanic rock from a plutonic?
To differentiate a volcanic rock from a plutonic rock, the easiest way is to compare its texture, for example, plutonic rocks will have a phaneritic texture, that is, their minerals will be visible to the naked eye.
While in a volcanic rock the minerals will not be visible to the naked eye without the help of a hand lens, so its texture is aphanitic or glassy.