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The igneous rock is a broad category of naturally formed rocks found on and within the Earth's surface.
Igneous rocks are formed from magma, a hot, molten material found below the Earth's crust or lava that erupts out of a volcano. 'Igneous' is a common terminology derived from the Latin word 'ignis' that means 'fire'.
One fun fact is that Rhyolitic magma contains the greatest amount of silica. Simply put, rocks are a composition of various undistinguishable minerals and other earthly substances that accumulate to form them. Quartz, mica, olivine, feldspar, and calcite are some commonly obtained minerals found within rock particles. Igneous rocks lack fossils as the fossils are already melted while forming the magma.
Rocks are classified into three broad categories, namely, igneous, sedimentary, and metamorphic rocks. These three types of rocks are caught in a loop wherein they change from one form to the other, resulting in an altered texture, color, and chemical composition. This process is known as the rock cycle.
In this article, the focus is laid on igneous rock, particularly on its types and formations. The entire oceanic crust and a large portion of the continental crust are composed of igneous rocks, basalt, and granite, respectively. These are strong rocks that have been used by humans on various occasions (building constructions, furniture, and more) since ancient civilizations.
The assignment of different names to each igneous rock is based on the locations where they are formed, the minerals that they are made of, the grain size, and cooling conditions. Some common igneous rock types include granite, basalt, diorite, gabbro, rhyolite, andesite, komatiite, and peridotite.
Keep reading to discover more about igneous rock. If you like this article, don't forget to check out space rocks and 3 Types of metamorphic rocks for your next read!
Igneous rocks are broadly classified into two groups, namely intrusive or plutonic and extrusive rocks. Whether an igneous rock is intrusive or extrusive can be determined by its composition, that is, the minerals it contains alongside magma (unerupted) or lava (erupted magma). The location where an igneous rock is formed, whether on or within the Earth's surface, also determines its type. The time taken by the magma or lava to cool down also decides the type of igneous rock. Let us find out more about the two types.
Intrusive igneous rocks are formed from magma (molten rock) present inside the Earth. Due to high temperatures, the magma cools slowly and gradually, taking thousands of years to form into rocks. Intrusive rocks are named so because the magma intrudes and cools down inside the cracks on other rocks. Slow cooling gives room for the individual crystals or minerals to grow large.
The individual crystals are shiny, have a flat surface with straight edges; you will be able to identify the minerals just by the look of them! The large mineral crystals give these rocks a coarse-grained texture and make them extremely strong that can scratch glass. Hence, intrusive igneous rocks are characterized by large crystals of light-colored minerals and a coarse-grained or phaneritic texture. Some examples of intrusive rocks are granite, diorite, gabbro, and peridotite.
Plutonic rocks are a subset of igneous rocks that form further inside the Earth, where the temperature is even hotter. These rocks take millions of years to form and result in even larger crystals that may measure up to 3.2 ft (1 m) in length!
Extrusive igneous rocks form when magma erupts out of the magma chamber present below the Earth's crust. This erupted magma called lava cools quickly due to low temperatures, leaving less time for the crystals to grow. Hence, mineral grains on extrusive rocks are comparatively smaller, which may be difficult to locate with naked eyes. As a result, these rocks do not have a coarse-grained but a fine-grained texture. Therefore, extrusive igneous rocks are characterized by dark-colored minerals with tiny crystals that are almost invisible and a fine-grained or aphanitic texture showing flow lines.
Examples of this type of rock include rhyolite, andesite, basalt, and komatiite. However, extrusive igneous rocks can be found in various textures. Obsidian or black glass is a type of volcanic glass that forms when lava materials cool quickly. Volcanic rocks such as rhyolite form visible grains upon a fine-grained surface. Pumice is volcanic glass froth that has a vesicular texture. Vesicular textures form when gas bubbles are trapped inside melted rock during solidification.
It is really difficult to assert which is the oldest rock on the Earth's surface. This is because the planet had undergone a number of disasters before it was identified as Earth. Therefore, to answer this question, it is important to dissect the temporal axis and consider one particular time frame with an initial point.
Some geologists consider that before the Theia-Earth impact (collision of the Earth and a Mars-sized planet), metamorphic rocks were the first to form below the Earth's crust due to cold accretion. The Earth's surface at that time was made of dust particles that settled to form sedimentary rocks. However, most of what was present before the Theia-Earth impact was completely destroyed.
After the Theia-Earth impact, the planet was filled with molten rock or magma. The magma or lava solidified to form the igneous rock. Active plate boundaries or hot spots are where the magma originates from.
Igneous rocks do not contain fossils because they cannot stand extreme conditions like heat in the case of igneous and cold for metamorphic rocks. This magma then cools down to form igneous rocks. Erosion and weathering break down these rocks break into smaller materials containing minerals. Rivers, wind, and other transportation forces carry these smaller materials and deposit them elsewhere. These particles or sediments settle down over several years to form sedimentary rocks.
The sedimentary rocks, after experiencing extreme heat and pressure, undergo structural changes, giving birth to metamorphic rocks. This cycle of rock formation and transformation has been repeating itself since the beginning of time. Hence, it can be safely concluded that igneous rock is the oldest rock on our planet.
Did you know that the oldest igneous rock that was discovered on our planet was actually a meteorite? This meteorite, known as Erg Chech 002, was originally andesite (a type of volcanic rock) that was found in the southwest Algerian desert very recently, May 2020. What will blow your mind is the fact that it is marginally older than the Earth!
Primarily, there are eight varieties of igneous rocks, namely, granite, diorite, gabbro, peridotite, rhyolite, andesite, basalt, and komatiite. These rocks form by cooling molten rock either on or within the Earth's crust. These rocks differ in their composition, texture, and location of formation.
Rocks such as granite and rhyolite have the same mineral composition but form in different locations, either on or within the Earth's crust. Hence, different names have been assigned to them. Again, the diorite and andesite rocks are named so, depending upon their texture where the former is coarse-grained and the latter fine-grained.
Similarly, a mafic rock composed of Pyroxene and Ca-rich such as gabbro and basalt has different names assigned to them because they exhibit different textures due to differences in the location of formation and the time taken by them to solidify. Basalt is also igneous rock of which most ocean floor is made of. The ultramafic rocks, peridotite, and komatiite are called by different names even though they are composed of the same minerals. This is because neither do they form at the same location nor take the same amount of time to form. To conclude, four different criteria are involved in the composition of one single rock. Hence, it is necessary to assign different names to these rocks because it will ease out the task of geologists in distinguishing and identifying them correctly.
Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestions for what do igneous rocks look like, then why not take a look at a rock made of shells and corals cemented together or facts about little rock Arkansas?
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