How Are Igneous Rocks Formed?

By Carly Dodd on October 22 2020 in Geography

Different kinds of igneous rocks. Image credit: Artography/Shutterstock.com
  • Igneous rock is formed when magma, which is liquid molten rock, cools or sets, solidifying into rock and rock formations.
  • Extrusive rocks are rocks that have formed on the surface of the earth. This occurs when magma bursts forth from the mantle or crust on to the surface.
  • Intrusive rock is rock that forms within small pockets beneath the earth’s crust.

Formation Of Ignerous Rocks

Diagram representing the formation of igneous rocks. Image credit: VectorMine/Shutterstock.com

Rocks can be categorized into one of three types: sedimentary, metamorphic and igneous. Igneous rock is formed when magma, which is liquid molten rock, cools or sets, solidifying into rock and rock formations. As this magma, or molten rock emerges to the surface, it experiences a change in temperature and pressure, which forces it to cool and crystallize, forming rock. The word igneous is derived from the latin word ignis, meaning fire. This describes how magma, or rock that has been melted by fire, is the basis of all igneous rocks. Much of the Earth's surface, and the rock found within the Earth’s crust is composed of igneous rock. It is the most common type of rock found on Earth, as it covers approximately fifteen percent of the Earth’s surface, but also makes up much of the inner layers of the Earth, below the crust.

Molten rock is known as magma when it is beneath the surface of the earth, and lava once it bursts forth from underground. This is why volcanoes are known to have lava flows, but molten rock underground is called magma. In both cases, this is rock that has been melted in one of three ways: through an increase in temperature, a change in composition, or a decrease in pressure.

Because igneous rocks are formed from melted rock, which is in a thick viscous form, it can take on a wide variety of textures, appearances and compositions when it cools and solidifies. Sometimes lava is expelled violently in a volcanic eruption, or in other cases fissures open up in the Earth’s crust and magma slowly leaks out. The speed in which it cools and solidifies greatly effects the type of igneous rock that forms.The rocks can also form either above ground, as in on the surface of the Earth’s crust, or while still below the surface. When rock forms above ground, it is known as extrusive rock, while magma that cools below the crust is called intrusive rocks. There is also a third classification, which is intrusive rock that forms in shallow depths just below the crust, and this is known as hypabyssal, or subvolcanic rock.

Formation Of Extrusive Rocks

Basalt is an extrusive ignerous rock. Image credit: Yes058/Shutterstock.com

Extrusive rocks are rocks that have formed on the surface of the earth. This occurs when magma bursts forth from the mantle or crust on to the surface. This generally happens in volcanic areas, when volcanoes erupt or ooze magma. As the hot liquid rock hits the open air on the surface, it cools and solidifies, forming rock. Because the most common way magma escapes is through volcanic activity, these rocks are often called volcanic rock. Rock tends to cool very quickly when it hits the open air. Even in warm climates, the temperature above ground is much cooler than the temperature below the crust, and is at a much lower pressure, so therefore the magma cools and solidifies rapidly. As the molten rock cools, it forms crystals within its rock makeup. When magma cools quickly, these crystals don’t have long to form, and therefore are much smaller, as the crystals stop forming when the rock is fully cool.

Igneous rocks with smaller crystals tend to be smoother and have a glassy appearance. The faster the rock cools, the more glass like it appears. One of the prime examples of this is obsidian, an extremely glassy black rock which forms nearly instantly, and therefore has no individual crystals in its makeup. This process of extremely hot magma cooling has produced a variety of rock types in very unusual shapes. These variations of pattern, texture and even composition vary based on the speed and temperature of the lava flow. Slow moving lava tends to form short steep flows, while quick moving flows produce longer thin variations.

One of the rocks most commonly associated with this volcanic flow is pahoehoe lava. This type of extrusive rock cools very quickly as it flows from a volcanoe or fissure. Because of this, the rock maintains a glossy rolling appearance, and looks as though it was frozen mid flow which, essentially, it was. Another example of this is Pillow Lava, which forms bubble like balls of solid rock. This happens in a similar process to pahoehoe lava, but instead occurs exclusively underwater. In other cases, lava explodes violently from a volcano, expelling it into the open atmosphere where it solidifies almost instantly.

In these cases, lava can cool midair, or as it lands, causing rough, bubbly or stringy looking rocks. These rocks and ash formations are known as volcanic pebbles, ash hailstones or tuffs, which are formed in the air and fall as rock. One of the main examples of this is Lapillistone. Other common extrusive rocks include pumice, pepertite or reticulate. All of these types have varying degrees of holes or pockmarks which give them a rough sometimes even mesh-like texture caused by gas pockets which evaporate and leave gaps in the rock. Extrusive igneous rocks also include andesite and basalt, basalt being one of the most common volcanic byproducts.

Formation Of Intrusive Rocks

Granite, an intrusive ignerous rock. Image credit: Aleksandr Pobedimskiy/Shutterstock.com

Intrusive rock is rock that forms within small pockets beneath the earth’s crust. Instead of breaking free, this magma cools and solidifies while still inside the earth’s crust. This means that intrusive rocks, which are also known as plutonic rocks, cool at a much slower rate than extrusive as they are surrounded by preexisting rock. Because of this slow cooling process, crystals in the rock have more time to form, and are therefore larger and usually visible to the naked eye. Rocks that have this crystalized texture are known as phaneritic rock. The visible crystals can range widely in shapes and sizes. Intrusive rocks as a whole, are very hard, compact rocks, Though their colour and crystalizations may very, they are always completely mineral based, as the heat and pressure under which they are formed prevents organic matter from remaining. Most intrusive rocks are coarse grained and exhibit no spaces or air pockets. There are five types of intrusive rocks, which are granite, pegmatite, gabbro, diorite and peridotite. Granite is the most common type of land based intrusive rock, while gabbro is the type found most often underwater.

Formation Of Hypabyssal Rocks

a sub category of the intrusive rock is the hypabyssal, or subvolcanic rock. It is a type of igneous rock that forms in very shallow depths, ie just below the earth’s surface. This often occurs in fissures, or near faults. These rocks are in many ways similar to other intrusive rocks, but have intermediate grain sizes and textures, generally half way between those found in extrusive and other types of intrusive rocks. These types of rock include diabase, quartz-dolerite, micro-granite and diorite.

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