There are three ways rocks are formed. Depending on the method or process of formation, they are called igneous rocks, sedimentary rocks and metamorphic rocks. Sedimentary rocks are formed from the sediments in water, and in some cases sediment in the air. Metamorphic rocks formed from the changes in existing rocks which are caused by altering pressure and temperature.
Igneous Rock Definition
Igneous rock is formed from magma. When magma erupts and cools down to form hard objects, they are called igneous rocks. Let us explore what leads to such formation.
The core of the earth, aka the center of the earth, has a massive raging fire. This fire is perpetual and is facilitated by radioactive reactions of a plethora of materials. The solid materials are in a molten state. As you would have a rock on the surface of the earth, it would melt at a thousand degrees Celsius or close to two thousand degrees Fahrenheit. Some rocks could be in semi molten stage but most are in liquid state. This molten or liquid state of rocks, including various minerals and radioactive materials, is magma. When magma erupts through the earth’s crust, it gets exposed to much cooler temperatures. As a result, the magma solidifies, rather quickly. This leads to the formation of igneous rocks.
Types of Igneous Rocks
There are two types of igneous rocks: intrusive and extrusive.
Intrusive igneous rocks are formed below the surface of the earth. As the magma rises to the crust of the earth, it experiences reducing temperatures. As a result, the molten materials start to solidify. More than solidification, they start to crystallize. The crystallization process is slow because the magma stays stagnant for a long time. This results in the formation of large rocks or crystals. Intrusive igneous rocks are formed over a long period of time and they are substantially larger. Some intrusive igneous rocks are granite, diorite, pegmatite, gabbro and peridotite.
Extrusive igneous rocks are formed when magma erupts as lava and the molten state is quickly affected by a sudden decline of temperature on the surface of the earth. The atmospheric pressure which is much weaker than the one inside the core of the earth and the surface temperature compel the molten lava to solidify quickly and that leads to the formation of small rocks or crystals. These are known as extrusive igneous rocks. Some extrusive igneous rocks are basalt, pumice, tuff, andesite, obsidian, rhyolite and scoria.
Presence of Igneous Rocks
According to some estimates, more than a quarter of all rocks found on the planet are igneous rocks. Almost all other rocks are sedimentary rocks. Metamorphic rocks are rare. Also, it is estimated that most of the earth’s crust, which is not accounting for the rocks found on the surface of the earth, is formed by igneous rocks. The heating and cooling phases at the time of formation of our planet would have been a chaotic process. Molten lava or magma would have erupted, causing explosions and highly radioactive as well as mineral rich materials to have been exposed. The rampant cooling would have crystallized the molten lava, leading to the formation of the rocks that now form the crust of the earth. The molten lava or magma still rest inside raging into a fire which at times erupts through volcanoes.
Common Igneous Rocks
There are numerous igneous rocks found in abundance across the world, particularly in areas that have experienced volcanic activity in the recent past. Igneous rocks are also studied to understand the world, the formation of the planet and what it constitutes. Here are some popular igneous rocks.
• Andesite is an extrusive igneous rock made primarily of plagioclase, hornblende, biotite and pyroxene.
• Diorite is an intrusive igneous rock made of feldspar, hornblende and pyroxene. It may also contain quartz.
• Basalt is an extrusive igneous rock made of pyroxene and plagioclase.
• Gabbro is intrusive igneous rock made of feldspar and pyroxene. It may also contain olivine.
• Obsidian is a volcanic glass. You cannot get crystals of obsidian because it forms rapidly from molten rock.
• Granite is an intrusive igneous rock made of quartz, mica and feldspar.
• Peridotite is an intrusive igneous rock made of olivine, amphibole, quartz and feldspar.
• Pegmatite is an intrusive igneous rock made of rare minerals.
Other popular igneous rocks are rhyolite, pumice, scoria, fire opal and tuff. There are more than seven hundred different types of igneous rocks. Igneous rocks hold the secret to our past, the formation and evolution of the world we inhabit, the evolution of humans, rock formation and how changing environments affect the planet. Igneous rocks are also used in construction, certain types of jewelry and as a source of necessary minerals. Over the years, many igneous rocks have been unearthed that contained fossils of extinct species and have offered enlightening insights of our past.