Earth

Rock Cycle

Ever thought about how are the rocks you see around yourself are made up of? How the mountains, the plateaus, the uneven lands, etc. form? In the following points to come, you will get an idea about how the formation of rocks, their types, and also the rock cycle.

Rock Cycle

Rock Cycle

Introduction to Rocks

Before entering into the topic of the rock cycle, let us first discuss a little bit about what rocks are. This will build a foundation while understanding the rock cycle. There are 3 main types of rocks i.e. the igneous rocks, sedimentary rocks, and metamorphic rocks. Rocks are classified according to the way they are formed. We consider factors such as hardness, cleavage( how the rocks split), lustre ( how shiny it is), colour, texture, etc.

Igneous rocks are of two types- intrusive and extrusive rocks. When magma cools outside the earth’s surface then they are called extrusive igneous rocks. If the magma cools inside the earth’s surface then they are called intrusive igneous rocks. Extrusive igneous rocks are used to build statues and buildings, while intrusive igneous rocks are used to make gravestones and countertops because they are durable. We will discuss the remaining rocks subsequently.

How rocks are Formed?

Rocks are constantly changing on earth. Most of the rocks on earth are only about 100 million years old. When we see any rock, it is quite obvious that it was of a different shape millions of years ago. The Earth has a very hot core, similar to that of the sun. Rocks constantly change their shape whose process is not visible to the naked eye.

Magma flowing out of volcanic eruptions eventually cools off either inside the earth’s surface or outside the earth’s surface as lava. The magma from minerals becomes the rocks specifically the igneous rocks. Some examples of igneous rocks are Granite and Basalt.

Igneous Rocks

As mentioned earlier these rocks are formed from the cooling of magma. As these rocks are formed from a liquid hence it is considered to be a primary rock or parent rock. Even though rocks are hard they can break apart through weathering or erosion.

Sedimentary Rocks

During the weathering process or erosion process, the igneous rocks break down. When this igneous rock mixes with sediments like tiny grains of sand clay, slit, or other bits of rock and it compacts or hardens with pressure, it then takes the form of another type of rock and we call it the sedimentary rock.

Sediments are blown around because of their lightweight and are carried by the wind just like seeds. Usually, the sediments settle at the bottom of the water bodies such as at the river beds, ocean beds, and lakes. Over time, these sediments harden and then grow as more and more sediments settle down. In this way, they become layered. Sandstone and limestone are good examples of sedimentary rocks. The Grand Canyon is also a great example of sedimentary rock. Limestone is a chemical sedimentary rock.

Metamorphic Rocks

Heat and pressure together change igneous and sedimentary rocks when the minerals in the rocks change over time by heating without fully melting the rocks. They form new rocks called metamorphic rocks. The heat requirement is near about 200oC and the pressure requirement is about 1000 bar.

The value of 1 bar is equal to 105 N/m2. In other words, the minerals change or morph. If there is enough heat or pressure, rocks can be folded and squeezed together and then they harden into metamorphic rocks. Slate Marble and quartzite are examples of metamorphic rock. Slate is a metamorphic rock that is formed from shale.

There are many metamorphic minerals which might be found in the metamorphic rocks but are not a direct outcome of metamorphosis. Metamorphic rocks make up about 13 % of the earth’s surface. The study of metamorphic rocks provides great information regarding the variation of temperature and pressure inside the earth’s core or at any near places.

When it comes to metamorphic rocks, another phenomenon known as foliation is very important. As we now already know that the metamorphic rocks are formed because of pressure and heat, hence the shear component of pressure is also responsible for the formation of various layers on the surface of the rocks. This phenomenon is called foliation. The layers are formed parallel to the direction of the shear component of the force or perpendicular to the direction of the net pressure. An example of a foliated metamorphic rock is gneiss. Quartzite falls in the category of non-foliated metamorphic rock.

The Rock Cycle

The igneous, sedimentary, and metamorphic rocks when melt again, they form magma. Then the process of melting and cooling to igneous rocks, weathering and eroding of the igneous rocks to sedimentary rocks and application of pressure and heat to form metamorphic rocks again is completed. It forms a cycle which is the rock cycle. This cycle goes on and on and never ends. In the rock cycle, igneous rock can directly become metamorphic rock also. Sedimentary rocks can become igneous or metamorphic as well and metamorphic rocks can become igneous as well as sedimentary.

Although the metamorphic rocks are generally formed deep inside the earth because of the need for excessive pressure and heat, they are also exposed to the atmosphere as well. Apart from water and wind, some other forces which are responsible for the conduct of the rock cycle are the tectonic plate movements, continental collision, spreading of the mid-ocean ridges.

FAQs about Rock Cycle

Q.1. What is weathering?

Ans: When rocks break apart due to weather elements such as heat, rain, and wind, we call this process weathering. For example, a really strong wind can blow sand particles against a rock and weather it down like a nail file does to nails or sandpaper does to wood.

Q.2. What is erosion?

Ans: When rock wears away with natural forces like the sea waves repeatedly crashing on the shore we call this process erosion. Moving water is the most powerful cause of erosion. Riverbanks are eroded by the flowing river water. Gullies are formed when heavy rains wash away soil and fields. Heavy rain can also cause the loosening of the soil and cause landslides. The Grand Canyon is a great example of the phenomenon of erosion.

Q.3. What is magma?

Ans: Magma is the molten substance formed inside the earth’s crust from which the igneous rocks are formed. The magma is formed due to the melting of metal near the mantle region due to the heat of the earth’s core. This magma looks for openings on the earth’s crust and comes out through them. A good example is a volcano.

Q.4. What is a chemical sedimentary rock?

Ans: These are rocks which when eroded get chemically precipitated in the solution and deposited elsewhere. For example, when water travels through the earth’s crust, weathering the rock, some of its minerals get dissolved. These dissolved minerals get precipitated as soon as the water or the medium gets evaporated. Limestone is an example of chemically precipitated sedimentary rock.

Q.5. What is the temperature at which the rocks melt?

Ans: At a temperature range of around 600oC to 1500oC, rocks melt to form magma. Magma is also known as lava on the earth’s surface.

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