Types of Rocks and Formation
What is a rock?
Rock is a naturally occurring solid material made of one or more minerals.
Some rocks are made of mainly one mineral, such as Chalk – made of a mineral called calcite. But other rocks have mixtures of minerals; for example, pink granite – is composed of feldspar, quartz, mica & amphibole minerals.
Types of rocks
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Igneous rocks
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Sedimentary rocks
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Metamorphic rocks
How are rocks formed?
The Earth is composed of four different layers (Earth’s layers):
- The crust,
- The mantle,
- The liquid outer core,
- The solid inner core.
The outer and inner core is composed mainly of nickel and iron. The crust (outermost and thinnest) and uppermost part of the mantle are collectively known as the lithosphere, from which the tectonic plates are composed.
Rocks are formed and reformed in the lithosphere. Rocks are categorized into three main types: Igneous, Metamorphic, and Sedimentary.
The process of formation of each type of rock is different. In the following sections, we will explore how other Rocks are formed.
How are Igneous rocks formed?
The ‘igneous’ word is derived from the Latin word “ignis” which means ‘fire.’
Volcanic eruptions form igneous rocks. When a volcano erupts, it throws out a lot of molten rock called magma or lava. When magma cools down and hardens, it becomes Igneous rocks.
In short, Igneous rocks are formed when magma or lava (molten rock) cools and hardens.
Igneous rock formations are found in Yosemite National Park in California, Sunset Crater Volcano National Monument in Arizona, Joshua Tree National Park, California, Acadia National Park, Maine, and Hawai’i Volcanoes National Park, Hawaii. (source)
Types of igneous rocks
- Intrusive igneous rock – erupts from a volcano and reaches the earth’s surface.
- Extrusive igneous rock – solidifies before it reaches the surface
Example of Igneous rocks
How are sedimentary rocks formed?
Weathering & Erosion – When rocks are weathered and eroded, they break up into tiny fragments called sediment.
Transportation & Deposition – These small rock fragments transported by wind or water are deposited in the sea or river beds.
Compaction & Cementation – The deposited sediments compress and harden over a long time to form many different layers made of many minerals. New sedimentary rock is created when the compaction and cementation process is finished.
Sedimentary rocks may also contain lots of fossils because the dead bodies of animals and plants get deposited in the sediment layers and harden as a rock.
Sedimentary rock characteristics
- Sedimentary rocks are formed on the Earth’s surface, opposite to igneous and metamorphic rocks, which are formed deep within the Earth.
- They cover 75% area of the Earth’s surface.
- They are generally non-shiny, soft and non-crystalline.
- Formations of Sedimentary rock are found in Arches National Park in Utah, Grand Canyon National Park in Arizona, etc.
Examples of Sedimentary rocks
Types of sedimentary rocks
- Organic sedimentary rock
- Clastic sedimentary rock
- Chemical sedimentary rock
How are metamorphic rocks formed?
The word ‘metamorphic’ is derived from the Greek word ‘metamorphose’ meaning “change of form.”
Metamorphic = Meta(change) + Morphe(form)
Metamorphic rocks are formed when either an igneous or sedimentary rock is exposed to intense heat and pressure. The intensive heat and pressure make the metamorphic rock much denser than the igneous or sedimentary rock.
Formation of Metamorphic rocks is found in Rocky Mountain National Park in Colorado, Shenandoah National Park in Virginia, Grand Teton National Park, and Harper’s Ferry National Historic Park on the East Coast.
Examples of metamorphic rocks
- Slate and schist are formed from mudstone, which is a sedimentary rock.
- Marble is formed from limestone, which is a sedimentary rock.
- Quartzite is formed from sandstone, which is a sedimentary rock.
- Gneiss is formed from granite, which is an igneous rock.
- Granulite is formed from basalt, which is an igneous rock.
Types of Metamorphic rocks
- Foliated Metamorphic Rocks
- Non-Foliated Metamorphic Rocks
Things to Remember
➤ Petrology is the branch of geology that studies rocks’ structure, composition, and distribution.
➤ Rock is a naturally occurring solid material made of one or more minerals.
➤ Rocks are formed and reformed in the lithosphere. The Earth is composed of four layers:
- The crust,
- The mantle,
- The liquid outer core,
- The solid inner core.
The crust (outermost and thinnest) and uppermost part of the mantle are collectively known as the lithosphere.
➤ Based on the formation process, there are three main types of rock found on the Earth:
- Igneous
- Metamorphic, and
- Sedimentary.
➤ The ‘igneous’ word is derived from the Latin word “ignis” which means ‘fire.’
➤ Igneous rocks are formed when magma or lava (molten rock) cools and hardens.
➤ There are two types of igneous rocks: Intrusive and Extrusive igneous rock.
➤ Granite, obsidian, basalt, scoria, and pumice are common examples of igneous rocks. Pumice is a very porous rock
➤ The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling down.
➤ Sedimentary rocks are formed by the deposition and solidification of sediment, mineral, or organic particles.
➤ The five most important processes are forming sedimentary rocks: Weathering, Erosion, Deposition, Compaction, and Cementation.
➤ Sedimentary rocks are formed on the Earth’s surface, opposite to igneous and metamorphic rocks, which are formed deep within the Earth.
➤ Sandstone, limestone, breccia, conglomerate, and shale are common examples of sedimentary rocks.
➤ There are three different types of sedimentary rocks:
- Organic Sedimentary Rocks,
- Clastic Sedimentary Rocks
- Chemical Sedimentary Rocks.
➤ The word ‘metamorphic’ is derived from the Greek word ‘metamorphose’ meaning “change of form.” Metamorphic = Meta(change) + Morphe(form)
➤ Metamorphic rocks are formed when either an igneous or sedimentary rock is exposed to intense heat and pressure.
➤ Slate, schist, gneiss, quartzite, phyllite, and marble are common examples of metamorphic rock.
➤ There are two types of metamorphic rocks: Foliated and Non-Foliated Metamorphic Rocks