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Types of Rocks: The Three Major Rock Groups

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  • 0:05 Rock Types
  • 0:34 Igneous Rock
  • 1:59 Sedimentary Rock
  • 4:00 Metamorphic Rock
  • 5:07 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Did you know that lava is molten rock that reaches the earth's surface? Furthermore, did you know that rocks exist in three general forms: igneous, sedimentary and metamorphic? This lesson describes these rock types and examples of each.

Rock Types

Scientists are always looking for ways to put things into categories, and rocks are no exception. Rocks are solid masses occurring naturally as part of our planet. As it turns out, rocks can be put into three fundamentally different types:

  • Igneous rock
  • Sedimentary rock
  • Metamorphic rock

This lesson will identify and describe these three types of rocks.

Igneous Rock

Let's start with igneous rock. Igneous rocks - for example, basalt and granite - are crystalline solids that form directly from cooling magma. Alright, that's fine, but what is magma, and what is its source?

Magma is molten rock made by the partial melting of rocks in the earth's interior under conditions of high temperature and high pressure. As rock melts, it becomes less dense and rises towards the earth's surface. Magma that reaches the earth's surface is called lava. Most lava flows are quiet, but some can be violent, such as the eruption of Mount St. Helens back in 1980.

Igneous rock that forms at the surface of the earth is referred to as extrusive or volcanic rock. These are named after the fire god, Vulcan. Basalt is a good example of extrusive igneous rock.

Igneous rock formed before it reaches the earth's surface is named intrusive or plutonic after the god of the lower world, Pluto. These rocks cool over long periods of time and, thus, develop large crystal structures, as we see in granite. Some intrusive rocks are subsequently exposed to the surface - but only after uplifting or erosion of the earth's surface, otherwise they would remain buried.

Sedimentary Rock

While igneous rocks make up the bulk of the earth's crust, they're often covered by relatively thin sheets of sedimentary rock. As the name suggests, sedimentary rock is formed from sediments or debris transported by liquid water, ice, or wind that become compacted and cemented together.

Sedimentary rocks are secondary rocks, as they are formed from the accumulation of small pieces of pre-existing rock - that is, parent rock. There are two main types of sedimentary rocks, based on the source of the sediment.

Sedimentary rock can form as solid particles from weathered rocks. These particles are called detritus, and detrital sedimentary rocks are accumulations of detritus. Therefore, the source of sediment for detrital sedimentary rocks is weathered rock from another form. For example, particles of sand from other rocks can form sandstone, and mud can form shale, which is the most common form of sedimentary rock.

Chemical sedimentary rocks are derived from material carried in solution to lakes and oceans. Under certain conditions, the dissolved material precipitates out of solution and, thus, settles to the bottom. Precipitation may occur due to physical processes such as evaporation or through living organisms. For example, limestone is the most abundant chemical sedimentary rock, and it's typically formed when calcite precipitates due to evaporation. Calcite is produced by living organisms to form shells which can, in turn, form sedimentary rocks. Now, interestingly, 90% of limestone is of this biological origin.

Metamorphic Rock

Much like a caterpillar can morph into a beautiful butterfly, rocks can morph into different rocks. Now in this context, metamorphism is the transformation of one rock into another. Like sedimentary rock, metamorphic rock is a secondary rock.

Metamorphic rocks are formed from pre-existing igneous, sedimentary, and even other metamorphic rocks. In short, any rock can become a metamorphic rock. How does this happen?

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