A'a lava
A'a lava

Astronomy, Guide to Space

How Are Rocks Formed?

3 Dec , 2009 by

You’ve come to find an answer to the question, “How are rocks formed?”. However, there are three types of rocks: igneous, sedimentary, and metamorphic rocks. Each type of rock has a different origin. Therefore, the question, “How are rocks formed?” begs three distinct answers. Here they are:

How are igneous rocks formed?

Igneous rocks are formed when melted rock cools and solidifies. Melted rock may come in the form of magma, when it is found underneath the Earth’s surface. It can also come in the form of lava, when it is released unto the Earth’s surface during a volcanic eruption.

Some examples of igneous rocks are granite, scoria, pumice, and obsidian.

Pumice, for instance, is formed when lava made up of melted rock, water, and trapped gas is ejected from a volcano during a violent eruption. As the ejected material undergoes very rapid cooling and depressurization, some of the trapped gas escape, leaving holes and gas bubbles on the solidified material.

How are sedimentary rocks formed?

Sedimentary rocks start forming when soil and other materials on the Earth’s surface are eroded and finally settle down, forming one layer of sediments. As time passes, more and more materials get eroded and settle on the older layers. Thus, layer upon layer is formed. The lower layers undergo intense pressure due to the weight of the upper layers, eventually evolving into rocks.

Some examples of sedimentary rocks are sandstone, limestone, shale, conglomerate, and gypsum.

Sandstone, for instance, is a result of depositions of sand from beaches and rivers. You can find them mostly in deltas, since this is where the rivers flow into the ocean.

How are metamorphic rocks formed?

To metamorphose or simply to morph means ‘to change in form’. Metamorphic rocks are actually products of rocks that have undergone changes. Thus, a metamorphic rock may have originally been an igneous, sedimentary, or even another metamorphic rock.

The changes occur when the original rocks are subjected to extreme heat and pressure beneath the Earth’s surface. They may also occur when the the original rocks are caught in the middle of two colliding tectonic boundaries.

Some examples of metamorphic rocks are marble, slate, schist and gneiss.

Marble, for instance is the result of the metamorphism of limestone and dolostone. When limestone metamorphoses, its calcite grains grow and interlock with one another. As such, marble is denser and harder compared to limestone.

We’ve got lots of articles about rocks here in Universe Today. For variety, we recommend these two:
Volcanic Rock
Moon Rocks

If you want to read more about Fray’s device that extracts oxygen from Moon rocks, we’ve got exactly what you need here in Universe Today. Just click on that link. We also have an article featuring secrets that may be revealed from moon rocks even 40 years since the Apollo missions brought them to Earth.

Here’s an article from NASA that debunks the hoax theory using the Moon rock arguments. Another article about Moon rocks from the same site.
Episodes about the moon from Astronomy Cast. Lend us your ears!
Shooting Lasers at the Moon and Losing Contact with Rovers
The Moon Part I


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A Physics teacher who turned freelance writer but still missed Physics just the same. He also writes for: APCmag.com 2live2blog.blogspot.com and Robot Reviews (News & Blog section, under the username eigenlance

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