Rocks

Making up the majority of the Earth's crust, rock is usually defined as a mixture of common minerals. Rocks can be hard or soft, as small as a grain or as large as a building. They have been an integral part of the history of mankind, first being used as tools for hunting and defense, and as a building materials to construct shelters and monuments. Combined with the effects of tectonics, weathering and vegetation, rocks define the natural landscapes we see around us. The minerals and metals we find in rocks are essential to the prosperity and cultural splendor of human civilization. There are many kinds of rock, and they can be classified in a number of ways. However, geologists classify rocks based on how the rocks were formed. The three classes are igneous rocks (formed directly from liquid rock), metamorphic rocks (formed by direct alteration of existing rocks), and sedimentary rocks (formed by eroded materials from other rocks)
Rocks:
Rocks are a composed of one or more minerals. A rock can be made up of only one mineral or, as shown in the figure, a rock can be made up of a number of different minerals.
So, rocks are composed of one or more minerals and minerals are composed of one or more elements.
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.
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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
Sources:
http://en.wikipedia.org/wiki/Igneous_rock http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/igneous_rocks.html http://geomaps.wr.usgs.gov/parks/rxmin/rock2.html http://geomaps.wr.usgs.gov/parks/rxmin/rock3.html Read more: http://www.universetoday.com/46594/how-are-rocks-formed/#ixzz2q5BtjOAU