Types of Rocks

 Learn more about Rocks

The three types of rocks

1.Types of Rocks

The three main types, or classes, of rock are sedimentary, metamorphic, andigneous and the differences among them have to do with how they are formed.

Sedimentary
Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils.

Examples of this rock type include conglomerate and limestone.

Metamorphic
Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface.
Examples of this rock type include gneiss and marble.

Igneous
Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock.

Examples of this rock type include basalt and obsidian.

https://www.learner.org/interactives/rockcycle/types.html

2.Uses of Rocks & Minerals

We have made use of rocks and minerals from the very beginning of time. If you look at how we named the eras of human development, STONE AGE, COPPER AGE, BRONZE AGE and IRON AGE, it is obvious how important rocks and minerals have been to us all. For more information of how we used rocks and minerals throughout history.

In MODERN TIMES, since about 1700 CE (or 400 years ago) we have been using rocks and minerals at an ever increasing rate as we build machines, cities and consume a great number of "things" in a lifetime.  We use things made from rocks and minerals every day. If something doesn't come from a plant or an animal, it has to be mined. According to the Mineral Information Institute, it is estimated that in a lifetime, a person living in North America will use up the following quantity of rocks and minerals:

http://www.rocksforkids.com/4uses.html

3.Oldest Known Rocks Give New Clues to the Earth’s History

With the help of a tiny fragment of zircon extracted from a remote rock outcrop in Australia, the picture of how our planet became habitable to life about 4.4 billion years ago is coming into sharper focus.

Writing today (Feb. 23, 2014) in the journal Nature Geoscience, an international team of researchers led by University of Wisconsin—Madison geoscience Professor John Valley reveals data that confirm the Earth’s crust first formed at least 4.4 billion years ago, just 160 million years after the formation of our solar system. The work shows, Valley says, that the time when our planet was a fiery ball covered in a magma ocean came earlier.

http://news.wisc.edu/oldest-bit-of-crust-firms-up-idea-of-a-cool-early-earth/