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     Rocks change over time depending on how much pressure, weathering, and erosion that they get.  Pressure is the pushing down of top layers of rock on lower layers of rock.  We use football as an example of this.  Let’s say that the football falls on the ground and a player leaps on top of it.  Think about the player.  He is just lying there, like a top layer.  He is feeling pretty good because he is the top layer.  Other players see this and jump on top of him, hoping to get the ball away. First one player leaps on top, and the player with the ball feels pressed down a little.  Little by little, as more players pile on top, he is pressed down so much that he feels pushed into the ground and flattened.  This is what we mean by pressure.
     Weathering is how the wind, rain, snow, etc. affect the rock.  Weathering isn’t how one rain storm affects a rock.  It’s what lots of rain storms do to it over a long, long time.  Most of the time, weathering causes bits and pieces of rock to separate from larger chunks and washes them into streams and rivers.  Erosion moves the pieces of rock from place to place using gravity, running water, or weathering.   Erosion wears away rock over lots of time.
     Rocks go through something called the rock cycle.  This is where rocks change over a lot of time.  Time, to a rock, is thousands and millions of years.  A rock can begin as one kind of rock and then change many times before it is mined.  Underground heat and the pressure we were talking about, plus weathering can change rocks over millions of years. 

     There are three kinds of rocks:

	Igneous
	Sedimentary
	Metamorphic

Rocks, minerals, and gemstones are mined in many different ways depending on how close to the surface they are found.  For example, if you found a certain kind of rock at the bottom of streams or rivers, it would be mined using surface mining like placer mining.  If it was about six feet underground, a different kind of surface mining like open pit mining would be used.  In open pit mining, layers of dirt are taken off the top until they get down to the rock they want to mine.  If it is deep in the Earth, different kinds of underground mining are used.  Mining companies use the kind of mining that will get them to the rocks, minerals, or gemstones the cheapest.  Mining companies will not mine for anything that won’t make them money. 
     Each of the rocks that we included on our site will include the following information:  What it is and where it’s formed, how and where it is mined, and what its uses are. 

The rocks we cover on this site are:

	Bauxite		Marble
	Coal		Peat
	Conglomerate		Sand
	Gneiss		Sandstone
	Granite		Shale
	Gravel		Slate

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Citations:

Fuller, Sue.  Rocks & Minerals.  London: Dorling Kindersley, 1995.