Rocks

The oldest known rocks on Earth are 3900 million years old - they are the Acasta gneiss rocks from Canada.

There are three main kinds of rock:

• Igneous rock( igneous means "fiery" ) - made when hot molten magma or lava cools and solidifies. Volcanic rocks( such as basalt ) are igneous rocks that form from lava that has erupted from volcanoes.
  
  
• Metamorphic rocks - rocks that have changed over time, such as limestone which is made into marble because of the heat generated by magma.
  
  
• Sendimentary rocks - rocks that are mase from the slow hardening of sediments into layers, or strata. Some sendimentory rocks, such as sandstone, are made from sand and silt. Other rocks are broken down into these materials by weathering and erosion.