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Teachers Guide 11 – Rocks and Soil What are Rocks and Soils? Rocks come from magma, which is the molten material found within the earth. When magma cools, either above or below the earth's surface, it crystallizes and forms the igneous rocks that can later be changed into metamorphic or sedimentary rocks. Rocks are made of one or more minerals. There are three main classifications of rock, based on the way the rock was formed: sedimentary, metamorphic and igneous (see Teachers Guide 10 – Rocks and Soils Glossary) Soil is formed of fine rock particles mixed with air, water and particles from dead plant and animal matter. There are three main types of soil which are classified according to the amount of sand and clay in them. Top 10 Facts 1. The oldest rocks ever to be found were formed about 4 billion years ago – only two pieces of rock this old have ever been found. 2. The largest meteorite ever discovered weighed 66 tonnes. It was named Hoba after the farm in Africa that it was found on. 3. The chalks that you draw and write with are made from limestone. 4. Pumice rock is so full of air that it floats. 5. If lightning hits a beach it can make the sand reach such a high temperature that it melts to form fulgurite, which is a glassy rock. 6. It takes over 500 years for just 2cm of topsoil to form. 7. There are approximately 10,000 different types of soil found in Europe. 8. The number of micro-organisms found in a cup of soil is greater than the number of people on Earth. 9. Some of our soil has in it the dusty remains of stars that have been pulled to Earth by gravity or another force. 10. Soil can filter and purify water (don’t try this at home, though!). Important vocabulary Sedimentary rock: Sedimentary rocks are formed from sediments that have settled at the bottom of a lake, sea or ocean, and have been compressed over millions of years. Metamorphic rock: Metamorphic rocks have been subjected to tremendous heat and/or pressure, causing them to change into another type of rock. Igneous rock: Igneous rocks are formed by magma from the molten interior of the Earth.