Geography & History Notes | KCSE, NECTA, ZIMSEC & GCSE

  They are formed from crystallization and solidification of molten magma. They may be deposited onto the earth’s surface or with in the earth’s crust i.e. extrusive or intrusive rocks respectively. The main process for the formation of igneous rocks is volcanicity. This is due to the radioactivity and connectivity with in the earth’s mantle. Lines of weakness or fault lines are created in the earth’s crust through which magma is either extruded or intruded. Igneous rocks are usually described as fire formed rocks, igneous rocks are characterized by being hard, don’t contain fossils and they are crystalline in nature. As magma is pushed from deep in the earth’s interior, it cools in different parts of the earth’s crust thus forming different types of rocks. They include Volcanic rocks or extrusive rocks Hypabyssal or intermediate rocks Plutonic or abyssal or deep seated rocks. Volcanic or extrusive rocks. These are igneous rocks formed on top of the earth’s surface. They are formed...

  A rock is an aggregate of mineral particles found in a soft, solid, or unconsolidated state. The earth’s crust consists of rocks and rocks consist of a combination of different minerals. All Minerals are formed from one or more of eight main elements. These are oxygen, silicon, potassium, sodium, calcium, magnesium, iron, and aluminum.                        SEDIMENTARY ROCK Sedimentary rocks are found in layers; they contain fossils and are very soft. These are weathered particles formed through deposition and lithification processes. Sedimentary rocks are formed when the sediments are accumulated, compacted, and cemented together. The sediments are compacted by compression to form sedimentary rocks. These are called stratified rocks . Characteristics of sedimentary rocks They are formed when particles or sediments are accumulated, compacted and cemented together. ...