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

  Rock cycle is the cycle in which rocks tend to change from one type to another. For instance, igneous rocks may change to metamorphic rocks or sedimentary rocks ; sedimentary rocks to metamorphic or igneous rocks, etc. It is a relationship in which rocks tend to change from one type of rock to another.   Necessary conditions for rock cycle to take place 1. the molten rocks are erupted from the interior of the earth and then cool and solidify to form igneous rocks. 2. the igneous rocks are subjected to denudation process to form sedimentary rocks. 3. either igneous or sedimentary rocks undergo metamorphism, due to prolonged heat and pressure, to form metamorphic rocks. 4. metamorphic or igneous rocks can undergo weathering process through erosion and transportation of sediments which are further deposited in layers in the ocean or lake floors where they are cemented and consolidated to form sedimentary rocks and vice versa. 5. metamorphic or sedimentary rocks can be ...

  The Earth’s lithosphere (crust and upper mantle) is divided into seven large and several smaller plates. These plates are constantly moving, and are driven by convection currents in the mantle. Plate boundaries mark the sites of the world’s major landforms, and they are also areas where mountain-building, volcanoes, and earthquakes can be found.   However, in order to account for such activity at the plate boundaries, several points should be noted  Continental crust is less dense than oceanic crust so it does not sink.  Whereas oceanic crust is continuously being created and destroyed, continental crust is permanent, and hosts the oldest rocks on the planet (the shieldlands).  Continental plates may be composed of both continental and oceanic crust (e.g. Eurasia).  Continental crust may extend further than the margins of the land masses (when continental crust is covered by an ocean, it is known as continental shelf).  It is not possible for pl...

  Most of the relief features are related with continental drift due to the fact that forces such as tensional and compression forces affected many parts of the world. Features which have been formed as a result of continental drift include; rift valleys, basins, plateau, highland ranges, coastal plains and associated features, block mountains etc. A rift valley is one of the spectacular features associated with continental drift. This is an elongated depression bordered by two in facing escarpments. It is formed due to either compressional forces or tensional forces or through differential uplift theory, basin and swell theory and the sea floor spreading theory. Plate tectonics also caused the vertical movement of the crust which led to the formation of Block Mountains. A block mountain is also known as a Horst. It is formed when the middle block bounded by more or less parallel faults is made to rise or is uplifted by the compressional forces. The best example of a block mountai...