Thursday, November 23, 2006

The Rock beneath our feet!

Rock can be broadly classified into three types. Igneus, metamorphic and sedimentary, with the Jurassic Coast being of the sedimentary type

Igneus, is the rock that originates from magma within the earth's crust, produced by volcanic activity, when it is found on the earths surface
Metamorphic is the rock that is transported into the depths of the earth by geological processes, where it is heated and compressed. This process eventually changes its composition

Sedimentary rock is created by processes that occur on the surface of the earth, by the weathering of existing rock by water, ice and wind. Sedimentary rocks may consist of the organic remains of marine organisms (limestone), land plants (coal) or chemical reactions and evaporation (salt). Sedimentary rocks that constitute the Jurassic Coast are formed largely of the remains of marine animals. Sedimentary rock is often created by the weathering and re-deposition of pre-existing rock, with the parent rock being of igneus, metamorphic or sedimentary type. Rock particles, once created, are generally carried to distant locations by water, wind or ice, before sedimentation occurs

In past times of high sea levels, shallow seas have flooded deep inland and produced vast continent-wide limestone deposits. Chalk is a form of limestone, that developed in the Cretatious era. This rock was formed by millions of tiny plates of single-celled microorganisms called coccoliths. Over a few million years thier calcite plates piled up on the seabed to form a carbonate mud, which is now a form of limestone, up to 300 metres thick. This chalk forms the spectacular white cliffs of Southern England and North Western France

Any fossil content within a sedimentary rock will indicate its age and the type of environment in which it was deposited. Sedimentary rock often contains organic and inorganic materials. For example, early Jurassic black mudstone, found in the cliffs of Lyme Regis, contains organic material in the form of fish teeth and ichthysaurus bones. The mud itself is made of small inorganic rock particles, but its black colour suggests a high carbon content, the result of decomposition of the soft parts of organisms

Most sediment is deposited underwater, and is then buried further before it becomes rock. The exposure of marine sediment above sea level is due either to tectonic plate processes such as earthquakes and volcanic activity or to a general decrease in sea levels such as is associated with glaciation. Once sedimentary rock is exposed, weathering begins, particles are transported, and the cycle begins again

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