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Rocks
Sedimentary Rocks
• Introduction
• Limestone
• Limestone landscapes & features
• Chalk
Sedimentary rocks are formed when eroded fragments of old rocks and dead organisms settle (usually
in seas or rivers) to form a sediment. Over millions of years, layers of sediment build up and are buried
one on top of the other. They are compressed, and their weight squeezes out the water. Eventually the
pieces of rock in the sediment become bonded together to form sedimentary rocks.
Sedimentary rocks are laid down in layers called beds or strata. Each new layer is laid down
horizontally over older ones. There are usually some gaps in the sequence called unconformities. These
represent periods in which no new sediments were being laid down, or when earlier sedimentary layers
were raised above sea level and eroded away.
Sedimentary rocks contain important information about the history of the Earth. They contain fossils,
the preserved remains of ancient plants and animals. The composition of sediments provides us with
clues as to the original rock. Differences between successive layers indicate changes to the environment
which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and
metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remnants.
Sedimentary rocks include common types such as chalk, limestone, sandstone and clay. Sedimentary
rocks cover 75% of the Earth's surface.
Economic Uses
Sedimentary rocks are economically important in that they can easily be used as construction material
because they are soft and easy to cut Sedimentary rocks often form porous and permeable reservoirs in
sedimentary basins in which water and important minerals such as oil can be found.
Limestone
Limestone is a sedimentary rock composed largely of the mineral calcium carbonate. Pure
limestone's are white or almost white. Because of impurities, such as clay, sand, organic
remains, iron oxide and other materials, many limestone's exhibit different colours,
especially on weathered surfaces. If metamorphism occurs e.g. during the mountain
building process limestone re-crystallises into marble.
Limestone is permeable since water can pass through the many natural cracks in the rock
and it is easily dissolved by acidic rainwater. In areas of limestone, distinctive landforms
will be created to give a landscape referred to as karst.
NOTE: Add image
of limestone rock
Karst Landscapes & Features
Above ground, there are areas of bare rock called limestone pavements which show distinctive
clints (blocks) and grykes (grooves) and swallow holes where streams disappear underground.
Below ground, large cave systems are formed by solution along the vertical joints and horizontal
bedding planes. In these caves may be narrow stalactites hanging from the ceiling and stumpy
stalagmites on the cave floor. Sometimes these join to form pillars.
Springs are formed where water re-emerges back on to the surface as the rock changes from
limestone to something impermeable.
Sometimes the roofs of cave systems collapse and leave narrow steep-sided limestone gorges. Some
of these are dry but others may contain streams, particularly in wetter periods.
NOTE: Add image
of limestone cave
Chalk
Chalk is a soft, white, porous form of limestone composed of the mineral calcite. It is relatively
resistant to erosion so forms tall steep cliffs where chalk ridges meet the sea e.g. Beachy Head
along the south coast of England
Chalk is porous which means that water can pass through and be stored in the pore spaces between
the particles of rock. When it rains, water infiltrates into the chalk layers and saturates the chalk.
The upper level of saturation is called the water table. Many water companies drill boreholes into
the chalk rock to supply water to nearby settlements.
Chalk forms distinctive landscapes with steep escarpments and gentle dip slopes. The hills may
have dry valleys which show the characteristics of a river valley but have no water flowing along
them. The North and South Downs in south-east England show all these features.
Economic Uses
Chalk is a valuable resource and much of it is quarried for the construction industry where it is
used in the making of cement.