HOW DIFFERENT TYPES OF ROCKS FORMED
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The
rock cycle is an ongoing process, beginning as rocks are pushed up by
tectonic forces, and eroded by wind and rain. The eroded rocks travel by wind
or moving water until they are deposited, settling into layers. Additional
eroded rocks may bury these layers until heat and pressure change the
underlying layers to metamorphic rock. More eroded rocks may squeeze and
press the layers into sedimentary rocks. Rocks can also be sunk down into the
lower layers of the earth by plate tectonic processes. Buried rocks may also
melt and recrystallized into igneous rocks. Metamorphic, sedimentary, and
igneous rocks may then be pushed up by tectonic forces, starting the rock
cycle again.
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EROSION AND WEATHERING
Weathering and
erosion slowly chisel, polish, and buff Earth's rock into ever evolving works
of art—and then wash the remains into the sea. The processes are definitively
independent, but not exclusive. Weathering is the mechanical and chemical hammer
that breaks down and sculpts the rocks. Erosion transports the fragments away. Working
together they create and reveal marvels of nature from tumbling boulders high
in the mountains to sandstone arches in the parched desert to polished cliffs
braced against violent seas. (science.nationalgeographic.com/science/.../weathering-erosion-article/)
Weathering and
erosion are geological processes that act together to shape the surface of the
Earth. In simple terms, weathering is a set of processes that break solid rocks
into fragments. Erosion is a group of processes that involve running water,
blowing wind, or moving ice that picks up and moves these fragments to a
different location.Weathering can be divided into two categories. They are physical
(also called mechanical) weathering and chemical weathering. Physical
weathering includes any process that breaks rocks into fragments. These
processes may include breaking the rock by collision with other rocks,
fracturing caused by release of pressure as material is removed by erosion from
above, growth of ice crystals in fractures, and growth of plants, which extend
their roots into fractures and pry the rocks apart. Chemical weathering occurs
when water from rainfall, streams, lakes, or oceans, or oxygen from the
atmosphere chemically react with the minerals in rocks to produce new minerals
that are more stable at the low temperatures and pressures near the Earth’s
surface, or in some cases, where the minerals completely dissolve as a result
of contact with water. The end products of chemical weathering are the minerals
that make up soils, such as clay minerals, quartz, and iron and aluminum oxides
and hydroxides. Basically, weathering takes large pieces of rock and breaks
them down to fragments that can more easily be transported by water, wind, and
ice.
sedimentary rock, rock formed at or near
the Earth’s surface by the accumulation and lithification of sediment or by the
precipitation from solution at normal surface
temperatures (chemical rock). Sedimentary rocks are the most common rocks
exposed on the Earth’s surface but are only a minor constituent of the entire
crust, which is dominated by metamorphic rocks.
(jersey.uoregon.edu/~mstrick/MinRockID/RockID/Sedimentary.html)
METAMORPHIC ROCKS
Metamorphic
rocks record how temperature and pressure affected an area when it was forming.
The rocks provide clues to their transformation into metamorphic rocks.
Metamorphic rocks are best identified when looking at the rock as you see them
in nature. You can clearly see the deformation and features that are characteristic
of an entire area. Metamorphic rocks were once sedimentary, igneous, or another
metamorphic rock. These rocks are physically deformed and chemically changed
due to different temperatures and pressures. The elements in the minerals can
actually "move" to form new minerals. The rock does not melt, or else
it would be considered an igneous rock. A rock looks different after it has
been metamorphosed. The rocks
texture and overall appearance changes also. It now has a squished look!
(www.classzone.com/books/earth_science/terc/.../es0607page01.cfm)
8 MAIN TYPES OF METAMORPHIC ROCKS
1. slate
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2. phyllite
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3. schist
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4. gneiss
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5. marble
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6. quartzite
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7. serpentine
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8. hornfels
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(skywalker.cochise.edu/wellerr/GLG101/GLG101-metamorphic-rocks.htm)
IGNEOUS ROCKS
Igneous rocks are called fire rocks and are formed
either underground or above ground. Underground,
they are formed when the melted rock, called magma, deep within the earth
becomes trapped in small pockets. As these pockets of magma cool slowly
underground, the magma becomes igneous rocks. Igneous rocks are also formed
when volcanoes erupt, causing the magma to rise above the earth's surface. When
magma appears above the earth, it is called lava. Igneous rocks are formed as
the lava cools above ground.
There are two types of igneous rocks.
- Extrusive - produced when magma flows on the earth's
surface
- Intrusive - produced when magma solidifies at depth
beneath the earth.
All
typical classification schemes rely on a combination of texture, particularly
grain size, and mineralogy. But, keep in mind they are process-oriented. Coarse
grained are plutonic, fine grained are volcanic. See figure below that
depicts a typical classification. Stress similar mineralogy of granite vs.
rhyolite, just differ in grain size. Compare granite to gabbro which have the
same grain size, but different mineralogy. Notice from figure how the
three common fine-grained rocks, rhyolite, andesite and basalt differ in their
chemistry. Rhyolite is very rich in silica while basalt has less silica, but
more iron and magnesium. Andesite is intermediate. (geology.csupomona.edu/drjessey/class/Gsc101/Igneous.html)
SRI LANKAN ROCK TYPES
Mainly sri Lanka rock types takes metamorphic type. more than that it contains sedimentary lime stone as well as metamorphic lime stone. Not only that it contains minute amount of sand stone also. There are some issues regarding the geology in our country. Some as follows.
·
Sri
Lanka’s National Building Research Organization says more landslides, rock
falls, subsidence and cut slope failures could occur in the districts of Kandy,
Nuwara-Eliya, Matale and Badulla, if the prevailing bad weather condition
continues.
- Once booming, bustling gemming hubs like Ratnapura and Matale are now virtual ghost towns, as miners migrate to other employment.
- · Excessive fluorides in groundwater are a serious water quality problem in some parts of Sri Lanka. The incidence of dental fluorosis shows a high correlation with the presence of groundwater in certain areas. Tube wells constructed in various rock types have shown different fluoride concentrations, possibly due to the different mineral constituents in these rocks and their relative capability of releasing fluoride ions into groundwater. (www.emeraldinsight.com/journals.htm)
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