Rock types - Geography Essay Example
* There are 3 types of rocks.
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* Formed when molten magma from mantle is pushed out through the earth’s crust often through volcanoes forming hard and layered rocks with speckled appearance. These contains minerals randomly arranged in large and interlocking crystals as magma cools slowly inside the earth’s crust. The cooling produces rocks that always contain crystals.
* 2 types: intrusive and extrusive rocks
*Intrusive rocks formed when molten magma from mantle cools slowly (underground) inside the earth’s crust forming rocks with large crystals such as that of granite’s. Granite is a very hard and decorative stone ideal for steps and buildings.
*Extrusive rocks formed when molten magma from mantle cools quickly above the earth’s crust forming rocks with small crystals usually dark in colour such as that of basalt’s.
* formed from layers of sediments laid deep down inside lakes or seas over millions of years. The layers are cemented together by salt crystals.
* Made up from fragments of older rocks that have been weathered or may have formed from the remains of living organisms.
* Formed in layers and these may give additional evidence about the conditions they were formed and may show evidence on the wave ripples on their surface.
S. Rocks with large and coarse-grained particles were probably deposited under active conditions, such as on a stormy beach
S. Rocks with smaller and fine-grained particles were probably deposited under quiet conditions, such as in a lake or deep sea.
* Sedimentary rock layers are often found tiled, folded, fractured (faulted) and sometimes even turned upside down which shows us that the Earth’s crust is unstable and has been subjected to very large forces.
* At the Earth’s surface, younger sedimentary rocks usually lie on top of older rocks.
* They often contain fossils of dead remains or plants and animals. This is used to work out the age of the fossil rock.
* Examples –
A. Sandstone – formed by cementing of the sand particles together used for buildings.
B. Mudstone or Shale – formed by the action of pressure alone on the mud particles. It tends to split into original layers very easily.
C. Limestone – A few fossilised shells can be found in limestone.
E. Conglomerates – containing pebbles.
* Calcium Carbonate is the main composition in limestone.
* Formed when existing (sedimentary) rocks are changed by heat and pressure (created by mountain building processes) over a long period of time – millions of years and they recrystallize. This occurs deep in the crust often near plate boundaries as earth movements can push all types of rock deep underground.
* Rocks that recrystallize have interlocking crystals that are aligned. The alignment gives evidence of the direction of pressure on the rock.
* They can be layered and can contain fossils if they are formed from Sedimentary Rocks. The may have really tiny crystals.
* Contact metamorphism – recrystallization of pre-existing rocks due to high temperatures and pressures.
This can happen when in igneous intrusion bakes surrounding rocks and may cause them to recrystallize.
Also, when the sedimentary rock, mudstone, is changed by contact metamorphism, it first produces slate (low grade metamorphism), then schist and then gneiss (high grade metamorphism). Each metamorphic rock formed still has randomly oriented crystals.
* If metamorphic rocks melt they turn into magma which may resurface and cool to form igneous rocks.
* Examples –
A. Marble [heated limestone] – very high temperatures will breakdwn the shells in limestone and they reform as small crystals which gives marble a more even texture and makes it harder.
B. Slate [squashed mudstone] – as mudstone is heated and compressed a lot, its tiny particles align in the same direction which allows slate to be split into thin sheets in that direction making it an ideal roofing material.
C. Schist [heated mudstone]- as mudstone is heated a lot and compressed, new minerals like mica start to form the rock schist containing a band of interlocking crystals.
ROCK cycle – takes millions of years to complete!
* The amount of rock on earth is always about the same, even though it is weathered away.
* Involves changing the 3 types of rock –
IGNEOUS, SEDIMENTARY and METAMORPHIC
from one form to another.
* This happens by the following processes:
1. WEATHERING: the breakdown of rocks
2. TRANSPORATION: movement of the eroded rock bits round the world by wing and water.
3. DEPOSITION: sediment being laid down.
4. BURIAL / COMPRESSION: squeezing and compressing the layers – which eventually form sedimentary rocks.
5. HEAT/PRESSURE: the rocks being further heated and squashed which turns rocks into metamorphic rocks.
6. MELTING: by intense heating, the rock completely melts that forms igneous rocks.
7. COOLING: the molten rock solidifying.
* This involves changes to rocks both inside and outside the Earth’s crust.
What is WEATHERING?
* Breakdown of rocks into smaller bits.
* There are 3 types of Weathering. – 3 distinct ways that rocks are broken up into smaller fragements.
A – Onion Skin Weathering – THE SUN
* This happens when the sun warms the surface of a rock by day making it expand and at night when there is no sun it cools making it contract.
* The expansion and contraction causes the layers of rock to break away, like peeling an onion.
B – Freeze-Thaw Weathering – THE WATER
* When water freezes in a crack of a rock it expands making the crack bigger.
* This causes the bits of the rock to break away.
Chemical – THE ACID RAIN
* The rain is naturally acidic due to carbon dioxide in the air but pollution make it more acidic.
* When the acid rain falls onto carbonate rocks such as limestone, a CHEMICAL REACTION occurs breaking the rocks down.
Acid + Metal Carbonate ï¿½ Metal salt + Water + Carbon Dioxide
Biological Weathering – THE ANIMALS & PLANTS
* Animals such as bunny rabbits burrow under rocks
* Tree roots grow through rocks
* These activities WEAKEN them. Weak rocks break up easily.
Useful Products from Rocks
Limestone (mainly Carbon carbonate)
* As Limestone is a hard sedimentary rock, it can be quarried and used as a building material.
* Powered limestone (a base) can be used to neutralise acidity in lakes and soils. Limestone (a base) reacts with acid to make salt and water in a neutralisation reaction.
* When heated in a furnace, it breaks down to quick lime (calcium oxide) and carbon dioxide as metal carbonates decompose on heating. Quick lime (Calcium oxide) reacts with water to make slaked lime (calcium hydroxide) which is used to reduce the acidity of soil.
Heat quicklime = CaCO3 (s) ï¿½ CaO (s) + CO2 (g)
Calcium Carbonate ï¿½ Calcium oxide + Carbon dioxide
This type of reaction is called thermal decomposition. Other carbonates behave in a similar way.
Slaked lime = CaO (s) + H2O (l) ï¿½ Ca(OH2) (s)
Calcium oxide + Hydrogen Oxide ï¿½ Calcium Hydroxide
* Heated together with clay in a furnace to make cement
* Heated with sand (silicon oxide) and sodium carbonate to make (soda glass) used in windows.
* Heated with sand (silicon oxide), sodium carbonate and boron oxide to make borosilicate glass such as Pyrex which are harder then soda glass.
* Mixed with iron ore and coke to form the charge which is loaded into a blast furnace. The limestone reacts with the high melting point non-metal impurities in the iron ore to form a molten slag which floats on top of the iron.
* Mining and Quarrying which creates visual pollution as ugly slag heaps are created which scar the landscape. They cause dust and smoke pollution and generate a lot of heavy traffic because of the transportation of rocks.
* Produced by roasting powered limestone with powdered clay in a rotary furnace.
* When cement is mixed with water, sand and crushed rock, a slow chemical reaction produces a hard, stone-like building material called concrete.
Cement + water + sand + crushed rock ï¿½ concentre (stone like building material)
* Made by heating a mixture of limestone, sand and sodium carbonate (soda).
* Used in industry to make bottles, containers etc.
* The wearing away of exposed rocks.
* The process of carrying away the rock fragments, either falling away due to gravity or being carried away by rivers.
* The rocks travelling down rivers get worn down as they go and they also wear away the river bed causing river valleys. The Grand Canyon is a grand example.