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The Rock Cycle
Yr 12 Geology
Yr 13 Geology
The Rock Cycle
The rocks and landforms we see around us come about as a result of processes acting on and within the earth: weathering, erosion, transport, deposition, burial, lithification, metamorphism, melting, intrusion, eruption, uplift. These processes make up the rock cycle. They can be represented in diagrammatic form as shown below. We will examine them one by one.
to an animated version. I suggest you use this as revision, so the link appears again at the bottom of this page.
Weathering: means the breakdown of rock into smaller fragments or dissolved minerals. There are two types of weathering:
many students think that because the root word is 'weather', weathering must mean something to do with storms and rain or extreme weather events. This is not true. Weathering happens as a result of exposure to
- air and water. Weathering can happen even in a desert.
chemical changes in minerals
due to exposure
to air and water
. This happens because most rocks form deep underground, without oxygen. When they reach the surface, many minerals (which are just chemicals) react easily with oxygen, particularly if water is also present. Carbon dioxide in the air is also important in chemical weathering, particularly of limestone. Carbon dioxide reacts with oxides of magnesium, sodium and potassium formed by the oxygen/water reactions to form carbonates and bicarbonates, which are washed into the sea. This is a major process removing carbon dioxide from the atmosphere, and a reason why sea level fall helps lower carbon dioxide levels by exposing further rocks to weathering.
The coloured band on the outside of the broken rock on the right is the result of chemical weathering.
Chemical weathering can do several things.
Some minerals may dissolve away, leaving others as unsupported grains to be washed away. Limestone and a few other rocks can dissolve away completely. Ions which can form soluble carbonates or bicarbonates, such as sodium , magnesium, calcium and potassium tend to be removed from highly weathered material.
When some minerals change, it may involve a change of volume and or hardness.This can leave other mineral grains unsupported to be washed away. Examples of this are feldspar minerals changing into soft clay minerals, or volcanic minerals changing into oxides and hydroxides which are much softer.
You can see the effects of chemical weathering on the outside surface of many rocks. Minerals dissolved by chemical weathering eventually get washed into the sea.
Another common misunderstanding:
some students think that chemical weathering must be caused by exposure to chemicals, such as acid rain. Although this
cause chemical weathering, plain old water and air is enough. The term 'chemical' is used because these things cause chemical reactions in the rock.
This slab was levered off by ice
This is the
breaking up of rock
, with fragments being physically moved apart. This can be caused by:
water freezing in cracks in the rock (water expands when it freezes)
plant roots forcing cracks apart
swelling minerals like clays expanding between grains as they get wet and dry out
expansion and contraction due to heating and cooling over the day and night.
Chemical and physical weathering combine to make rock softer and more fragmented, and make erosion easier.
This is the
of broken up (weathered) material, and is combined with transport. The
rate of erosion
increases with rainfall and mountain height (relief). This means that the highest mountains are in the places of greatest uplift, as mountains will grow until the rate of uplift and the rate of erosion are equal. For example, the most rapid uplift in NZ is occurring near Mt Cook. Erosion can be caused by water, ice or wind, or wind-blown dust and such things.
This is where weathered and eroded material, together with material like volcanic ash and plant fragments, is moved by water, wind or mass movement. More detail on these processes is given in the section on
Deposition and burial
This is when transported material is deposited. Most transported material ends up on the continental shelf or the area between the continental shelf and the deep ocean. However, some gets deposited in other places like swamps, river deltas, lakes, desert dunes and valleys.
As more material is deposited on top, the sediment undergoes burial. The deeper it is buried, the more heat and pressure it is subjected to, and it begins to change into rock.
This means the change of deposited material (sediment), into rock. It happens as a result of the increase of temperature and pressure with burial.
The next processes in the rock cycle are part
plate tectonic activity.
Review plate tectonics here
As burial depth increases, the heat and pressure increase further and the minerals in the rock begin to change. This is termed metamorphism, and it forms metamorphic rocks.
Melting, intrusion and eruption
Further increase in temperature may result in part of the rock melting. Non-melted material continues to descend and is re-absorbed into the mantle.
The melted portion of the rock gets squeezed out to form magma. This forms a body of molten rock which is called a pluton or an intrusion. Sometimes it can make its way to the surface and be erupted. Other times it stays underground and solidifies there. It can cause some metamorphism in the rocks around it.
Devil's Boots, near Collingwood
Uplift is upward movement of deep down rocks as a result of
. This is the result of faults (breaks in the earth's crust) and folds (places where the earth's crust is bent into
). This movement can be up, down or sideways. It is the upward movement that results in uplift and forms mountains, which weather, erode and provide the material to continue the rock cycle. Uplift and erosion can expose deep down metamorphic rocks and intrusions.
Uplift is driven by the processes of plate tectonics. There is a more detailed description of plate tectonic processes in NZ
The picture on the right shows a limestone formation known as the "Devil's Boots" in Golden Bay. A nearby river run along a fault line; the river undercut the limestone producing the overhang. However, an earthquake uplifted the formation from the river leaving something that looks like a giant, upside down set of boots sticking out of the earth. Nearby caves were recently underwater, and contain ontly the youngest type of stalactite, called
, shown left at Gardner's Gut cave in Waitomo. These young features indicate the cave was recently underwater.
Revise what you have read here with this
More on the Rock Cycle from the UK Geological Society
Rock cycle exam type questions.
Answers are typed in the same colour as the background. Drag the cursor over the space where the answer is to read it. I suggest you try writing your answers first, then compare them to my ones. I have tried to answer the questions fully to show a good example of 'excellence' answers.
A sedimentary rock, such as a mudstone, can be transformed into a metamorphic rock, such as a schist.
By deep burial, many kilometres underground where it is subject to high temperatures and pressures and therefore where the minerals of the mudstone become different, new minerals in the sandstone.
A metamorphic rock, such as gneiss, could be transformed into a sedimentary rock, such as conglomerate.
The gneiss would need to be physically weathered to break it up, then transported a short distance and deposited and buried while it was still made out of big fragments of gneiss e.g. near a rocky glacial coast like Fiordland
Metamorphic rocks such as schist are formed deep under the ground. Discuss the process which could cause sediments on the sea floor to become schist, kilometres underground, then be exposed again at the surface.
First of all, sediment needs to keep being deposited for millions and millions of years so that enough thickness built up to bury them kilometres deep in the crust, where heat and pressure would build up enough to cause changes in the minerals. This is how the sedimentary rock changes when new metamorphic minerals are formed from the old sedimentary ones. Later on, there is uplift. Kilometres of overlying rock are eroded away over millions of years, until the rocks from deep down are exposed at the surface.
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