Download Rock Cycle - PowerPoint Notes Pages File

Learning the processes of “The Rock Cycle” helps explain rock classification
This presentation is for classroom use only
1
Please note, rock types not rock names are required at this year level
2
The rock cycle describes the processes by which material is recycled within the crust and
mantle (asthenosphere). In some areas the crust has been through the complete cycle
more than once whereas in others only part of the cycle has been completed. Tectonic
movements can disrupt the cycle at any time.
(Graphic courtesy of geolsoc.org.uk)
3
Weathering is a destructive process.
Weathering is the breakdown of rock into smaller pieces called CLASTS. These clasts
remain with the parent rock.
Weathering can be:
• Physical by heat/cold, rain & wind.
• Chemical by acid groundwater and acid rain. Volcanoes produce most of the carbon
dioxide in our atmosphere. Past massive volcanic eruptions have been tied to mass
extinctions. Rotting plants, animals and their faeces produce humic acid
• Biological from living things. Vegetation prises apart rocks. Animals moving over
rocks can break them. Humans mine, quarry, farm and build by breaking rock.
Weathering in the tropics is fast. A fresh basalt can be broken down enough to produce
soil for planting yams in less than 4 years. Towards the poles weathering takes place
very much slower. Scrape marks from glaciers melted 1.2 million years ago are still
fresh.
The image is of the soil profile from a cutting in weathered Tamala Limestone near
Fremantle. The rock is being chemically and biologically weathered.
4
Weathering is critically Important in the development of economic iron ore reserves in
our north. Although most of the sediments were laid down about 1.8 billion years ago,
the original sediments contain bands of silica rich sediment which makes processing
more expensive. Later warm wet tropical weathering caused the silica to dissolve leaving
valleys with iron enriched sediments filling them. These high grade “channels” are the
cream of our Hamersley iron ore deposits. On Mt Whaleback the deposit is so rich that
when they wanted to erect a radio antenna they could weld it directly to the iron rich
rock.
The image is of weathered Banded Iron Formation in Karijini National Park.
5
Plant roots follow cracks in rock and physically force them apart. They utilise water
trapped there and minerals released from weathered rock.
Mosses and lichens (fungi + algae) also break down rock to create soil for later
colonising plants.
The image is of a fig breaking apart rocks on Mt Robinson in the Hamersley Range
6
Acid rain is created by solution of sulphur oxides and carbon dioxide in water. This reacts
with carbonate rich rock such as marble and limestone.
Acid groundwater has extended root channels into soft sandy limestone. Water has both
dissolved and re-deposited calcium carbonate along these “swallets” (solution channels)
as its pH changed.
Sculptures which were exposed to weathering without obvious adverse effects for over
one thousand years suddenly rapidly broke down with the advent of the Industrial
Revolution and the coming of the motor car. Burning fossil fuels releases carbon dioxide
and sulphur oxides. Students might research the scientific controversy surrounding “The
Elgin Marbles”.
The image is of Tamala Limestone in a sand quarry behind Rockingham.
7
E = away rodo = I gnaw or take away
Erosion moves clasts. Wind and water are vectors of erosion.
The rate of erosion depends on the force of erosive power. The silts and clays in the
inland of WA are moving towards the Eucla Basin very, very, very slowly. Unless there is a
flash flood or a rainy period (geologically) they move over millions of years. Clasts
broken from the Stirling Range may be flushed down to the sea in a few hundred years.
8
We loose one cricket pitch of top soil every year because of run-off gullies from our
roads. (5 loaves of bread per day). Run off water from roads and sealed areas like car
parks is channeled into small forceful streams which cut through topsoil. Exposed roots
die leaving soil open to further erosion by wind.
Roots protect the soil from erosion.
The image shows an erosion gutter at the side of the highway near Mingenew in the
Central Midlands.
9
Erosion by salt water.
Please note the waves cut notches at sea level. This “swash” zone has maximum erosive
power. Years of erosion leaves a wave cut platform just below this point.
Because our sea levels have risen and fallen during pulses of freezing and thawing
during the last glacial period (locally 1.2million to 16 thousand years ago) several
notches can be found at different heights along our shorelines.
Our wonderful mineral sand deposits are the result of winds blowing from the land
interacting with ocean currents to doubly sift a sand with low concentrations of minerals
and create bands rich in high density minerals.
The image is of sea cliffs near Point Peron.
10
Erosion by wind
The erosive power of wind is greatest towards the base. That is why when you walk
along a beach on a windy day sand stings your ankles but not your knees.
This explains the undercut of the granite face at Wave Rock.
The image is of the “Hippo’s Yawn” at Wave Rock near Hyden in our wheat belt.
11
As pebbles/clasts are moved by wind or water they clash against each other. The further
they travel the more rounded they become.
These pebbles came from volcanic rocks erupted during the Upper Proterozoic over 560
million years ago. Water carried them and caused them to become rounded. During
Permian times (255 mya) they were laid down as part of bright red conglomerates at the
base of the new deserts of the super continent Pangaea. They are now exposed as sea
cliffs and fall out to be further rounded by the sea crashing on the modern pebble beach
. Because they are igneous rocks they are made from interconnecting crystals. This
makes them hard and more resistant to weathering than the red sandstone matrix.
The pebbles are from a beach at Auchmichtie in Scotland where the Scottish Declaration
of Independence was written in 1340.
12
There are exceptions to the above generalisation. Not all rock types weather and erode
equally. Igneous rocks tend to be harder and more resistant to breakage due to
interlocking crystals. Rocks with originally flat bedding or schistocity will form platy
clasts.
13
As currents wind or water travel, they lose speed and carrying power because of friction
with the ground they are passing over. Larger clasts are dropped out first. When
seasonal rain stops, the decrease of river flow towards the end results in a general
decrease in grain size from the base of the sediment to the top. Similarly dunes
demonstrate graded bedding as wind power falls.
About 400 million years ago a river deposited these sediments over one year. Sediment
size decreased as the carrying power of the river decreased. This general decrease in
sediment size can be used to establish “WAY UP”.
The image is of a piece of metamorphosed quartzite (meta-sandstone).
14
If a mix of silt, sand and pebbles is laid at the top of the flume and either wind
(hairdryer) or rain (watering can) is applied, the gradation of sediment size down the
slope is obvious.
15
Sediments are loose clasts and often water. As succeeding beds are lain down on top,
spaces between the clasts decrease and water is squeezed out. Over 90% of water is
lost upwards in the first kilometre of burial.
The groundwater remaining contains dissolved minerals, often silica and lime. These
cement the clasts and rock is formed.
16
Sedimentary rocks can be generally classified as:
1. Clastic (made from broken rock)
2. Biogenic (made from living things)
3. Chemical (deposited from water)
Sedimentary rocks are according to clast size and shape.
Rock which has not travelled far has unsorted angular clasts. The image on the left is
loose scree which fell directly under the parent outcrop a few years ago.
Rock like this has been compacted and cemented by groundwater to form breccia rock
(Italian: broken). Breccia is indicative of faulting and massive uplift close by.
The images are of (left) scree from Mt Robinson and (right) iron rich breccia from Mt
Nameless beside Tom Price.
17
If the clasts have been further eroded they are rounder but still unsorted. Pebbles have
been compacted and cemented within a finer matrix to form conglomerate rock.
This ancient conglomerate has rounded pebbles in an unsorted matrix. It probably
indicates an unconformity due to rapid uplift in the Upper Paleozoic. Rivers brought the
clasts down from the new mountain sides rapidly depositing them when they reached
the plains or lakes.
Every rock tells its own story!
The pebbles/clasts on the left image and the clasts in the rock on the right are
approximately the same age. The pebbles are from Scotland and the conglomerate rock
is from Wiluna.
18
Still decreasing in clast size as we move downhill or down wind.
On the left are dune sands showing classical large scale dune bedding. Sand thrown up
by the sea is wind blown to form classic dune bedding in the present day.
On the right is similar sand compacted and cemented by silica to form sandstone.
The image on the left is from a sand quarry behind Rockingham and the image on the
right is of Donnybrook sandstone.
19
An activity for making replica breccia, conglomerate and sandstone can be found in the
Year 8 WASP materials. www.wasp.edu.au
20
Mudstones are deposited far from source where the force of the erosive power (river) is
depleted and only very fine particles can be carried. This is often out to sea or in coastal
swamps. It is made of clay particles (alumino-silicates) and may show general
horizontal bedding structures.
The image shows mudstone from the Irwin River in the Central Midlands.
21
Our limestone may have been initially biogenic (made by living things) but is
subsequently affected by groundwater. This hard rock is probably only 70,000 years old.
The unbroken shell content suggests that it was assembled from the deposition from a
storm surge. General beach shells are broken up by waves.
The rock is from Redgate Beach near Margaret River.
22
Before the Stirling Ranges were raised up to form mountains, this area was a warm
tropical sea where simple sponges with silica “stiffeners” in their skeleton swam. When
they died the tiny pieces of silica (just the same as glass) rained down onto the floor of
the sea. Over eons these deposits became an open network of angular silica spines
which became compacted and cemented to form spongelite. Spongelite is extracted to
mop up chemical spills.
23
Coal is made from fossilised plant material. Our WA coal is Permian whilst most coal is
Carboniferous, much older.
This coal is from the Collie coalfields.
24
Burial can not only compact and cement rock. Temperature increases 25°C for every
kilometer of depth. Unstable minerals will melt and re-crystallise into forms which are
more stable under those conditions. Igneous rocks are completely melted and recrystallised. Crystals are interlocking mostly producing strong hard rocks.
Igneous rocks are classified according to their chemisty (silica rich or silica poor) and
crystal size.
25
Extrusive = volcanic (cooled at or near the surface)
Intrusive = Intermediate or deeper still plutonic (cooled below or deeper)
Felsic = iron & silica rich
Mafic = magnesium & silica rich
Felsic rocks make up most of the continental crust of the Earth. Granite is the most
common example. Felsic rock tend to be less dense and light coloured.
Mafic rocks make up the oceanic crust, mantle and core of the Earth. Basalt is a
common example. Mafic rock tends to be dense and dark.
26
Igneous rocks have lots of names but only general classification is required.
27
These three rocks have similar chemical composition but their mode of cooling makes
them appear different
Obsidian or volcanic glass is extrusive and chills instantly to glass (crypto-crystalline)
Obsidian was used to make stone knives, spears and arrows.
Pumice is violently expelled with gas and water from volcanoes. It is extrusive and cools
rapidly, within hours. The gassy bubbles are instantly sealed making pumice able to float.
(Beware. Pumice bought from pharmacists is often reconstituted and cemented with
mortar. It sinks)
Granite is intrusive (plutonic). It cools over millions of years at great depth and has large
obvious crystals
The obsidian is from the USA, the pumice from Mt Tarawera in New Zealand and the
granite from the Darling Scarp behind Perth.
28
When a batholith (magma chamber) rises it causes doming in the rocks above. Cracks
appear and molten material is injected.
Dolerite intrudes vertical cracks in country rock to create dykes. Dyke is a northern
European word meaning wall.
These dykes were injected into softer sandstone which later eroded.
You can see how the sheep hide behind the sheltering dykes, the house was built in the
lee of the dyke and the fishermen look for salmon in the quiet water on the western
edges of the dykes.
Who is stronger – man or rocks?
These dykes are in the south end of the Island of Arran in Scotland.
29
All these rocks have the same chemical composition. They are rich in magnesium and
iron.
Basalt is extruded from a volcano as lava. It may take tens of years to cool. The rapid
cooling results in small crystals which can be seen by using a hand lens.
Dolerite has the same composition as basalt but the crystal size will depend on how
deeply the intruded rock is buried. It may cool over thousands of years into a rock where
crystals can just be seen by eye. Dolerite either forms vertical dykes or horizontal sills as
the molten rock intrudes between sedimentary beds.
The same magma can take millions of years to cool at depth. This gabbro has large well
formed crystals.
Every rock tells a story
30
Metamorphic rocks have been partially re-crystallised. Re-crystallisation results from
increased temperature and/or pressure due to burial. The melted portions take on a
more stable crystalline form.
Metamorphic rocks retain traces of their original deposition despite their subsequent
change.
31
The marble on the right still retains traces of its fossils.
Marble is used in kitchens and sculpture because the metamorphism re-crystalised it
and made it stronger and more resistant.
The fossilised limestone on the left is from near Geraldton whilst the source of the
marble (meta-limestone) on the right is unknown.
32
Sandstones when buried in the Earth experience a 25OC rise in temperature for every 1
km depth of burial. The more silty layers of this example have changed into green micas
which are more stable at the temperatures and pressures during their burial. A
sandstone has metamorphosed into a quartzite.
The sandstone on the left is from Rajasthan in India whilst the quartzite (metasandstone) comes from Toodyay
33
Progressive regional metamorphism changes the mudstone on the left into the slate in
the center and the schist on the right. They all have the same chemical composition but
have metamorphosed to suit the heat and pressure forces they have experienced.
The mudstone is from the Irwin valley, the slate (low grade met-mudstone) from the
Pilbara and the schist (high grade met-mudstone) from Bindoon.
34
Even igneous rocks are changed by metamorphism. The discrete dolerite dyke on the
left hand side has been re-melted into mafic blebs by intense regional metamorphism.
The dolerite dyke is from Lesmurdie and the gneiss from Northam.
35
When materials become hot they expand and become less dense. They will rise. When
they cool they will sink. Convection currents below the base of the crust cause rock to
heat and rise or cool and sink.
36
When materials get hot they expand and become less dense. Mountains rise up, there is
folding and faulting ......and the cycle begins again.
The image is of the Hamersley Gorge.
37
38