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The Rock Cycle
Yr 12 Geology
Yr 13 Geology
When you have read this, try using the focus questions
. You can fill in your answers online; SHC students will receive an email of their answers with comments from me.
Conglomerate (breccia) containing volcanic rocks, Taupo Bay
Sedimentary rocks are formed from grains of minerals and rocks, dissolved minerals and other material, such as plant matter or animal remains, which have been transported and deposited as sediment. After deposition, as the sediments become buried deeper and deeper, they become transformed into rock by a process called
Origins of sedimentary materials
Most materials that become sedimentary rock are derived from the processes of
, during which solid rock is broken down into smaller fragments. Other material that can become sedimentary rock includes volcanic debris, and plant and animal remains. Volcanic debris gets into sedimentary rock either through the fall of ash over a wide area, or through transport of volcanic fragments (for instance, when a volcanic cone or island becomes unstable and collapses). The volcanic grits found interbedded with the sandstones and mudstones in the cliffs around Auckland are an example of this. The volcanic conglomerate in the photo above right is another example; as you can see from the angularity of the fragments and large variation in size, it was close to the volcano (part of the 'ring plain' around it, like the modern Taranaki area).
Coal seams in siltstone near Westport
Material of biological origin is also common in sedimentary rocks. Plant material can be found as fragments in sandstones or other sedimentary rocks, and if it makes up the bulk of the rock it is termed
. The shells of many marine organisms are made of
, and form
or calcite-rich sediments such as
). These shells can include
microscopic shelled plankton
. Other organisms make their shells from
), and their shells compact into a hard form of sedimentary quartz called
. The remains of the soft parts (living tissue) of various organisms is also sometimes incorporated into the sedimentary rock to form
. The oil and gas is often squeezed out of these rocks by heat and pressure to escape at thesurface as gas seeps such as the
in Northland (picture below), or be trapped underground in oil or gas fields.
Dissolved minerals can be important in some sedimentary rocks. Calcite is the most common, but dolomite and limonite (a form of hydrated iron oxide), siderite and barite are others that occur quite often.
Mineral grains often survive several cycles of erosion and deposition, so a sedimentary rock can be formed from material that was eroded from other
Runaruna mud volcano,
sedimentary rocks. These “second generation” sedimentary rocks are often very rich in clay minerals (because more and more of the feldspar minerals will break down into clay over time), and mineral grains are often stained and very worn down and rounded. By contrast, a sedimentary rock formed from fresh volcanic ash will contain large numbers of grains which are clean, shiny and angular.
There are four main transport mechanisms for sediment.
• By water (stream, river, underwater currents)
• By wind/air (wind blown sand and dust)
• By mass movement (landslide, rockfall, volcanic cone collapse etc.)
• By ice (glaciers acting as “bulldozers”, or “dropstones” – stones dropped out of icebergs as they melt)
The next section examines these one by one.
River water laden with glacial silt
This is by far the most important transport mechanism. Streams and rivers and underwater currents can carry sediment a long way.
The amount of sediment and size of particle carried depends on the water current speed. Sediment will flow until the water slows down enough to deposit it. Sediment down to silt size requires moving water, but very fine mud takes so long to settle it can go a long way out to sea even where the water is barely moving. See the animation linked from the rock cycle page to illustrate this.
A “muddy looking” river like the Wanganui will be carrying large amounts of sediment in suspension. More sediment moves along the river bed, particularly when it is in flood.
Another important and less well known mechanism in water is the
. This is an underwater “river” of sediment-bearing (or “turbid”) water flowing across the sea bed. The turbid water is denser than the normal water, and can flow tens of kilometres underwater at several kilometres per hour. Turbidity currents usually originate when the “pile” of sediment dumped near a river mouth by the slowing water becomes unstable and collapses. Because the sediment pile eventually builds up again, sedimentary strata formed by turbidity currents tend to consist of thick layers of sandstone or coarser sediment (formed by a single turbidity current event) alternated with thinner layers of very fine sediment (mudstone) that was carried out to sea in suspension. The cliffs around Auckland are a good example of these turbidites (also called “flysch” or “papa”), and the main greywacke ranges of both the North and South Island are composed of them.
Giant dunes, Te Paki (90 Mile Beach)
A characteristic of water-born sediments is that the particle size often decreases the further the sediment has been transported, because the water slows down as it spreads out and the gradient decreases. In many cases, fragments also become less angular and sharp the further they are transported, because sharp corners get knocked off as the particles tumble over each other.
The best known examples of these are the dunes of a beach or desert. The Awhitu Peninsula, on the western side of the Manukau Harbour, is formed from many layers of dunes emplaced during periods of different sea level. Another important wind formed deposit is called loess. This is wind-blown dust, often “rock-flour” formed when glaciers grind their way over hard rock. There are significant deposits of loess in the South Island.
Giant landslide, Arthur's Pass
This is a general term for landslides, rock avalanches and other related phenomena. It can be locally important; much of the “ring-plain” of Taranaki was formed by gigantic rock-avalanches that occurred when the volcanic cones got too big and became unstable. There are similar deposits mixed with the sandstones and mudstones around Auckland; either the avalanches entered the water or occurred underwater. West Tamaki Head, near St Heliers, is formed from such a deposit. Twenty million years ago, when these rocks were laid down, there were a number of volcanic islands to the west and north of Auckland (the Waitakeres and at Dargaville).
Franz Josef Glacier
The main mechanism for ice transport is when glaciers act as “bulldozers”, pushing and grinding rocks ahead and aside to form moraines. Quite a bit of rock gets incorporated into the glacier. If the glacier flows into the sea, the trapped rock can be carried a long way in the icebergs until they melt. They then get “dropped” onto the seabed and incorporated into whatever sediments are already on the seafloor. These rocks are usually conspicuously different from the surrounding sediment, and examples have turned up in sediments that have later been formed into rock and lifted up onto land. The term for these rocks is dropstones. Isolated large boulders pushed into place by glaciers and many kilometres from where they originated are called erratic boulders.
This is the term used to describe the conversion of sediment into rock.
The first stage in the process is simply the rock being squeezed, so that the spaces between the grains (pore space) is reduced and the sediment becomes harder and rock-like. If there is a good variation in the particle size, rocks formed by this step alone can be fairly solid because smaller grains fit “between” larger ones . Much of the soft sedimentary rock called “papa”, found from Taihape to Wanganui and north Taranaki, has only undergone this degree of lithification.
With more pressure, some mineral grains start to dissolve at the grain boundaries and be recrystallized into the pore space. This “
” the grains together, producing a far harder rock. The hard sedimentary rocks that make up New Zealand’s main mountain ranges have undergone this degree of lithification, and in fact have begun the first step towards metamorphism because some new minerals have formed. New minerals formed before metamorphism are termed
minerals; zeolite and prehnite are two examples of such minerals and geologists tend to disagree whether rocks containing these minerals should be termed metamorphic or sedimentary.
Calcite (shell) fragments become cemented together very readily even at quite shallow depths of burial, so can form hard limestones without getting very deep. However, for rock types other than limestones, the hardness gives a rough guide as to how deeply the sediments have been buried.
Uneven cementing can produce hard
in the sedimentary rock. The Moeraki Boulders are a good example (photo
). The 'meteorite' on display outside the Waiomio Caves north of Whangarei is actually a
concretion. Concretions get left behind when the softer surrounding rock weathers and erodes away.
Classification of sedimentary rocks
The diagram above shows a system for classifying the rocks mentioned in the Achievement Standard (breccia is not, but has come up in a Level 2 question). There are many other sedimentary rocks not shown here, for example, marl is a calcareous mudstone (one made out of at least 50% calcium carbonate from shells). Most sedimentary rocks made from fragments have a large range of grain sizes, so are more complex than this diagram implies.
Certain types of sediment (or sedimentary rock) indicate particular environments of deposition.
Kahikatea swamp, NZ
Coal is formed in swamps or peat bogs. For plant matter to make up the bulk of the rock it requires a shortage ('deficit') of other sediment input, which usually occurs a landscape of low relief (i.e. few hills or mountains, and mostly flat). This landscape has poor drainage and rivers fan out to form swamps.
Most NZ coal was formed from 35 - 50 million years ago. The swamps would have been very similar to the kahikatea swamps that once covered the Hauraki Plains and are still found in parts of Westland (pictured left). Coal is often interbedded with siltstone, because silt is common in rivers and rivers change their courses over time.
A cay (shell fragment island)
Limestone pinnacles, Punakaiki
Limestone also requires an environment with little other sediment input so that the main sediment forming material is the shells of marine organisms (shellfish, corals, bryazoa etc.), usually living in shallow and biologically rich seas. Coal and limestone are often found together when they were formed by an “old”, worn-down landscape that was gradually being submerged to form shall
Punakaiki limestone, close up
ow, sediment-poor seas or lagoons. The south-eastern USA is the sort of environment where coal and limestone form today: the swamps of the coastal states will eventually become coal and the coral of Florida limestone. Notice that this is well away from an area of volcanism or mountain building.
At the time of the formation of NZ limestones, most of NZ was underwater. It would likely have consisted of small islands of shell fragments in a shallow, warm and biologically rich sea. The picture above right is such an island in the Coral Sea, near Australia.
The 'pancakes' illustrated right are caused by chemical weathering at slightly different rates on layers in the limestone.
Sedimentary environments for rocks made from fragments
These sedimentary rocks are formed by transported material. Some generalisations about this are:
finer particles means slower water speed when deposited
more angular particles have not been transported as far as more rounded ones
However, this is very dependend on sediment source and other factors.
Coarse materials such as conglomerate or breccia indicate relatively short transport distances and a landscape of high relief (mountainous) for the sediment source. Very course conglomerates have generally been deposited by mass movement (landslide or underwater rockslide) or very high energy currents (white-water river). Other mechanisms include lahars and underwater lahars. They may vary over time and space, for example on the Canterbury Plains; when eventually the gravels form sedimentary rock they will vary from conglomerate size close to the Alps to sand sized on the quieter river meanders. Extremely poor sorting indicates a very high energy environment of deposition, such as a rock avalanche, a rugged mountain river or a rocky, exposed coast.
Other conglomerates will show sorting. For example, a formation called the
is found in the sedimentary rocks of Auckland (the Waitemata Group rocks). This shows 'graded bedding', with conglomerate at the base (up to fist-sized fragments) at the base to grit-sized above this and grading up to sandstone at the top, over a distance of 1-8 metres. It was deposted by an underwater 'lahar' , formed from underwater and island volcanoes to the north, west and some to the east of present-day Auckland.
is usually formed by underwater currents - moderately fast river currents or turbidity currents in the sea. A few sandstones are wind-deposited, for example those on the large barriers enclosing the Manakau, Kaipara and other west coast harbours of the northern North Island.
A very large proportion of sandstones are formed by underwater currents of muddy water called
and most of the rocks of NZ's main ranges were formed this way. When such sandstones are hard and grey they are called
.Read more about NZ greywacke
indicates a quieter water flow, such as a meandering river or an estuary. This is why coal seams are often interspersed with siltstone - the swampy river changed course, burying plant material in silt.
Extremely fine grained sedimentary rocks such as mudstone imply long distances of transport and/or a low energy environment (e.g. slow moving water like a lake or an estuary, deep ocean). Mud carried out to sea can stay in suspension for some distance, then slowly settles. This is why layers of mudstone are found between layers of sandstone very frequently. The sandstone is formed by periodic underwater currents (turbidity currents) and the mudstone has settled out on top of these in the intervening time (decades to centuries).
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