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The Rock Cycle
Yr 12 Geology
Yr 13 Geology
These are rocks whose mineral composition and/or texture, have been changed as a result of heat, or a combination of heat and pressure
Heat and pressure in the crust
Most people know that the earth gets hotter the deeper down you go. The rise in temperature with depth is called the
. The average geothermal gradient in NZ is between 20°C and 30°C per kilometre of depth. The graph below shows this relationship. The pink area shows the range - it varies quite a bit. The line becomes steeper because as you get deeper the rock is a better heat conductor, so it doesn't get hotter as fast.
How do we know how hot it is deep down?
The deepest boreholes in NZ only go to a depth of about 5 km. However, geologists have calculated what temperatures should exist at various depths using measurements of the amount of heat energy per square metre given off at the earth's surface, and the heat conductivity of rock (measured in the lab).
The temperatures worked out in theory match borehole measurements up to the deepest holes. So, although we can’t directly measure the temperature at depths much more than 5 km, the fact that the temperature rise over the first 5 km agrees with the ones calculated suggests that the calculations for greater depth should be accurate. There are also some quite complex indirect ways of measuring the temperature at depth by doing experiments with minerals and physical chemistry. The detail of that is University level material.
Sylvia Flats hot springs, Lewis Pass
The non-volcanic hot springs, such as those on the East Coast north of Gisborne and all those in the South Island, are warm because water from great depths rises up, often on fault lines, too rapidly to cool all the way back to surface temperatures. The hot springs at Sylvia Flat in Lewis Pass (photo on the right) are on a branch of the Alpine Fault (the sandflies there are dreadful).
More on this...
Hot springs in Rotorua and at Ngawha in Northland get their heat from bodies of magma near the surface. The geothermal gradient there is very high, but the effect is very local. The magma will be causing
in the surrounding rocks (see below) as well as the geothermal features seen on the surface.
Laboratory experiments suggest that new minerals start to form at temperatures of around 200°C, which translates to a depth of around 7 km in a 'normal' situation.
Pressure is caused by the weight of the column of rock directly overhead, so you can work it out if you know the density of the rock. At a depth of 10 metres underground the pressure is about 3 times that of the air around you. One kilometre down it is about 300 times atmospheric pressure (0.3 kB or 30 MPa). So at the depth where rocks begin to metamorphose, the pressure is thousands of times atmospheric pressure.
An increase of 25°C and 300 bars (atmospheres) per km depth is called a
normal P-T gradient
. Rocks that undergo metamorphism due to this normal increase in pressure and temperature are said to have undergone
If there is an igneous intrusion nearby the geothermal gradient around it will be unusually high. The rocks metamorphosed by this undergo a type of metamorphism called
, with high temperature and low pressure. The intrusion at Paritu, on the end of the Coromandel Peninsula, has contact metamorphosed the sedimentary rocks around it. Contact metamorphism is fairly localized (tens or hundreds of metres around the edge of the intrusion).The rocks formed by this process are known as
, but this name is not required by the Achievement Standard.
High pressure metamorphism:
Sometimes rocks get dragged down in a subduction zone without strong heating. These are high pressure metamorphic rocks (dark blue region on the diagram below). The nearest place to NZ where they occur at the surface is in New Caledonia, and the rocks produced are known as
. Note on the graph below how they form to the left of the graph, at low temperature. There is some evidence that such rocks are forming right now under Fiordland, where the subduction zone is unusually steep and rocks are being plunged to great depths quite quickly. They are not mentioned in the Achievement Standard.
Metamorphic rock types:
Different sorts of metamorphic rocks are formed at different pressures and temperatures. The relationship of metamorphic rock type to temperature and depth is shown in the diagram below. Marble is not shown because it can form in a wide variety of circumstances. The large black names are the ones in the AS (descriptive terms), and in brackets are how geologists would tend to identify these rocks.
Slate, schist and gneiss are metamorphic rocks mentioned in the AS. These rocks are distinguished mostly by their texture. Geologists base their classification of metamorphic rocks on a more detailed description of the new minerals formed, and often use terms such as “greenschist” or “amphibolite” or even more specific terms. A few of these terms are given in brackets in the diagram. They are often used in geological literature because they tell geologists something about how the rocks were formed or what minerals were in it. A geologist would usually call the schist of Otago a greenschist, for example.
normally, rocks come too quickly to the surface to 'change back' i.e. a gneiss would not normally change back to a schist. However, it does occasionally happen, and is called
Geologists can see subtle signs of it when they look at the rock under a microscope - the 'ghosts' of past minerals can be seen.
Metamorphic rocks in NZ
Most of the main ranges of both the North and South Islands are made of rocks called greywacke and argillite. Greywacke is a sandstone, and argillite a mudstone, right on the border between being sedimentary and metamorphic. In most places, some of the features of the original sedimentary rock are still visible so many geologists do not consider them truly metamorphic. They do contain some new minerals. In places the argillite is slate-like in appearance. Near the Alpine fault, the sediments that formed these rocks have been uplifted much further in the past and we see rocks from deeper down. These form the schist of Otago and elsewhere in the South Island. In places, particularly around Fiordland, there is gneiss from deeper still.
Metamorphic rock types from the Achievement Standard
Slate (schistose argillite), Benmore hydro
is a fine-grained mud or siltstone that has just begun to be metamorphosed. It breaks into thin, flat sheets that were once used for writing on and making rooves. The term slate is generally applied to a very smooth-breaking and fine grained
rock, and this is relatively rare in NZ. The slate on the left, from Otago, is from where the greywacke begins to show some 'schistosity', and contains some metamorphic minerals. A geologist would probably call this a fine-grained schist, but as it has no visible mica crystals and forms the scatches shown, it is a slate in terms of the Year 11 course.
Slate-like rocks can also be found in the Marlborough.
Rocks that would often be called slate elsewhere tend to be described as either argillite or schist in NZ because geologists here usually base their name on the minerals present rather than the appearance, which is often more broken up or less smooth than European slate. This is why slate is in quotation marks in the diagram above - NZ geologists would probably not use the term.
A characteristic of slate is that when you scratch a piece of slate with another piece it leaves a white mark which is easily rubbed off when you wipe it with a damp thumb or cloth. Thin pieces of slate about the size of a paperback book were once used in schools to teach writing, because the marks could be wiped off and they could be used over and over, saving cost on paper. Elizabethan iPods!
Schist, Franz Josef
Schist is a layered metamorphic rock similar to slate, but with larger crystals. You can usually see the shiny crystals of the mineral
on the surface. For example, at Franz Josef glacier (picture on right) the mica in the schist makes it look quite golden when small bits of rock are lying in the streams. Quite a few visitors think they have found gold.
Schist has been buried deeper than slate, and subject to hotter temperatures and higher pressure (see diagram above). This frequently causes minerals to separate into layers.
For geologists, most NZ schist is termed “greenschist”, and is often (but not always) greenish in colour because of the presence of the minerals chlorite and epidote. A decorative purple schist sometimes used as a building stone is formed from metamorphosed manganese–bearing rocks that are common in the greywacke sedimentary rocks that the schist was formed from. Once these purple rocks were manganese nodules on the sea-floor. The gold of Otago is weathered from schist, and was formed from lava and related rocks from mid-ocean ridges that have been mixed up with sedimentary rocks and then metamorphosed by deep burial and later uplift.
New Zealand geological maps will label schists as "chlorite II" or "chlorite III' and other such labels (see Level 2 website for more details). These labels refer to particular assemblages of minerals in the rock. Those minerals can be used to plot the rock fairly exactly on a P-T graph such as the one further up the page.
Gneiss, Manapouri underground hydro
(left) tends to lack the layered structure of schist. Mineral grains can be up to several cm in size, with large mica crystals feat
uring prominently. You can find gneiss in Fiordland, often close to granite. The photo on the left was taken in the tunnel that leads to the Manapouri underground power station.
Most New Zealand gneiss is found in Fiordland and along the Alpine Fault, where tectonic activity has uplifted rocks from well over 20km deep to the surface. New Zealand greenstone (pounamu) has been formed by gneiss-grade metamorphism of special igneous rocks called
(see igneous section for more details on this).
Sacred Heart Church, Takaka - built of local marble
is metamorphosed limestone. Because limestone recrystallizes so easily, it can be a bit difficult to tell whether a very white limestone is truly a marble or just a crystalline limestone unless it has the characteristic “marbled” appearance, caused by metamorphic minerals. The only marble in NZ is the Takaka/Mt Arthur marble found in and around Abel Tasman National Park. It is a sturdy building stone (see picture abovet) and is often a grey colour because of graphite crystals mixed with the calcite. This graphite was once coal, deposited in swamps near the limestone from which the marble formed. The Mt Arthur marble is the thickest carbonate rock formation in the country and contains the largest and deepest cave systems.
Marble can be anywhere on the P-T diagram above from about where slate is through to the lower part of the gneiss area. Geologists can work out the metamorphic grade by looking for non-carbonate minerals under a
At very high temperatures and pressures calcium carbonate starts to chemically react with surrounding rocks to produce new and different minerals. Limestone also undergoes some unusual changes under contact metamorphism and some economically important ore deposits are formed this way. Chemical reactions under heat and pressure are responsible for forming pounamu near the Alpine Fault, from reactions between ultramafic igneous rocks and various other rocks, including carbonates, during gneiss-grade metamorphism.
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