THE ROCK CYCLE
This tectonic rock cycle is a clickable map (19/07/95 -map still under construction) that offers more detailed descriptions of the rock types found within their tectonic settings, with links to more detailed information about each subject
If we examine the rock cycle in terms of plate tectonics, as depicted in the figure above, we see that mafic (tholeiitic) igneous rocks form at sea floor spreading ridges. Fluid intrusion of these rocks, both during and after formation, results in some low grade metamorphism. As the rocks cool, and more magma is introduced from below, the plate is forced away from the spreading ridge, and acquires a sediment cover. As shown in the figure, in this case, the plate is eventually subducted under a continental plate. In the trench of the subduction zone, at relatively shallow depths, high pressure – low-high temperature metamorphism of the plate and its sediment cover occur. As the plate travels deeper, high temperature conditions cause partial melting of the crustal slab. Fluid intrusion plays a key role in partial melting. As the partial melt rises, and intrudes into the continental plate, the surrounding country rock is contact metamorphosed at high temperature conditions. This melt is either driven to the surface as volcanic eruptions, or crystallizes at depth to form plutonic igneous rocks. Sedimentary rocks form from the weathering, erosion, transport and deposition of arc material onto the continental platform and shelf.
Igneous rocks form by direct crystallization of minerals from a magma melt; we see a surface expression of magmatic activity at sea-floor spreading ridges and other rift zones, volcanic arcs (subduction zones) and hot spots (intraplate volcanism).. Intrusive (plutonic) rocks crystallize at depth, whereas extrusive (volcanic and pyroclastic rocks) rocks crystallize after the magma reaches the earth's surface. In general, extrusive rocks have a finer grained texture than intrusive rocks.
Igneous rocks are often classified according to the percentage of SiO2. The figure below is a general guide to igneous rock classification, showing the rock names and the differences in mineralogy.
Sedimentary rocks form from pre-existing rock particles - igneous, metamorphic or sedimentary. The Parent rock undergoes WEATHERING by chemical and/or physical mechanisms into smaller particles. These particles are TRANSPORTED by ice, air or water to a region of lower energy called a sedimentary basin. DEPOSITION takes place as a result of a lowering of hydraulic energy, organic biochemical activity or chemical changes (e.g., solubility).
Once deposited, the sediments are LITHIFIED (turned into rock) through COMPACTION (decrease in rock volume due to weight of overlying sediment) and CEMENTATION (chemical precipitation in pore spaces between grains which "glues" the rock together. The primary mineralogical and textural characteristics of the rock can be modified as the sediments are buried deeper in the earth's crust and undergo an increase in both temperature and pressure. These low pressure, low temperature changes are termed DIAGENESIS.
Metamorphic rocks are formed where a parent rock, called the protolith, is subjected to changes in pressure, temperature or chemistry (such as addition of fluids).
The rock cycle picture shows several areas where metamorphism is common.
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In subduction zones: as the oceanic plate descends into the mantle, both the sediments and the basalt floor are subjected to high pressure and low to high temperature conditions. Fluids may play an important role chemically change the rocks' composition.
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Adjacent igneous intrusions - contact metamorphism, where cooler (country) rocks are altered by contact with a hot igneous intrusion, is another common type of metamophism. This type of metamorphism commonly produces a
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At spreading centers (mid-ocean ridge): Fluids play an important role in hydrothermal alteration associated with magma emplacement on the sea floor at mid-ocean ridges.
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Contact (thermal) metamorphism of country rocks during igneous intrusion or beneath thick flows.
Igneous Rocks
Sedimentary Rocks - Weathering and Erosion.
Sediments have come from the weathering and erosion
of surface rocks.
These surface rocks were originally igneous rocks,
formed from the cooling of .
Weathering breaks a rock down into small pieces.
The two main processes are exfoliation and freeze-thaw weathering.
Exfoliation occurs when rocks are heated and expand
in the heat of the day,
and then cool and contract in the cold of night.
Thin slithers of rock tend to flake off the surface,
and these can then be broken further into tiny pieces.
Think of exfoliation as flaking.
Freeze-thaw weathering occurs when water gets into cracks in rock
and then freezes (below 0 °C) in cold weather.
Water expands when it freezes and turns to ice,
and the expansion can cause the rock to split and fragment.
In warmer weather (above 0 °C) the ice melts (called thawing),
and new cracks are exposed allowing the process to repeat itself.
Erosion is a slow process of wearing away weathered rock
by action of the wind, rivers and waves.
Metamorphic rocks
Both slate and schist (pronounced "shist")
are formed from the sedimentary rocks and .
Mudstone and shale are formed from mud or clay.
They are made of very small flat particles like tiny plates,
which have often become during the sedimentary rock formation.
Metamorphic Rocks - Marble.
Marble is formed from limestone.
At high temperatures and pressures,
the which formed break down and recrystallize.
The resultant rock is marble,
which is much harder than limestone, with a smooth even texture.
Marble is a valuable rock for construction and decoration.
It can be polished to produce an attractive mottled surface.
Marble is still calcium carbonate,
and in the same way as limestone.
Granite pics -
Bsalt -
