METAMORPHIC ROCKS

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6. METAMORPHIC ROCKS - Slate & Marble etc.

Metamorphic rocks are formed from pre-existing sedimentary rocks or igneous rocks. Under the effect of heat and pressure these pre-existing recrystallise without melting into another type of rock with different physical properties. The formation of the metamorphic rocks marble, slate, gneiss, quartzite and schist are described.

6. Metamorphic Rocks - formed through the action of heat and pressure!

Metamorphic rocks are formed from other pre-existing rocks by the action of heat and pressure. The original rock is usually igneous, but can be sedimentary, but the rock never melts, but is recrystallised into another form over long periods of time before the metamorphic rock is formed. The chemical composition is unchanged. if the rock melts and then solidifies, it becomes an igneous rock.

Large scale movements of the Earth's crust can cause mountain ranges to form very slowly over millions of years. These replace older mountain ranges worn down by weathering and erosion. Metamorphic rocks are associated with the Earth movements (tectonic activity) which created present-day and ancient mountain belts. They are evidence of the high temperatures and pressure created by these mountain-building processes.

6(a) A metamorphic rock is one that is formed directly from a pre-existing rock which is usually a sedimentary rock, but it can also be an igneous rock.  Heat and pressure are the 'driving forces' for metamorphic rock formation in which the grains of pre-existing rocks are re-crystallised without melting. The metamorphic rock usually retains the same chemical composition as the original rock, but the crystal structure will be different. The pre-existing rocks involved are usually deep in the crust where they are subjected to great pressure. The high temperatures often needed, are due to rocks being near the hot mantle, or when an igneous intrusion rises, or volcanic rock heats other surrounding rock and when continental plate meets oceanic plate (see (3) in Fig 2. below). However, if the temperature is high enough to melt the rock, which subsequently cools to solidify again, the new rock is an igneous rock and cannot be classified as a metamorphic rock.

Fig 2. Find the metamorphic zone!

6(b) The link between metamorphic rocks and igneous intrusions is shown in the two diagrams above. The rising magma heats up the surrounding sedimentary (or igneous) rocks producing metamorphic rocks such as marble, slate, gneiss or schist. Note: There are high-low grades of metamorphism depending on the high-low temperatures and pressures particular pre-existing rocks are subjected to. For example, the rocks become 'less metamorphic' the further you go from the igneous intrusion as you go to a lower temperature.

6(c) Slate is formed from sedimentary rocks such as shale, mudstone or clay deposits and the re-crystallised layers are easily split - hence its use in roofing. Sometimes, but rarely, fossil traces are preserved in the layers through the crystallisation process.

6(d) Marble is a hard rock formed from the action of heat (high temperature) and high pressure (deep underground) on the sedimentary rocks limestone or chalk which recrystallises into small dense crystals without melting into the harder crystalline form we call marble. It will still give carbon dioxide with acid but has a much more even texture, as well as being physically harder and stronger than limestone or chalk. Not surprisingly, marble has found favour with sculptors for thousands of years in many civilisations.

6(e) Gneiss, quartzite  and schist are metamorphic rocks formed by the action of heat and pressure on pre-existing igneous or sedimentary rocks. They can form from igneous rocks* like granite or basalt, from metamorphic rocks* like slate or from sedimentary rocks like shale, mudstone or sandstone, and chemically they are mainly 'silica' SiO₂. * Note, the original pre-existing rock does not have to be sedimentary!

• Gneiss is a metamorphic rock, with pronounced crystalline layering, formed by the action of heat and pressure on igneous rocks.
• Quartzite is formed by the action of heat and pressure on the sedimentary rock sandstone.
• Schists are formed in regional metamorphism, they tend to be course grained and easily split into layers.
• NOTE: The terms ....
• Regional Metamorphism refers to large scale metamorphic rock regions associated with mountain building from tectonic activity.
  • see for example (3) in Fig 2. below.

• Contact Metamorphism refers to localised metamorphic rock formation around an igneous intrusion. (see Figs 5, 6 and 8)

Fig 6. Igneous and intrusion and metamorphic rock formation

Fig 5. Another example of metamorphic rock formation

Fig 8. A dome formed by an igneous rock intrusion

6(f) Metamorphic rock has the same chemical composition as the original rock it was formed from (in terms of % elements). This is because no minerals are added or lost in the recrystallisation process. For example, the Ca:C:O ratio is the same in the sedimentary limestone rock as it is in the resulting metamorphic rock marble, because chemically they are both mainly calcium carbonate CaCO₃.

Note: Fossils are rare in metamorphic rock but their trace can sometimes be preserved in e.g. slate, despite the effects of heat and pressure involved in their formation.

It is not impossible for the 'traces' of fossils in pre-existing sedimentary rock to be preserved through the re-crystallisation process in metamorphic rock formation.

However the fossils are likely to be distorted or destroyed by the heat and pressure factors involved in metamorphic rock formation.

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