Andhra Pradesh PSC: Introduction to Metamorphic Petrology
Metamorphic petrology is the study of rocks which have been changed (metamorphosed) by heat and pressure. They are broadly categorized into regional and contact. Metamorphism is an extension of the process which forms sedimentary rocks from sediment: Lithification. However, all types of rocks; igneous, sedimentary and metamorphic, can all be metamorphosed. During metamorphism no melting takes place. All the chemical reactions which take place occur in the solid-state.
Factors Controlling Characteristics
The characteristics of a metamorphic rock depend on the following factors:
- Composition of parent rock
- Temperature and Pressure of metamorphism
The composition of the parent rock does not usually change during metamorphism (if it does it is then called metasomatism). The changes are the due to the minerals changing. A basalt which has around 50% of silica will produce a metabasalt with 50% silica.
Temperature and pressure affect the rock in terms of the mineral assemblage which is stable at the pressure and temperature obtained. The minerals stable at the pressure and temperatures that metamorphic rocks reach are simulated in a lab. This allows geologists to look at a mineral assemblage and give a (good) estimate of the pressure and temperature that the sample was exposed to. This gives tectonic information which is useful in other branches of geology.
Fluid changes the chemical composition of the rock being metamorphosed and hence is called metasomatism. The addition of fluid can radically change the rock.
Time has an important role as a rock which is heated to an extreme temperature for a short (years) period of time will not be altered too much. A rock heated for a longer period of time (millions of years) will show changes.
The classification of metamorphic rocks is split into contact and regionally metamorphosed rocks. After this it is divided according to the “amount” of metamorphism that has taken place and/or on the mineral content.
- Contact Metamorphism (based on mineral content)
- Parent Rock-Metamorphic rock-Dominant Minerals-Characteristics
- Limestone-Marble-Calcite-Interlocking grains. Fizzes in weak acid
- Quartz-Sandstone-Quartzite-Quartz Sugary texture
- Shale-Hornfels (Spotted Rock) -Micas-Dark colour
- Regional Metamorphism (name based on degree of metamorphism)
- Texture-Rock Name-Characteristics
- Slatey-Slate-Splits easily into sheets
- Between slate and schistose-Phylitte-Silky lustre, splits into wavy sheets
- Schistose-Schist-Pearly looking. Silky to touch
- Gneissic-Gneiss-Wavy, white and dark layers
Causes of Metamorphism
Caused by heating from an external source. Contact metamorphism occurs next to an igneous body. The degree of metamorphism decreases away from the body. This occurs at fairly shallow depths, as temperature not pressure is the dominating factor.
Regional metamorphism is caused by high pressure and temperatures usually during mountain building (oregenesis). The extremes of regionally metamorphic rocks are a high pressure, low temperature rock (called a blueschist) and a high pressure and very high temperature rock (called a granulite). If the rock is heated to the point of melting, but doesn't actually melt, it is called a migmatite.
Introduction to Igneous Petrology
Igneous rocks are formed form the cooling of molten rock, magma. They are crystalline, which means they are made up of crystals joined together. There are many different types of igneous rocks but they fall into two (very) broad categories; intrusive and extrusive. Intrusive rocks are igneous rocks which form at depth. They cool slowly, taking tens of thousand of years to cool. They have large crystals, tens of millimetres in size. Extrusive rocks are those which have erupted from volcanoes. They have very small crystals, not visible to the naked eye, as they cooled quickly. Of course there is every grain size possible in between these two extremes.
The chemistry of igneous rocks is quite complicated. It depends on two things; evolution and silica saturation. In this tutorial we will concern ourselves with the effect of evolution only, the silica saturation will be assumed to be constant. Igneous rocks evolve as they cool. This process is called differentiation. The mechanism for this process is as follows:
Liquid rich in minerals A, B and C.
Remove mineral A as it crystallises at a higher temperature than B and C. Liquid is relatively enriched in minerals B and C.
Remove mineral B as it crystallises at a higher temperature than C. Liquid is now completely mineral C. The minerals are removed in order of Bowen's Reaction Series.
As you can see, if you remove olivine, the magma will become more enriched in pyroxenes etc. This process continues until only quartz is left. This leads us to the following, simple, identification.
Textures & Names
Igneous rocks have many textures which tell us about their cooling histories and/or chemistry. In general rocks which have cooled rapidly are fine grained, that is with grains which are not visible to the naked eye. Rocks which have cooled slowly have large grains, sometimes as large as several centimetres across:
Textures & Names
This size variation arises as grains grow around a nucleus of some sort, i.e.a minute grain. The slower the cooling the more time grains have to grow and amalgamate. Grains which show their true shape are said to be euhedral. Grains which show no shape are anhedral. Using this information, the order of grain growth can be worked out. For example, a rock may have large euhedral quartz grains, which are surrounded by anhedral feldspar. The quartz grew first as it had space, the feldspar then grew around the quartz.
Other features seen are:
- Porphyritic texture-large grains (phenocrysts) surrounded by much finer grains (groundamss). This implies that the large grains grew slowly at depth, the magma with the grains in it, then rose up in the crust, cooling much more quickly forming the fine grains (the matrix).
- Exsolution-occurs within grains on certain minerals (pyroxenes and feldspars). This can give an indication of pressure and hence depth.
- Xenoliths-bits of the rock into which the magma intruded
- Cumulate layer-when a mineral grows which is denser than the magma, it will sink to the base of the chamber causing a cumulate layer. Minerals may form from liquid trapped between the grains-interstitial minerals.
Igneous Rock Formations
Igneous rock bodies are either intrusive or extrusive. Extrusive bodies are lava flows. If these occur under water they form pillow lavas. On land they can form lava tubes, aa (pronounced ah-ah and looks blocky) or pahoehoe (which looks ropey).
Intrusive bodies form when magma is injected into existing rock layers. A dyke is a body which cuts across the country (host) rock. A sill is parallel to the bedding layers. The baked margin is an area in the country rock, in contact with the igneous body, which has been thermally metamorphosed. The chilled margin is the area in an igneous body, in contact with the country rock, which cooled quicker than the rest of the rock due to the temperature difference between the magma and the country rock. These features are not always visible. The scale of these bodies is from millimetres to tens or even hundreds of metres.
The largest of igneous bodies is a pluton or batholith. These are massive, hundreds of kilometres in size. The moors of Cornwall and Devon are outcrops of a massive batholith.