How Does Sedimentary Rock Become Metamorphic Rock?
There are two main types of rock: sedimentary and igneous. Sedimentary rocks are formed from the accumulation of sediment, which is basically pieces of eroded rocks or minerals. If you’ve ever been to a beach, you probably know that sediment can be found in lakes, streams, rivers, and other bodies of water as well. When these sediments are buried under layers of other materials such as soil or sand, they undergo chemical changes that turn them into metamorphic rocks. Metamorphic rocks have special properties that distinguish them from their non-metamorphic counterparts.
How Does Sedimentary Rock Become Metamorphic Rock?
- Metamorphism is the name given to the change that occurs when a sedimentary rock transforms into metamorphic rock. The word metamorphism comes from the Greek words that mean alterations in a land mass.
- Conditions must be right for metamorphism to occur: heat, pressure, and time are all needed for metamorphism to take place. These conditions are found throughout geologic history and can be brought on by many different events such as tectonic plates colliding, tectonic plates sliding past each other, or pressure changes brought on by volcanic activity or earthquakes.
- The rocks that form rocks within sedimentary rock units experience two main types of changes during their formation: physical and chemical. Physical changes include mineral alignment, cementation, and recrystallization (a process where a crystal dislocation moves between two parallel planes of other crystals). Chemical changes include an increase in silica content (layers build up of quartz) and a decrease in carbonate content (layers build-up of calcite or aragonite) as well as the precipitation of new minerals (the addition of new minerals such as hornblende or biotite). These processes form parts of the mineral list we will discuss later on in more detail.
- If a metamorphic rock is exposed on the surface of the Earth, it may be changed by weathering or erosion. Weathering occurs when rain and other precipitation cause chemical changes to the rock. Erosion occurs when the rock breaks down into finer particles and is carried away by water or wind.
- Metamorphic rocks occur throughout the Earth’s crust, but most of them are found at higher elevations where they have been thrust up and over sedimentary layers. When metamorphic rock is thrust up, it begins to cool rapidly because it is now exposed to air currents as well as sunlight. This rapid cooling causes crystals in these rocks to recrystallize quickly – this causes existing crystals to crack and break apart. In addition, new minerals (such as hornblende or biotite) can form during this process when they replace existing minerals that have broken down while still in contact with other crystals nearby in a mineral pair (these two minerals have similar chemical compositions but are arranged in two separate locations).
- Metamorphic rocks that are mostly found at high elevations also tend to be a lot harder than those found at lower elevations since they contain more minerals that result from recrystallization and new mineral formation such as hornblende or biotite (at lower altitudes rocks that aren’t exposed often contain fewer minerals formed through recrystallization). Some of these higher-level metamorphics we will discuss later on in more detail.
What Are The Conditions for Metamorphosis?
- The most important condition is elevated temperatures. To allow for this rapid cooling, the rock must be exposed to air currents which create thermal gradients (temperatures change as you move from warm to cold) which then cause crystal recrystallization.
- Another condition of metamorphism is an abundant supply of water and a source of heat. Water acts as a heat source, causing warm rocks to slowly cool and cool rocks to quickly cool over time as it evaporates into vapor or water turns into steam, both processes acting on rocks at a quick rate. If a rock contains special mineral combinations that are stable at high temperatures, high-temperature metamorphism can occur under dry conditions; however, water is still required for metamorphism without the aid of minerals that require high temperatures and can form new minerals such as hornblende or biotite.
- A third condition that has a considerable effect on the intensity of metamorphism levels is the quantity and type of sedimentary layers present in the area with which the rock must come in contact when it is pushed up through them (placement is part of thrusting). An abundant supply of sedimentary layers could potentially slow down the rate at which rocks are thrust up into mountains and stop them from cooling completely while they are not exposed to air currents; however, this would mean that hot magma would not flow out through cracks in these layers and hot rock would accumulate at deeper locations where it could solidify and be thrust up again.
- A fourth condition that has a significant effect on metamorphism levels is the presence of granite beneath the sedimentary layers. Granite is quite resistant to physical deformation and almost completely resistant to chemical deformation. As the rock gradually cools and recrystallizes, it can be the last to solidify and thus has a higher chance of transmitting heat from around it (such as through contact with sedimentary layers) to the rock above it. Because it does not take much heat for granitic rocks to melt, less heat must be supplied during metamorphism than would otherwise be required if there were no granite present. Although granitic rocks tend to have very low permeability so that they cannot transmit much heat through their structure, they can become extremely slippery when they are hot enough (and therefore act as a lubricant). The ability to cool slowly also means that granites will have time to harden before being thrust up into mountains or splinter into pieces that get pushed up by other lithologies above them.
- The fifth condition for metamorphism is a lack of strain on magma when it reaches the upper crustal level, preventing intrusions from entering deep below the surface (usually in areas lacking lithologies that are resistant to high temperatures). This is because a high temperature of magma does not necessarily increase its mobility. Figure 2 below shows how heat can be distributed through a crust by the fission of neighboring rock and by convection currents due to differential melting.
- A sixth condition that affects metamorphism levels is the eutectic composition of the magma. Magmas can have many different compositions, depending on which chemical elements are involved and how easily or not they have crystallized. This means that some magmas will be more easily melted than others and may therefore form crystals rapidly at lower temperatures when they intrude into sedimentary layers, whereas other magmas will solidify slowly but form crystals slowly if at all at these temperatures more difficult to melt when encountered with less chemically resistant layers on top of them (such as granite).
Which Rocks Turn into Metamorphic Rocks?
- Metamorphic rocks are formed by events that increase the temperature of a rock at an interface between two lithologies because of heat loss to the surrounding rock. The most common way this happens is through contact with another rock or igneous intrusion, which can cause heat transfer within a lithological boundary.
- Metamorphic rocks can also be formed when a rock is heated by its surroundings at some depth below the surface, as in a geothermal gradient (such as occurs beneath volcanoes). This type of metamorphism can also occur during collisions between tectonic plates and becomes particularly active along oceanic ridges and mountain ranges where there is little sedimentation or erosion to prevent heat from escaping from the crust to the overlying mantle (such as in Tibet).
- A third way that metamorphism takes place is underwater, where it enables sediments to form limestones, anhydrite, and other sedimentary layers with high particle density. This results in metamorphic rocks that have a high pH, as does the water.
- The fourth kind of metamorphism happens when sedimentary layers are melted through hydrothermal reactions at water temperatures. These types of rocks are not usually abundant and occur only in areas where there is a hot spring or another geothermal gradient below them (such as Yellowstone). These rocks are composed of minerals characteristic of high-temperature environments, such as calcite and aragonite (weathering products), which give them their distinctive coloration.
- The final source of metamorphic rocks is when magma heats up as it enters and then cools within sedimentary or igneous layers or vice versa. In this case, magma may crystallize very quickly if it reaches a layer with less chemically resistant material above it, such as granite at depth; alternatively, magma may solidify slowly if it reaches a lithologically more stable rock layer above it, such as limestone from the seafloor.
How Can You Identify Sedimentary Rocks That Have Turned Into Metamorphic Rocks?
- Sedimentary rocks that have been metamorphically altered to a high degree can be identified by their chemical and physical characteristics. They are generally composed of pulverized rock fragments and may contain fragments of other rocks within them, such as fossils.
- High amounts of heat can also make minerals brittle, or even impossible to form, making them invisible to the naked eye as they shatter when subjected to pressure or stress (thus making them less “metamorphic”). Because the heat from deep in the Earth turns melts into rocks, any geological features with these types of minerals will not be obvious on the surface.
- Some sedimentary rocks that look metamorphic are actually composed of metamorphosed sediments (such as sandstones), which generally have low amounts of heat and pressure from deep within the Earth that did not affect them all that much when they formed. For example, if you were able to excavate a pocket in sandstone that had been heated only slightly, even without melting the sandstone, you could still be able to see that it originally looked like a sedimentary rock (albeit a metamorphosed one).
- Metamorphic rocks can also be identified by their appearance and how they were formed. Usually, they will be composed of small crystal-rich grains oriented in the same direction or in parallel layers, which are usually composed of minerals and appear to have been cemented together, as well as extremely thin layers (such as skin-thin scale) caused by pressure or strain on the rock’s interior. Metamorphic exposures are different from sedimentary exposures, as they will generally show evidence of having been changed by heat – such as minerals that have been melted into crystals or crushed into fragments of glassy material – whereas a metamorphic exposure will often look like a “layered” section of sedimentary rock; this is because sedimentary rocks are typically made up of lumps, whereas metamorphic rocks are usually made up of grains that are not aligned together, nor do they include such small crystal-rich grains.
- While some metamorphic rocks can be identified by proximity to the area, others cannot, as they can occur far away from where the metamorphism occurred (such as in a different rock). In these cases, it is best to identify metamorphic features using references from a rock formation near or at the same latitude as where you’re looking (such as an identification key for type or formation). You may also be able to find out about metamorphic features by asking a geologist if there has been anything similar in the area before.
Metamorphic rocks are formed when sedimentary rocks are buried and heated to extreme temperatures. They are created by the breakdown of minerals within the rocks, and are transformed into different types of minerals. There are two main types of rock: sedimentary and igneous. Sedimentary rocks are formed from the accumulation of sediment, which is basically pieces of eroded rocks or minerals. If you’ve ever been to a beach, you probably know that sediment can be found in lakes, streams, rivers, and other bodies of water as well. When these sediments are buried under layers of other materials such as soil or sand, they undergo chemical changes that turn them into metamorphic rocks. Metamorphic rocks have special properties that distinguish them from their non-metamorphic counterparts. There are three main conditions that need to be satisfied for sedimentary rocks to become metamorphic rocks. All sedimentary rocks have distinct properties that make them different from igneous rocks. However, once they become metamorphic rocks, they will begin to look like igneous rocks.
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