Metamorphic Rocks (& toast)


There are three major groups of rocks. Igneous rocks form when magma solidifies and sedimentary rocks form by weathering and erosion of rocks and minerals near Earth’s surface. Today we consider the third major group, the metamorphic rocks. Metamorphic rocks often host economic mineral deposits and form bedrock under much of North America. Metamorphic rocks form when one type of rock undergoes physical or chemical changes to form a different rock. For example, this limestone may undergo metamorphism to form marble that might look like this. Or the minerals in this sample of granite may be reorganized by metamorphism to line up parallel to one another like this. On a more basic level, when we make toast, we are subjecting bread to a form of metamorphism. Rocks undergo an analogous transformation when we place them adjacent to a heat source. We have 2 learning objectives for this lesson. First we will consider how metamorphism occurs and the second, we will identify some examples of typical metamorphic rocks. Metamorphic rocks form by the process of metamorphism. These changes occur in the solid state after a rock has formed. The changes occur as the rock is subjected to changing conditions, typically changing temperature and pressure, but the presence of hot fluids can also be important. Processes analogous to metamorphism are actually pretty common in our daily lives. When we cook food we subject it to higher temperatures that cause the composition and texture of the food to change. Think of baking a pizza or toasting a marshmallow or cooking a roast. Temperature is a key factor in determining the changes that occur in a rock. If temperatures are too high the minerals in the rock will melt to produce magma that will form an igneous rock. If temperatures are too low, the reactions necessary to change the rock won’t take place. So we can consider metamorphism to take place in a temperature “window” that falls between approximately 200 to 800 degrees Celsius. These conditions typically occur at depths of several kilometers below Earth’s surface, in close proximity to a heat source such as a magma chamber, or in association with plate boundaries. Temperature and pressure increase with depth in Earth’s crust. Assuming a standard geothermal gradient, the temperatures necessary for metamorphism would typically be found at depths of 7 kilometers or greater. Magma exists within Earth’s crust at temperatures over 1200 degrees Celsius. These very hot materials bake the surrounding rocks that are in contact with the magma body via a process known as contact metamorphism. Convergent and divergent plate boundaries represent environments with a range of temperature and pressure conditions suitable for different degrees of metamorphism. High temperature and high pressure metamorphism occurs in the over-riding plate at convergent boundaries. In contrast, the oceanic crust of the descending plate experiences relatively low temperature and high pressure conditions. Finally, the oceanic ridge is characterized by high temperatures and circulating hot fluids. Each environment has its own particular assemblage of metamorphic rocks that are formed over a large region of the tectonic plates. Hence this type of metamorphism associated with plate boundaries is known as regional metamorphism. In response to the conditions associated with burial, magma, and processes at plate boundaries, rocks may change shape, minerals may grow larger or rotate and realign, and new minerals may grow due to chemical reactions in the rock. So, how do we recognize metamorphic rocks? First, many metamorphic rocks contain tabular or sheet-like minerals that are either physically rotated or grow so that they are perpendicular to the direction of pressure or tectonic stress. For example, imagine squeezing a balloon or a marshmallow. Either one will expand in the flattened direction, perpendicular to stress. Minerals in some metamorphic rocks do essentially the same thing. This process creates an alignment of minerals known as a FOLIATION. In this example we see the original rock in the lower part of the sample, a granite. And in the upper part of the sample you can see that the minerals have been realigned to form a near-horizontal foliation. Here are two more examples of foliated metamorphic rocks. Can you identify the foliations in each example? Here you go. We generally interpret foliated rocks to have formed under regional metamorphism conditions. As the degree of metamorphism increases with rising temperature and pressure, a typical rock will progress through a series of stages from low grade to high grade metamorphism. For example, if we metamorphose the common sedimentary rock shale, it would first be transformed to slate, then to phyllite, to schist, and finally to gneiss. During this progression the size of grains in each rock would become larger and the rocks would exhibit a well-defined foliation as the clay minerals in the shale would be readily transformed into tabular silicate minerals. But what happens if the original rock does not contain clays or other tabular minerals. For example, relatively pure sandstones are composed almost exclusively of quartz grains. The metamorphism of sandstone just changes the rock texture, forming larger grains in a rock known as quartzite. A similar thing happens in limestone which is converted to marble. In each case, there may be significant pressures but the rocks will not develop a foliation. So, to summarize, metamorphism represents changes in composition and texture that occur in the solid state after a rock has formed. These changes typically occur due to increasing temperature and pressure. Metamorphic rocks form as a result of deep burial, plate tectonic processes or proximity to magma. We have foliated metamorphic rocks such as slate, phyllite, schist and gneiss and non-foliated rocks such as marble and quartzite. We had two learning objectives for today’s lesson. How confident are you that you can successfully complete each of these tasks?

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