Lecture 12 Metamorphic Rocks
Goals
• Metamorphic Processes
• Types of Metamorphism
• Plate Tectonics and Metamorphism
• Continental Crust and Mountain Building
When we introduced the rock cycle, three rock types were introduced including igneous, sedimentary and metamorphic. We now turn to the third of these rock types, metamorphic. These rocks represent transitions from the other two types of rocks resulting from pressure and temperature increases. As we shall see these pressure and temperature changes are not enough to melt the rocks for then we would form new igneous materials. Instead, the application of increased pressure and temperature changes the sedimentary and igneous rocks just as baking a cake changes the initial flour, eggs and water into a new form. Our interest in metamorphic rocks is related to the story they tell about the pressures and temperatures that they have been subjected to. The source of these increased pressures will come from burial of rocks or from collisions of plates during plate tectonics. Thus the information on temperature and pressure that might be recovered from metamorphic rocks can be used to understand past tectonic activity. An understanding mountain building and continental structures which can result from plate collisions is also an important outcome of these investigations.
A. Metamorphic Rocks and Processes
Pressure and Temperature are the key contributors to metamorphism or the changes imposed on igneous and metamorphic rocks. These contributors result in changes in the mineral assemblages and texture of the rocks. Depending on the temperatures and pressures that the rocks are exposed to they are characterized as low-, intermediate and high grade metamorphism.
Important contributing factors to metamorphism are the following:
Rocks, both igneous and sedimentary, have many open spaces within them which we call pores or fractures. These pores or fractures are filled with fluids that include gases (carbon dioxide) and salts (sodium chloride or calcium chloride). These fluids serve as a transporting medium or catalysts for some chemical reactions during metamorphism.
Stress is a force applied over a surface area such as a pressure. Stress in a solid can have different magnitudes in different directions. The texture of silicate minerals can be affected.
B. Types of Metamorphism
Remembering that the key to metamorphism is increases of pressure and temperature we can identify two types of metamorphic processes. In each the pressure and temperature of the rocks are raised.
Burial Metamorphism - Sediments as they are deposited become buried. Often this occurs in a water environment and so the rocks are rich in fluids. If the burial of sedimentary rocks are deep enough then they are accompanied by metamorphism. With so much water, the pressures applied to the sediments are most often uniform and the resulting rocks show little foliation.
Regional Metamorphism - The forces associated with this type of metamorphism find their roots in plate tectonics where large pieces of continental and oceanic crust are moving and hitting one another. The stresses involved are differential and involve the deformation of large areas (~100 km x100 km). Foliation is important. Collisions between plates often results in metamorphism of sedimentary rocks at the plate boundaries. Slate, phyolite, and schist are low, intermediate and high grade metamorphic rocks resulting from such collisions. Greenchists result when basalt undergoes low grade metamorphism.
C. Plate Tectonics and Metamorphism
We have already seen that plate interactions can supply the pressures and temperatures responsible for metamorphism. When continents collide the sediments at their boundaries undergo either low or intermediate grade metamorphism forming greenschists or amphibolites. When crustal rocks are subducted at a convergent plate boundary, the rocks undergo very rapid changes in pressure with only moderate temperature changes. In this case, blueschists are formed. Finally, magma rising through the crust can cause metamorphism as a result of rapid increases in temperature but only moderate pressure increases, known as contact metamorphism. Thus one can look at exposed metamorphic rocks and determine the types of tectonic processes that were operating in the past. For instance blueschists are found in the Coast Range of California as a result of past subduction. Today such rocks are forming in areas such as the Aleutians. Similarly, rocks indicative of continental collisions are found in the Appalachians. Today such rocks are forming in the Himalayas.
D. Continental Crust and Mountain Building
Continents can be divided into two primary components which are related to their formation and the types of rocks. The two parts are cratons and orogens.
Cratons - Core of ancient rock several billion years old.
Orogens - Elongated regions surrounding cratons that have been intensely deformed by continental collisions.
Youngest orogens are mountains still today. Continental shield is a collection of cratons and orogens.
North America is made up of a number of cratons and orogens which reflect the tectonic history of this continent over the past 1.8 billion years.
We see from this discussion that continental margins are where all the action is in terms of deformation and metamorphism. We generally describe five types of continental margins in order to better understanding these processes.