Geology Coursework

Coursework

1. The premise of plate tectonic theory is that the outer part of the Earth is made up of individual slabs and plates that slide around on the mantle. Where these plates interact, results in a wide variety of geological phenomena. This is a widely embraced theory by scientists.

2. The evidence in the geologic record that Wegener used to support continental Drift is the match between Africa and South America looked conclusive. Patterns also matched up, particularly in the southern hemisphere such as plant fossils and climate zones. The rocks in South America as well as mountain ranges matched up with the respective geological formations in Africa. As expected, there were patterns between other continents as well to support the idea of continental drift.

3. The mechanism for continental drift proposed by Wegener was not accepted because the explanation for why continental drift occurred was not popular among geologists. Geologists could not understand how continents, as explained by Wegener, could plow through the oceans. In fact, the reasons for continental drift, proposed by Wegener, were flawed. However, continental drift is the explanation for the current position of the continents.

4. “Seafloor Spreading” contributes to the understanding of continental drift because it explains a different way for how the continents drifted. It was the ocean floor, not the continents, which allowed for drift. This was a tentative idea because like Wegener’s idea, there was little evidence behind it.

5. The orientation of the magnetic field on Earth was reversed to what it is currently. This is known to be true because there are rocks, aged at over 700,000 years old that are reversed magnetized compared to younger rocks which are magnetized in accord with the Earth’s current magnetic field.

6. The faults that cut across the oceanic ridges are a result of seafloor spreading. Transformed faults, as they were called, were faults that resulted from the new crust and shifting of the seafloor in the opposite direction. This was further evidence of the idea of seafloor spreading. Faults contribute to disasters such as the Indian Ocean Tsunami.

7. The age of the sea-floor relates to the distance from the oceanic ridges as a positive relationship. The further the amount of distance, the older the sea-floor is.

8. The questions still unanswered about plate tectonics are mostly centered around how plate tectonics works. While it is known that the Earth is confecting, it is not known how plate tectonics relates to this natural state of the Earth. On the surface level, the main questions deal with how quickly plate tectonics occurs as a process.

Video 13 Volcanism Summary

Volcanoes were once thought to be representative of hell on Earth from varying mythological tales within different cultures. Scientists have since discovered that volcanoes exist not only on Earth but also in space on moons and other planets in the solar system like Venus. Volcanoes also help provide oxygen and help scientists understand what is taking place within the inner layers of the Earth.

There are different types of volcanoes and thus different eruptions follow. Further, the most volcanic activity takes place underwater. In Hawaii, the line of volcanoes does not lie on divergent plates. Because of their unique formation, it is currently believed that there is a “hot spot” within the mantel of the Earth underneath these volcanoes. In fact, the volcanoes are aged in order from the Southeast to the Northeast.

Most volcanoes both on land and under the sea lie on convergent plate boundaries. These volcanoes also erupt more intensively than the ones on divergent plate boundaries. During the eruption, the most common substances are water vapor, carbon dioxide, sulfur and nitrogen. These elements help make the eruption explosive. The eruption of a volcano is similar to the opening of a soda bottle with the pressure. Composite volcanoes, usually lying on convergent plate boundaries, erupt most intensely because of the acidic content of where these volcanoes lie.

Further, the more explosive a volcano, the more cone-shaped the dome will be. Therefore, shield volcanoes, which are very broad and gently sloping, have more room for an eruption. This increase in diameter makes for a less intense eruption. It is also possible, though not common, for volcanoes to develop through the shifting of the Earth’s crust and doesn’t necessarily need to rest over a convergent or divergent plate boundary. The reason behind this is thought to be due to plate tectonics.

Damage from volcanoes can be devastating (see Yellowstone Super Volcano). In fact, it is estimated that currently, up to half a billion people are at risk should a volcano erupt. As a result, scientists have dedicated time and research in order to understand what precedes an eruption. Forecasting eruptions have been made more accurate through monitoring seismic activity below the surface of the Earth because this change has been shown to increase the chances of the eruption. However, because volcanoes are very tumultuous it is hard to monitor them up close. Therefore, most observation is done from a distance.

Geophysicists look for electrical conductivity (an increase in the water below the surface) to measure the increased chance for an eruption. There are also measures of magnetism for rocks below the surface. These readings help scientists chart and map what is occurring beneath the surface of the Earth. This data helps predict if an eruption is imminent and how intense that eruption may be. The Katmai project, which is currently being worked on, is one of the leading research ventures in the science of volcanism to attempt to understand the phenomena of volcanic explosiveness. Although dangerous, volcanic eruptions have numerous agricultural benefits, the most important being an increase in soil fertility. It is because they are dangerous but also beneficial to humanity that scientists have devoted time and research into studying and explaining the mystery of volcanoes.

Video 14 Intrusive Igneous Rocks Summary

Most rocks that are found on Earth are the result of the cooling of magma. This is known as intrusive igneous rocks. The study of this kind of rock occurred in the 1780s. The two competing scientists on these rocks were James Hutton and Abraham Werner. Werner was convinced that the Earth was once covered completely by water and all rocks formed from there. Hutton believed that the cross strata of rocks explained that something injected these conflicting rocks.

Magma can produce one of two results. If cooled underground, the rocks become intrusive or plutonic and if magma erupts aboveground, the rocks become extrusive or volcanic. Many types of plutonic rocks exist. Iron and magnesium-rich rocks are called Mafic. Felsic rocks contain silica and alumina, such as granite. Most rocks are a combination of these, however. Interestingly enough, there are equivalent counterparts to every plutonic rock with volcanic rocks. The textures, however, are very different because of the rocks cool and harden under very diverse settings. The larger the crystals, resulting from the cooling magma, the slower the cooling occurred. This allows scientists to measure the rate of cooling in rocks as well as how the rock was formed.

Magma has been shown to crystalize one, maybe two mineral types at a time. Given the shapes of the rocks, it is possible for geologists to categorize and pinpoint when the magma crystallized. The minerals that crystallize first are ones that can withstand the intense heat, keeping in mind that those minerals that crystalize first are doing so in the liquid stage of cooling when magma is still very hot, upwards of 1100 degrees Celsius. Further, the greater the water content in magma, the lower the temperature of the magma. Therefore, the range of temperature and consequent cooling rate alters the formation of subsequent rocks.

Plate tectonics is believed to be directly linked to the igneous rock formation. Different types of magma are found within different plates. Rocks higher in silica are found in subduction zones whereas basaltic rocks are found in mid-oceanic ridges. At convergent plate boundaries, it is most common to find andesite and diorite rocks. It is believed that most of the magma responsible for these formations are from the melting of basalt. Water is also a key explanation because an increase in water is responsible for an increase in magma production.

Basaltic magma is thought to be produced through the process of subduction which occurs beneath the Earth’s surface. Depending on the composition of the magma, it can either rise to the surface through volcanic eruptions or remain enclosed in rock formations. Either way, magma will rise because it is less dense than surrounding elements. Intrusions come in many forms. Two of them are dikes and sills. Dikes cross strata whereas sills do not. The larger intrusions are usually plutons and can vary in size from a couple of kilometers to thousands of kilometers.

Batholiths are used by scientists to attempt to understand the tectonics of the Earth’s crust. In studying batholiths, scientists are able to chart the history of magma and its activity as well as the intrusions that follow. Thus, batholiths have become a very useful tool for understanding the formation of igneous rocks and tectonic processes. One such rock is Komatites, which are rocks that suggest temperatures of the Earth’s interior used to be higher than today. Another, more rare rock on Earth, Anorthosite, was found to be common on the moon. This rock is found embedded in different plutonic formations often with large crystals and suggests that the moon and the Earth may have been very similar at one time.

The Earth is continuously evolving whereas the moon is “dead” and relatively unchanging. As shown, not only can the formation of rocks show the process of the cooling of magma and provide clues to how plate tectonics works but charts a history of how the Earth has changed in its contribution to rock formation.

Video 17 Sedimentary Rocks: The Key to Past Environments Summary

The Grand Canyon is a great wealth of knowledge in the study of sedimentary rocks because of the canyon’s layers upon layers of rock. It is like a gigantic timeline that explains climate as well as the topography of prior times.

Sediment is what occurs after weathering and erosion by wind, water, and ice. Living things also can affect how sediment occurs. Over time, when new sediment is added to existing sediment, there is a fusion and sedimentary rocks form. One of the most common is “clastic” and are commonly categorized by size ranging from boulders to pebbles to grains of sand. Clay would be the smallest, containing the same consistency as flour. Further, sediment is moved in several ways such as through water or landslides. Therefore, the further away sediment gets from its original home (a mountain range for example), the smaller and finer the sediments will be.

Sedimentary rocks can also form from chemical precipitation out of water. Desert lakes and lagoons are great examples of this occurrence. Minerals are often found here, resulted from a combination of varying chemicals, such as calcite, gypsum, and salt. This also happens in the ocean but with more contribution from living things. Limestone is formed often from deceased organisms and is one of the most common rocks found on Earth. Near continental shelves, there are microscopic organisms that use the silica to make shells. Upon death, the silica transforms into chert and diatomite.

Swamps are also a great place for the formation of sedimentary rock. Much of this is formed through vegetation and coal is ultimately the result. Peat is the first stage but with enough layers, coal emerges. Ultimately, all environments of deposition produce sedimentary rocks. Water, however, is a crucial element in this formation. This could be why the Grand Canyon, with the Colorado River, is such a great area for rock study.

Because sedimentary rocks build upon each other it is possible for geologists to examine these rocks and look for differences or similarities. Using the principle of uniformity allows scientists to see how the Earth has changed overtime. If there is a significant streak of similar rocks it could be concluded that the particular region remained largely unchanged. If there are noticeable differences, it is likely that there were significant changes, such as a loss or addition of water.

The sedimentary column that exists today illustrates much more time, within the bedding itself than the stack of sediments that have been preserved. Wind, in particular, has a way of angling and contributes to cross-bedding, where one deposition crosses over into another. This helps scientists see whether or not there was an interruption during the deposition process. The formation is perpendicular to the water current and also tends to angle downstream. This allows scientists to figure out what the direction of the water was. This information allows for quicker sorting and pinpointing of the source as well as the rocks themselves.

Through bedding, cross-bedding, and the study of erosion scientists are able to create a picture of the past landscape of a particular region. For example, the Ridge Basin contains fine-grain deposits which indicate that these deposits were deposited in a lake. Due to the lateral and thinness of the beds, it is suggested that water was a large part of the sedimentary process. There are also many limestones and mud cracks, indicating that freshwater was the source of the process. There are also significant variations within the layers of sediment. This variation is referred to as scour-and-fill which shows that the velocity of the river varied at different times. Finally, because the Ridge Basin rests on a fault, it is thought that the creation of the basin could have occurred from a stretch of the Earth’s crust.

Sedimentary rocks are very important because they are used everywhere in modern society. They are the primary use of fossil fuels but are also used in public buildings, concrete, roads, tile, brick, and steel. Sedimentary rocks also contain the most complete record of the history of the Earth. The Grand Canyon, as mentioned above, is one of the countless areas where this study can be conducted. At the bottom of the Canyon, the rocks allow for investigation of the past of up to 2 billion years ago. Throughout this time, there were many variations and allows scientists to shape the past landscape of the canyon. Even as the canyon continues to change, the study of the new sedimentary rocks will illustrate the canyon’s current state.

Video 18 Metamorphic Rocks Summary

Mountains, an important structure in human history, are often formed where there is a significant amount of tectonic plate activity. Much of the makeup of mountains, therefore, is due to metamorphic rocks.

There are a wide variety of metamorphic rocks. It is believed there are so many kinds because the composition depends on sedimentary and igneous rocks and how they form and respond to the metamorphic process in different areas. In order to trace this process, geologists use the term “protolith” which refers to the rock prior to metamorphism. An example is limestone is the rock prior to marble. However, more metamorphic rocks than protoliths exist, suggesting that there is more to the explanation than just what precedes metamorphism.

The pressure is not equally applied underground as it is above ground. This unequal distribution of pressure greatly affects the ultimate shape of a rock. Many metamorphic rocks are shaped by this pressure. For example, swirling crystal makeup within the rocks freezes the metamorphic process in time. The stress of the pressure distorts the planes and what results is a swirl and several rings, each illustrating the pressure. When temperature and pressure increases, minerals combine and grow opposite of the areas with the most intense pressure. The resulting pattern helps to show where the stress was the greatest.

Recrystallization, occurring due to an increase in temperature, produces new mineral types. This usually causes larger growth and actually looks like igneous rocks. For example, limestone can be metamorphosed into marble, usually due to an increase in temperature. If the temperature increases enough and the rock is situated deep under the Earth’s surface, the rock, and melt. Intermediate rocks, rocks made of igneous and metamorphic features, often result. Scientists have named these mixed rocks Migmatites. The most common of which is granite.

Another factor in the creation of metamorphic rocks is there are changes in composition. Much of this is due to the departure of water and carbon dioxide. Because water and carbon dioxide contributes heavily to the composition, it makes sense that this loss would change the shape of the rocks. In addition, there is often an intrusion of magma. Ultimately, the factors of formation are due to protoliths, fluid presence, as well as temperature and pressure changes.

Most metamorphic rocks form in one of two settings. The first is when cold rock connects with magma, called contact metamorphism, often restricted to a smaller region. The second is on convergent plates where motions of plates generate metamorphic conditions spanning thousands of kilometers. One of the most noticeable differences is temperature is often all that is required for convergent plate metamorphism but both temperature and pressure are needed for the regional or contact metamorphism.

A good analogy to metamorphism is cooking. Metamorphism works based on what is used at the start (ingredients) and how those “ingredients” are used, the actual cooking. Therefore the protoliths as well as factors such as temperature and pressure makeup what ultimately become metamorphic rocks. However, because metamorphic rocks are constantly changing, there is an intensity range of metamorphism. Slate, for example, transforms through increased pressure and temperature to become Phyllite which is a higher grade metamorphic rock. Further increase in pressure and temperature causes a higher grade rock like Phyllite to morph again. The makeup of different metamorphic rocks gives clues as to how intense temperature and/or pressure was.

The theory of plate tectonics helped immensely in the understanding of how metamorphic rocks are formed. When two tectonic plates collide, the consequent metamorphic rocks leave clues as to what degree the pressure and temperature occurred. However, the process of metamorphism in rocks is very slow. For example, the convergence of plates created an upheaval of rocks in Northeastern North America and created what is now the Appalachian Mountain Range. The consequent rocks that were formed show the pressure in particular of the plate activity that was occurring around them. Often times, these spiral patterns of pressure are able to show millions of years of activity and the process of metamorphism. Ultimately, metamorphism is a consequence of plate tectonics because so much of metamorphism occurs around or on plate margins.