Short Essays on Earth Science Chapter

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subduction zone is where two tectonic plates come together and one goes beneath the other. This is most common where an oceanic plate meets a continental plate, and the oceanic plate is pushed underneath the continental. As a result, subduction zones produce the biggest earthquakes in the world, and are also largely responsible for volcanic activity and tsunamis.

At the site of subduction, in which one tectonic plate (the oceanic) goes beneath another (the continental), an accretionary wedge can form. Essentially the upper tectonic plate scrapes off a wedge-shaped portion of sediment and other materials from the subducted plate.

A hot spot is a region below the earth's crust where the magma has an extremely high temperature. As a result the magma will melt the ocean floor creating a volcano, and ultimately a volcanic island. Because of the movement of the tectonic plates, the hot spot will create an arc of islands created from this volcanic activity, where the chain indicates the movement of the plates. The Hawaiian islands are an example of a volcanic island arc created by a stationary hot spot onto moving tectonic plates.

4. The intensity of an earthquake is a measurement of the amount of shaking and movement that is measured at any given location. As a result, any measurement of an earthquake's intensity is going to vary based on the overall distance from the earthquake's epicenter. An extremely strong earthquake centered in Los Angeles may very well be felt in San Diego, but it will be felt at a much lesser intensity in San Diego than it is felt in Los Angeles, or at any spot between the two. Intensity is therefore measured using the Modified Mercalli Intensity Scale, which is largely a descriptive scale. It is based on subjective factors that ordinary people can use to evaluate the earthquake. For example, an MM3 score indicates an earthquake that can be felt indoors as a light vibration and that may cause hanging objects to swing. An MM4, which is an increase in intensity, would entail feeling it indoors but not outdoors as a strong vibration or jolt, and it would cause windows or plates and glasses to rattle, but not fall or break. Magnitude, however, is an attempt to have a more objective scientific measurement of earthquakes, and is measured on the Richter or Moment Magnitude scale (both of which use similar numbers to indicate magnitude). It is measured not by personal observation but by the use of a seismometer. Magnitude scales measure the waves that are emanated from the earthquakes epicenter, and thus indicate the level of energy that is being released at the exact spot the earthquake occurs. As a result, the MM3 and MM4 earthquakes described above in terms of intensity would measure as between 2 and 3, or between 3 and 4, on the Richter scale. The strength of the intensity scale is that it allows ordinary people to describe earthquakes in terms of perceived effects, but the weakness is that it is dependent upon personal testimony. A magnitude scale is a more scientific measurement, but it will do very little to explain to the general public why a major earthquake in the middle of the ocean can be utterly unnoticed by human observers until a tsunami arrives several hours later.

5. The Dakota Sandstone is in a layer determined to be at the end of the Jurassic, right before the start of the Cretaceous. The Wasatch Formation is in a layer from the end of the Cretaceous, just before the start of the Paleogene. As a result we can estimate the Dakota Sandstone to be 145-157 million years old, and the Wasatch Formation to be about 66-100 million years old. This is based on the largest possible dates for the late Cretaceous and late Jurassic period but a comparison with other types of formation can probably narrow down the date much further.

6. Caldera is the Spanish word for cauldron, and it denotes a specific type of volcanic crater found generally in a large central volcano within an area of volcanic activity. It forms when the central magma chamber of the large volcano empties, often because of eruptions in the surrounding volcanic area, which suddenly causes the large central magma chamber to collapse downward, forming a large crater.

7. Magma is composed of molten minerals which will form crystals as the liquid rock cools. If the magma cools quickly, the crystals form quickly and will only form in small sizes.

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With slow-cooling magma, these crystals can become extremely large. When the magma cools at such a slow rate, this is an indication it cooled closer to the earth's core where the temperature remains higher.

8. There are four basic types of volcanoes, which are the cinder cone (which are the stereotypical volcanoes we imagine, the cylinder-shaped mound), the stratovolcano (which is larger than the cinder cone with layers of lava flow, ash, and solid rock), the shield volcano (which are large with small slopes, and have lava that covers a large area emerging from a single vent), and the lava dome (which is formed by lava too thick to flow, and often emerges inside the crater of a stratovolcano). The geologic record will offer some clues to help identify the past volcanic activity. For instance, the magma content of non-explosive volcanoes -- like the shield volcano and the lava dome -- is generally only 50 to 60% silicon with higher levels of magnesium and iron, and produces basalt or andesite as the prevailing volcanic rock. Explosive volcanoes have instead a higher 60 to 70% silicon content, and their signature volcanic rock is dacite or rhyolite. But there are also additional signifiers which can indicate volcano type. For example, a tremendous disturbance in the geographic record with damage to surrounding areas and the melting together of different areas will likely indicate a stratovolcano, which often has the most powerful eruptive force and can easily melt and damage the layers of rock in its immediate vicinity. Likewise, a stratovolcano eruption of that size will also be identifiable due to mineralized deposits of volcanic ash, which may surround a very large area.

9. At the full or new moon, the earth and moon and sun are all in a line, thus permitting the gravitational pull of all three to work in concert. This creates high tides that are higher, low tides that are lower, and a stronger pull on the water to create faster currents between. The tidal flat would therefore be larger during a full or new moon, and any organisms that inhabit the tidal flat would have a greater area to inhabit.

10. At a certain point as waves come close to the land, the lower portion of the wave will lose energy into the sea-floor and thus will decrease speed, while the upper portion of the wave maintains its speed -- the wave will then "break" when the momentum of the upper portion overtakes that of the underwater portion and tumbles forward. In general this occurs at a point where the water depth is about one and one-third times the size of the wave height, so as a result with waves of consistent height the "break" phenomenon will seem to occur at the same point off shore.

11. One reason why summer temperatures on the west coast are lower than on the east coast, even at the same latitude, is due to the earth's rotation. One aspect is the oceanic currents which respond to that rotation, so as a result the west coast is cooled by water from the Pacific, while the east coast is actually warmed by the Atlantic. Also position on the continent has an effect, where a "continental" climate like that of the east coast has a greater difference in temperatures overall.

12. The search for millions of years worth of undisturbed sedimentary layers would be better conducted off the coast of New England rather than the Pacific Northwest. The reason for this should be obvious. The nearest tectonic plate fault line to New England is in the middle of the Atlantic Ocean, and runs through Iceland. It is nowhere near the coastal shelf where the sediments would be drilled. Whereas the Pacific Northwest sits basically right on the Cascadia Subduction Zone, which is a particularly active area of collision between tectonic plates. Not only is the region between Vancouver and San Francisco particularly active in terms of seismic phenomena, North America's most recent noteworthy active volcano, Mount Saint Helens, sits in the Pacific Northwest as well. The area of New England features no such seismic activity. The reason for using this as a determination is that, of course, sedimentary layers would be disturbed past utility by seismic activity -- it would be impossible to determine what sedimentary layers actually dated from millions of years ago in the Pacific Northwest, if subduction had caused older sedimentary layers to.....

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