General earth science
1. Compare and contrast primary (P) and secondary (S) waves. How are they used to determine the nature of the layers of the Earth? How might they be used to determine the epicenters of earthquakes?
The Earth surface has been determined to confer elastic properties which are observed as seismic waves that spread towards the inner core. Compressional or primary (P) waves are known to move at a fast speed between 1.5 to 8.0 kilometers per second along the surface of the Earth. P waves are mainly responsible for the shaking of the ground, and the shaking is generally similar to the direction of the P waves. Shear or secondary (S) waves are transmitted in a slower manner, at approximately 60 – 70% of the velocity of P waves. Unlike P waves, the action of S waves is perpendicular to the direction of its transmission (Shearer, 1999).
P waves have the ability to move through the Earth, as well as the ability to bend in response to variations in the density of the Earth’s layers. P waves can travel across the Earth’s mantle and core but these waves change in form as soon as it reaches the centre of the Earth. P waves are also helpful is determining the depth of each layer of the Earth because they are sensitive enough to change their direction upon reaching the periphery of each layer. It has been suggested that this change in direction of the P waves is created as a refractory response to the change in densities of each layer of the Earth.
The velocity of waves frequently fluctuates when transmitted across the surface of the Earth, but the speed ratio between a P wave and an S wave is generally stable. Such feature has facilitated seismologists to determine the epicenter of an earthquake. This employs the chronological measurement of the appearance of a P wave in relation to the onset of an S wave (Bolt, 1993). In addition, the location of the seismological observation station is also considered. The distance of the epicenter of an earthquake is thus calculated by determining the time between the appearance of the P and S wave and multiplying that time interval by 8 kilometers/second. The product is thus the distance of the epicenter from the seismological observation station.
2. Compare and contrast cinder cone volcanoes, composite cone volcanoes, and shield volcanoes. List examples of each.
Cinder cone volcanoes are the most common and simplest type of volcanoes that are generally created from Strombolian eruptions. The cones of these types of volcanoes show steep sides made up of basaltic segments. These cones transmit lava directly to the environment through the presence of vents. It has been established that stronger volcanic eruptions generated taller and steeper cones of a hundred meters in height and width. A crater is typically present at the top of these volcanoes. Cinder cone volcanoes are formed by the mixture of ash and cinders, which are small volcanic pebbles that have been generated from the liquification of volcanic rocks. The Paricutin volcanoe in Mexico and the Surter volcanoes of Iceland are examples of cinder cone volcanoes.
On the other hand, composite cone volcanoes or stratovolcanoes are characterized by large, highly symmetrical cones. Similar to cinder cone volcanoes, composite cone volcanoes also have vents, but may be present in higher numbers. In addition, composite cone volcanoes are different from cinder cone volcanoes in the manner of putting out their lava. Composite cone volcanoes exude lava through cracks along its craters or from the sides of its cone. One unique characteristic inherent to composite cone volcanoes is the process wherein magma is contained underneath the Earth’s crust and is only expelled to the immediate environment once the amount in the reservoir is beyond its containment capacity. Japan’s Mount Fuji and Mount Rainier in Washington are examples of composite cone volcanoes.
Shield volcanoes are flat, dome-shaped volcanoes that are generated from basalt lava flows that originate from one to several vents. The height of shield volcanoes increases by the accumulation of thousands of basaltic flows over a long period of time. In addition, shield volcanoes have larger widths than cinder cone and composite cone volcanoes because of their inherent characteristic of being generated from lava flows. The Mauna Loa and the Kilauea volcanoes in Hawaii are examples of shield volcanoes.
3. Compare and contrast these plate boundaries: divergent, convergent, and transform fault. List examples of each type.
A divergent fault is the linear perimeter between two tectonic plates. It rises upward through the force generated by magma that is situated below a divergent fault. The San Andreas fault in California is an example of a divergent fault. On the other hand, a convergent fault is a surface edge that exists between two plates that are physically colliding with each other. It is different from a divergent fault because the direction of movement of these plates is horizontal. The Japanese island arc presents the characteristics of a divergent fault. A transform fault is the border line that exists between two plates that are slipping by each other’s surface. Transform faults are susceptible to earthquakes because they have a tendency to accumulate pressure from the sliding plates, which in turn, generally result in a slip that may cause the plate to shake, which is a feature of earthquakes. South America’s Andes mountain range shows the features of a transform fault.
4. Describe the contributions to the theory of continental drift from Archie Carr, Allan Cox, Harry Hess, Drummond Matthews and Alfred Wegener.
Archie Carr, together with Patrick Coleman, proposed the seafloor spreading theory in 1974. Their concept was based on their research on the survival of sea turtles as they dwell in oceans. They observed that the sea turtles would passively migrate to other places by the force of continental drifts, which comprise more than half of the Earth’s surface area (Carr, 1987). It has also earlier been determined that specific volcanic rock layers are magnetized towards either the North or the South poles, resulting in magnetic reversals that occur intermittently (Meyerhoff and Meyerhoff, 1972). Allan Cox investigated these patterns of magnetic reversal by studying the flow of volcanic lava during eruptions. He was successful in identifying the connection between these magnetic fields and plate tectonics (Wesson, 1972). On the other hand, Drummond Matthews discovered that the Earth’s crust is enfolded by mid-ocean ridges that were magnetized to a specific polarity. This discovery led them to propose that the seafloor surface is generated on all sides of the cracks of the mid-ocean ridge, which in turn results in continental drifting (Davis, 1977). Alfred Wegener proposed the continental drift theory which describes geological origins of continents based on the similarities of mountains and rock strata and flora and fauna of different countries. His theory describes that a single continent (“Pangaea”) existed approximately 300 million years ago, hence explaining the similarities among the current continents. Now due to continental drift, this single large continent separated and moved towards different directions, resulting in the current continents around the Earth.
Bolt BA (1993): Earthquakes and Geological Discovery. Scientific American Library, W.H. Freeman, New York, 229 pages.
Carr A (1987): New Perspectives on the Pelagic Stage of Sea Turtle Development. Conserv. Biol. 1(2):103-121.
Carr A and Coleman P (1974): Seafloor spreading theory and the odyssey of the green turtle. Nature 249:128-130.
Davis RA (1977): Principles of Oceanography, 2nd edition. Addison-Wesley. ISBN 0-201-01464-5.
Meyerhoff AA and Meyerhoff HA (1972): The new global tectonics: Age of linear magnetic anomalies of ocean basis. Am. Assoc. Pet. Geol. Bull. 56:337-359.
Shearer RM (1999): Introduction to Seismology. Cambridge University Press, Cambridge, UK, 260 pages.
Wesson PS (1972): Objections to continental drift and plate tectonics. J. Geol. 80:191.