Question
Briefly describe what happens at a subduction zone
Explain how an accretionary wedge forms.
How are faults, hypocenters, and epicenters related? Faults that are experiencing no active creep (relatively consistent yet minor movements) may be considered “safe”. Rebut or defend this statement with what you have learned so far about faults
Sample paper
Earth Science
Number 1
Briefly describe what happens at a subduction zone
A subduction zone is the result of meeting of the earth’s tectonic plates. The earth’s crust comprises of massive plates known as tectonic plates. These plates may move along each other or slide over each other due to tectonic forces acting on them. When tectonic plates move away from each other, they form a boundary at the point of separation. When they move towards each other, tectonic plates collide, where one sinks beneath the other plate.
Number 2
Explain how an accretionary wedge forms
An accretionary wedge occurs along the surface compressions formed by the converging plate boundaries. When the two tectonic plates meet, one of them sinks underneath the other. As one of the tectonic plates undergoes subduction, loose sediment from the plate is scrapped off (Christiansen, 2014). The loose sediment forms the accretionary wedge after it piles up near the point of subduction. Folds may develop along the accretionary wedge. As more sediment is removed from the sinking tectonic plate, the wedge or mass of loose sediments grows towards the trench formed from the sinking plate (Christiansen, 2014). With time, the accretionary wedge collapses towards the trench. This results to the formation of the accretionary wedge, which is looks deformed in shape.
Reference
Christiansen, E. H. (2014). Dynamic Earth. New York, NY: Jones & Bartlett publishers.
Number 3
Faults, Hypocenters, and Epicenters
Faults, hypocenters, and epicenters are closely related terms that define various aspects of an earthquake. The fault refers to a point on the earth’s crust where a slip occurs leading to a release of energy and hence an earthquake. When faults slip over each other or collide, release of energy occurs leading to an earthquake. The hypocenter is a point on the earth’s crust where the seismic waves initially occur. When a slip occurs or when rocks break along the fault line, release of energy occurs, leading to seismic waves, which causes the ground to shake (Hung et al., 2010). The epicenter refers to a point located on the ground surface and directly above the hypocenter.
The statement that faults experiencing no active creep may be considered safe is wrong. Creep movements reduce the likelihood of an earthquake occurring due to stress on the rocks around the fault line. Faults that are experiencing no active creep may indicate two things. First, the sheer stress generated by rocks may be relatively small to cause the rocks to break along the fault line. In such a scenario, the fault may be considered inactive. On the other hand, the rocks could be locked in a way that hinders creep. As such, elastic strain slowly accumulates on the rocks along the fault lines. In future, the rocks may be unable to bear the strain and undergo breakup. This can lead to an earthquake even though there was no creep.
Reference
Hung, C. J., National Highway Institute (U.S.), & Parsons, Brinckerhoff, Quade & Douglas. (2010). Technical manual for design and construction of road tunnels–civil elements. Washington, D.C.: U.S. Dept. of Transportation, Federal Highway Administration, National Highway Institute.
Explain why Africa and South America are moving apart
Contrary to what many think, the earth’s crust is not made up of a single continuous plate, but rather several plates that float on the molten rocks of mantle. Due to high temperatures within the earth’s core, the mantle exists in molten state (Codrington, 2005). Closer to the earth’s surface, the mantle becomes increasingly solid. The molten rocks of the mantle move slowly due to convectional currents. This slow movement causes the plates to drift either apart or close to one another. The plates that make Africa and South America are slowly drifting apart due to currents within the mantle. A part of South American’s coast line bears a similar shape to that of West Africa’s coast line, indicating that the two continents may have been joined together but slowly drifted away due to currents within the mantle (Codrington, 2005). These molten rocks under both continents are moving in opposite directions, thus causing drifting apart.
Reference
Codrington, S. (2005). Planet geography. Sydney: Solid Star Press.
