EARTHQUAKES AND VOLCANOES:
A GLOBAL PERSPECTIVE
The Mid-Ocean Ridge (MOR) system is the single largest feature on the surface of the Earth. It was discovered by submarine crews during WWII when they encountered a vast undersea mountain range while crossing the Atlantic Ocean. Associated with these mountains were deep valleys which, together, dwarfed anything seen on land. After WWII, the ocean floor was surveyed in detail and revealed a very complex topography, unexpected by geologists. Why did this feature occur where it did, and how can it be explained? The development of the Plate Tectonics theory during the 1960s lead to our present understanding of the MOR and its key role in creating new oceanic crust.
One interesting discovery was that of linear features or faults that are generally oriented at right angles to the ridges. They apparently offset sections of the MOR system where the deep valleys occur. Some of these structures are very long, extending almost half the way across the ocean basins. These features include:
Below is an image showing the age of the oceanic crust in the Atlantic Ocean.
Answer questions using the image below.
Where can the oldest oceanic crust be found? What is its age? Compare this to the oldest age for continental crust from Australia and North America, which are well over 3.5 billion years. Using your understanding of plate tectonics processes, explain why oceanic crust is so (relatively) young compared to that of continental crust.
In 1965 the geologist J. Tuzo Wilson named these features transform faults and his interpretation of them became a cornerstone in the development of plate tectonic theory. Below is a diagram showing, in detail, the relationship between the MOR spreading center (called the ridge axis) and the transform fault.
Diagram from: http://www3.uakron.edu/geology/facpages/db/oceanography/plate/
The following images show the topography of the ocean floor in the vicinity of a MOR. Identify the tectonic features (transform faults, ridge axis or spreading center, and anything else) indicated in each diagram. Indicate or describe the directions of plate motion for all spreading centers and transform faults.
Diagram 1. A 3-dimensional view of the topography of a section of the MOR in the north Atlantic. Explain the features you see and relate them to the diagram above.
Diagram 2. A large-scale view of the north Atlantic.
Question: Which of the features that you just identified would produce the most earthquakes, and why?
This is an image showing the topography of the South Pacific. Identify the features.
This is an image showing the topography of the East Pacific Rise, where the Cocos plate to the right of the MOR is being subducted beneath North America. Identify the features.
This is the same image from Diagram 4, but it has been overlain with earthquake epicenters, color coded by depth.
Answer the following questions.
a. In a MOR system, what specific feature is associated with the most frequent earthquakes? How would you describe the depth of the majority of quakes from the MOR?
b. Provide an explanation for why earthquakes would be more frequent here than any other place along the MOR system. Speculate about the reason they are the primarily of the same depth range.
c. There is an earthquake located near the object identified as point B in the previous image. What could be the cause of this earthquake, since it isn't on a fault?
d. There are a large number of earthquakes in the upper right (northeast) corner of the image. How would you describe the pattern of earthquake depth? What type of feature is this?
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