Geophysical Observations#

Part I: Plate Tectonics#

The theory of plate tectonics posits that the Earth’s lithosphere (crust and upper mantle) is broken into a number of jigsaw puzzle-like plates which move relative to one another over a plastically-deforming (but still solid) asthenosphere (and mid and lower mantle). The boundaries between plates are narrow zones marked by a variety of topographic and tectonic features, and there is significantly less (but still some) tectonic activity in the interior of plates.

You will be exploring some of the evidence on which plate tectonics is based, and analyze data that are used to interpret plate tectonic processes.

Topography of the continents and bathymetry of the sea floor#

We are all relatively familiar with the topography of the Earth’s surface above sea level, but less so with the bathymetry of Earth’s surface below the sea level. Before this bathymetry was known, most people assumed that the sea floor was relatively flat and featureless, and personal experience with lakes and rivers suggested that the deepest part would be in the middle.

Actual mapping of the sea floor, however, revealed some surprises. Such mapping began in the 1930’s but accelerated during World War II with the advent of submarine warfare. Princeton Geosciences Professor Harry Hess played a pivotal role; as captain of the USS Cape Johnson, he used the ship’s echo sounder to “ping” the seafloor and measure depth profiles as the transport ship traversed the Pacific Ocean. After the war, this data led him to propose the process of seafloor spreading, a hypothesis crucial to the development of the theory of plate tectonics.

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Image: Harry Hess with a blackboard

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Image: USS Cape Johnson

Modern methods to measure bathymetry include:

  • Multi-beam echo sounders that map a wide swath of the seafloor.

  • Satellite measurement of variations in sea level due to variations in gravitational pull over bathymetric features – sea level is slightly lower over low spots on the sea floor and slightly higher over high spots.

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Image: (left) Various methods of seafloor measurements made onboard a ship (right) Satellite measurements of variation in sea level due to variations in gravitational pull over bathymetric features.

Clipping input data to the valid range for imshow with RGB data ([0..1] for floats or [0..255] for integers).
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Image: topographic map of Earth

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The lowest point is at (lon, lat) = (-65.45, 20.00) and 8081.00 m below sea level.
The highest point is at (lon, lat) = (-121.77, 46.85) and 4161.00 m above sea level.

Part II: Mountain building and isostasy#

Mountain ranges are created at present subduction zones (for the Andes or the Himalayas) or past subduction zones (for the Appalachians). These are regions where the lithosphere is thickening due to the compressional stress, crustal thickening and volcanism, facilitated by the relatively lower strength of the lower crust (i.e. think of strength envelopes of continental crust).

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Figure: Anatomy of a subduction zone setting. Source: USGS

In the schematic above, note that the continental crust at the mountain belt is thickened not only upward but also downward into the mantle asthenosphere. Mountain building causes relief, the elevation difference between peaks and adjoining areas which may already be above sea level.

Let us first explore surface topography from the [ETOPO1 model] and focus on the Andes mountain range in South America. We will make a cross-section that cuts through the mountain range so that we may calculate the local refief.