Plate tectonics is a unifying framework for understanding the dynamic geology of the Earth. The theory states that the outermost layers of the Earth (the crust and uppermost mantle) make up the brittle lithosphere of the Earth. The lithosphere is broken up into a number of thin (on average 50-100 km) plates, which move on top of the asthenosphere. The asthenosphere is solid, but ductile and flows over geologic time scales. Below are a series of activities and questions that are intended to introduce you to some of the most compelling observations supporting plate tectonic theory.
But how do we define plates and plate boundaries? How do we know plates are moving? How can we track their positions in the past, and how can we predict their positions in the future?
To begin to answer some of these questions, this assignment guides you through an examination of patterns on Earth – the distribution of earthquakes and volcanoes, the distribution of seafloor rocks of various ages, the topography of the earth’s surface above and below sea level.
Read the following on the United States Geological Survey website:
http://pubs.usgs.gov/gip/dynamic/preface.html (Links to an external site.)
http://pubs.usgs.gov/gip/dynamic/historical.html (Links to an external site.)
http://pubs.usgs.gov/gip/dynamic/developing.html (Links to an external site.)
http://pubs.usgs.gov/gip/dynamic/tectonic.html (Links to an external site.)
http://pubs.usgs.gov/gip/dynamic/understanding.html (Links to an external site.)
Watch this short video summarizing Plate Tectonic Theory
https://www.iris.edu/hq/inclass/animation/plate_boundaries_three_types_described (Links to an external site.)
Earthquake Evidence (44 pts total)
An earthquake is a vibration of Earth caused by the sudden release of energy, usually as an abrupt breaking of rock along planar fractures called faults.
Earthquakes originate at a point called the focus (or hypocenter), which is not at the surface of the earth, but instead at some depth within the earth. The epicenter of an earthquake is the point directly above the focus on either the land surface or seafloor; the depth of an earthquake has nothing to do with water depth, but instead is the depth in the solid earth from epicenter to focus.
Only rocks that are cold and brittle (the earth’s lithosphere) can be broken in earthquakes. Rocks that are hot and ductile will stretch and deform slowly over time without breaking (the earth’s asthenosphere) – and thus do not produce earthquakes. So observing where earthquakes occur, both horizontally and with depth, tells us something about where stress is concentrated, and also about the material properties of the earth.
Earthquake distribution and magnitude
Records of earthquake distribution including the depth at which they originate (focus) provide clues to plate tectonic processes. Using the series of maps found here:
1. Map #1 shows the global distribution of earthquakes by location (epicenter) and depth. Describe the pattern you see in the distribution of earthquake epicenters over the Earth’s surface. Are they randomly distributed? Or do they appear to be aligned along some sort of boundary and clumped in specific regions? (4 pts)
2. Look at Map #2 describe the relationship between the distribution of earthquakes and the tectonic plate boundaries. (4 pts)
3. On Map #2 different colors of the dots representing earthquake epicenters refer to the depths of the earthquakes, that is depth to the hypocenter as described above. What color are the shallowest earthquakes? ________ The deepest?_______ (2 pts)
4. Using Map #2, look closely at the earthquake depth pattern in the middle of the Atlantic Ocean between the North American and Eurasian plates versus the pattern in the Western Pacific Ocean between the Pacific and Australian plates. The earthquake depth patterns associated with these two areas are very different. Describe the depth range of earthquakes that occur at each of those plate boundaries from shallowest to deepest that occur there (e.g., 0-150 km).
North American and Eurasian plate boundary (2 pts) ________
Pacific and Australian plate boundary (2 pts) ________
Using Map #3, what is the relative plate motion at each boundary, moving towards each other or moving apart ?
North American and Eurasian plate boundary (2 pts) ________
Pacific and Australian plate boundary (2 pts) ________
Using what you learned in the readings and video, what is the type of plate boundary at each, convergent, divergent or transform boundary?
North American and Eurasian plate boundary (2 pts) ___________
Pacific and Australian plate boundary (2 pts) ____________
5. Using Map #3, describe the location of two other places on earth that have the same type of plate boundaries as the ones in the middle of the Atlantic Ocean. Use plate names to describe the boundary. For example, in the middle of the Atlantic ocean a plate boundary exists between the the South American Plate and the African Plate. (6 pts)
6. Using Map #3, describe the location of two other places on earth that have the same type of plate boundary as those in Western Pacific. Use plate names to describe the location of the boundaries. For example, in the Western Pacific ocean, a plate boundary exists between the Pacific Plate and Australian Plate. (6 pts)
Using the real-time earthquake monitoring site, http://www.iris.edu/seismon/ (Links to an external site.), answer the questions below. Note that on this site the size of the circle indicates how large (energetic) the earthquake was on a 10 point scale. The timing is indicated by color (red for earthquakes that occurred today). Find the earthquake magnitude and location on the map by scrolling over the circles and noting the information that shows up in the lower left of the screen view. You can also click on specific regions and earthquakes to get earthquake information in table form.
7. Ignoring the small pink dots, which are the last five years of earthquake history, what type of plate boundary (convergent, divergent, transform) do most of the recent earthquakes occur on? (4 pts)
8. Click on the “Last 30 days of Earthquakes” button, then click on the “Mag” (magnitude) header to sort by magnitude. What is the largest earthquake? List its location, magnitude, the date it occurred and the type of plate boundary on which it occurred. If there are two of the same magnitude, just pick one. (6 pts)
Volcanic Activity Evidence (6 points total)
A volcano is an opening in the Earth’s surface through which melted rock (magma), volcanic ash and/or gases escape from the interior of the Earth.
9. Using Map #4, which shows active volcanoes (white triangles) and Mid-ocean Ridge volcanoes (white lines) as well as earthquakes. Describe the relationship between the locations of most active volcanoes and locations of earthquakes: (6 pts)
Topographic Patterns (28 points total)
Map #5 shows the topography or shape of the surface of the earth. Ignore for now the Antarctic and Greenland data. Note the scale at the bottom of the map gives height above ( ) and below (-) sea level.
Topography of the earth BELOW sea level
We are all relatively familiar with the topography of the Earth’s surface above sea level, but less so with the bathymetry (topography underwater) of the Earth below sea level. Before this was known, most people assumed that the seafloor 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, showed 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 as the ship traversed the Pacific Ocean between battles. After the war, this data led him to propose seafloor spreading, a process crucial to the development of the theory of plate tectonics.
Modern methods to measure bathymetry include multi-beam echo sounders that map a wide swath of seafloor, and 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.
On Map #5, the bathymetry is shown in shades of blue: the darker the blue, the greater the depth. Examine the middle of the Atlantic Ocean between North/South America and Eurasia/Africa.
10. In one sentence, describe the bathymetry of the seafloor in the middle of the Atlantic Ocean (3 pts)
11. Now turn using Map #6 describe the relationship you observe between earthquakes and bathymetry in the middle of the Atlantic (3 pts)
Scan around to see the shallowest points in the interior (as opposed to edges) of the Indian, Pacific and Southern Oceans.
12. Using Map#7 and comparing it to your observations on Map #3, determine what type of plate boundary exists along these relatively shallow regions (convergent, divergent, transform)? (3pts)
13. Using Map #8 describe where you find the very deepest areas of the Pacific (shaded very dark blue almost black)? Look carefully as they are very narrow and somewhat difficult to see. A great way to see seafloor depth or bathymetry is on Google Earth if you have it or can download it. (4 pts)
14. Using Map #9, describe the range of earthquake depths at these deep parts of the Pacific. (4 pts)
15. Using your observations from Map#3 above, what predominant type of plate boundary occurs along these deep areas of the Pacific Ocean (convergent, divergent, transform)? (3 pts)
Topography of the earth ABOVE sea level
Use Map #8 again to look at the distribution of mountains on earth.
16. Where are the tallest mountains in North America? In South America? (be very general…east part of continent or west part of continent) (4 pts)
17. Where are the tallest mountains on the Eurasian continent (grey shading; you can name the mountain range)? (2 pts)
Now using Map #10 which shows the plate boundary How do the locations of these mountain belts relate to the location of the plate boundaries? (2 pts)
Seafloor Age (20 points total)
Map #11 shows the ages of volcanic rocks that have erupted and cooled to form the ocean floor. Focus on the Atlantic Ocean. Note that the age bands generally run parallel to the mid-ocean ridge which are active volcanic ridges. Seafloor age is a critical piece of evidence for plate tectonics; these are used to reconstruct how ocean basins have developed over time and predict how they may evolve in the future.
On average, continental crust is 2 billion years old; the oldest rocks are ~4.0 billion years old, and some of the grains in those rocks are even older.
19. What is the age range of the seafloor? _______________________________ (4 pts)
20. On average, which is oldest – the continental crust or the ocean crust? ________________ (4 pts)
21. Where is the oldest seafloor in the Atlantic (there are two places, either is fine)? (4 pts)
22. Where is the oldest seafloor in the Pacific? (4 pts)
23. Look at the seafloor between South America and Africa. What happens to the age of the seafloor as distance increases away from the ridge? (4 pts)
Plate Boundaries (2 pts total)
Use Map #10 again.
24. Do the plate boundaries correspond to the edges of continents (yes or no)? (2 pts)