There are a lot of tedious steps, but once finished it is fairly simple:
Plate tectonics is a unifying framework for understanding the dynamic geology of the Earth. The theory posits 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 plates, which move on top of the asthenosphere (middle mantle). The asthenosphere is solid, but flows plastically over geologic time scales. Plate interiors are relatively stable, and most of the tectonic action (earthquakes, volcanism) takes place where plates meet – where they collide at convergent boundaries, move away from one another at divergent boundaries, or slide past one another at transform boundaries.
Reconstructions of the Earth’s tectonic plate locations through time are available, for example, at:
http://www.scotese.com/newpage13.htm (Links to an external site.)
http://www.ucmp.berkeley.edu/geology/tectonics.html (Links to an external site.)
But how do we define plates and plate boundaries? On what are plate reconstructions and animations based? 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 answer these questions, this assignment guides you through an examination of patterns on Earth – the topography of the earth’s surface above sea level, the bathymetry of the ocean floor below sea level, and the distribution of earthquakes and volcanic rock ages. These patterns reveal plate boundaries, just as they did for geologists first developing plate tectonic theory in the 1960s. You’ll then use geologic data to determine long-term average plate motions, to predict how our dynamic planet will change in the future.
To do this, you’ll use the program Google Earth, and Google Earth layers compiled from various sources.
A. Getting started with Google Earth
Once the DynamicEarth.kmz is loaded, click and drag to move it from “Temporary Places” to “My Places.” Then save “My Places” by clicking File/Save/Save My Places. DynamicEarth.kmz will now be available every time you open Google Earth Pro on your computer.
When you exit, Google Earth Pro should save “My Places” for the next time.
But you should manually save “My Places” whenever you make significant changes to it, as Google Earth Pro does not autosave during a session.
You now have an interactive view of the Earth! Take some time to explore the Earth with Google Earth and figure out how the navigation works using the keyboard, your touch pad, your mouse. For example:
Expand and contract the folders and subfolders, turn various items on and off, etc. For example, with the Dynamic Earth/Volcanoes of the World layer displayed, right-clicking on a volcano (double-clicking with a Mac) brings up an information box about it.
B. Topographic Patterns
Uncheck all of the layers and focus on topographic features of the Earth.
Topography of the earth ABOVE 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 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 Google Earth, the bathymetry is shown in shades of blue: the darker the blue, the greater the depth. You can get Google Earth Pro to draw topographic profiles by a) using the “Add Path” tool to draw a path across a region of interest; b) saving that path to My Places and c) right-clicking on the path in My Places and choosing “Show Elevation Profile.” In order to see a bathymetric profile of the sea floor, (as opposed to a topographic profile on land), there is one more important step to take. In the information box for the path you create, click on “Altitude”, and then choose “clamped to the sea floor” instead of “clamped to the ground”. Otherwise your profile will simply show you a flat line for the sea surface.
C. Seismic Patterns
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.
Expand the Lab 1/Earthquakes item and click “on” the “Twenty years of large earthquakes” layer to show the epicenters of large earthquakes (those with magnitudes >= 6.0) during a 20-year period.
|In the vicinity of ridges.
(Scan 1500km or so on either side)
|In the vicinity of trenches.
(Scan ~1500 or so km on either side)
|Describe the depth or range of depths of earthquakes, and the distribution (symmetric or asymmetric?)
Is there any pattern to the depth distribution?
D. Volcano Patterns
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.
E. Plate Boundaries
The theory of plate tectonics holds that the Earth’s lithosphere is broken into a finite number of jigsaw puzzle-like pieces, or plates, which more relative to one another over a plastically-deforming (but still solid) asthenosphere. The boundaries between plates are marked by active tectonic features such as earthquakes, volcanoes, and mountain ranges and there is (relatively) little tectonic activity in the middle of plates.
Unclick all the layers, and then click on the “plate boundary model” layer (click the box to show it and then click the + or arrow to expand the legend). This shows plate boundaries and the names of major plates.
Find the boundary between the African and South American plates
Travel westward across the South American plate to its boundary with the Nazca plate
F. Plate motion
Motion across the mid-Atlantic ridge: the South American plate vs. the African plate
Turn on the “Seafloor age” and the “Plate Boundary” Google Earth (GE) layers. The “Seafloor age” layer 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 spreading 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.
G. Putting it all together:
Prepare a report documenting this lab activity. Your report should discuss how plate tectonic theory relates to earthquakes, volcanoes, and the bathymetry (sea floor topography) of oceans. Along the way, include answers to all of the questions in this lab. Your paper should be accompanied by the two drawings of your ocean floor profile sketches in questions 3 and 4. Your paper should be well organized and written in flowing paragraph form, instead of just a numbered list of questions and answers. Use APA format, including a title page, and citing and referencing any sources that you use to support your work, apart from this lab sheet