Download Module 3, Investigation 3: Plate Tectonics Introduction Welcome

Module 3, Investigation 3: Plate Tectonics
Introduction
Welcome!
You’ve now looked at evidence for shifts in the positions of continents and expansion of the
ocean basins, but what actually causes these things to occur?
In this investigation, we will examine what is happening below ground to cause these shifts.
You will need to refer to your notes about volcanoes, earthquakes, and the Earth’s structure to
see the whole picture.
Folder 1: Plate Geography & Structure
What is a tectonic plate?
The entire Earth’s surface is divided into pieces called "lithospheric" or “tectonic plates” and the
movement of these plates over geologic time is called “plate tectonics”.
There are seven or eight “major” plates, depending on how they are defined, and numerous
minor plates. Turn on your Plate Geography folder to see the names and locations of the
various plates. Note that the plates are typically named for the continent that occurs on them,
but most plates include both land and ocean portions.
The plates include two layers, the upper mantle below and crust above. The thickness of the
plates depends on their age. In the last investigation, you saw how new oceanic lithosphere is
constantly being formed at the mid-ocean ridges. In the ocean, plates are often less than 15 km
thick, while on the older land masses, plates can be more than 200 km thick.
Another important difference between the two types of plates is that oceanic crust is denser
than continental crust. This is because oceanic crust is made of basalt, which has a higher
proportion of dense minerals such as magnesium, while continental crust is granitic, which has
a higher percentage of less dense minerals such as aluminum. Why is this so important? You
will soon see!
Folder 2: Rates of Plate Movement
How fast do plates move?
There are several methods for measuring the movement of tectonic
plates. The most widely used is the global positioning system (GPS),
which relies on twenty-four GPS satellites that orbit the Earth every 24
hours. Networks of permanent, extremely sensitive, GPS receivers or
“monuments” are used to make repeated measurements of their
locations. Changes in monument location are then plotted against time
to determine the rate at which plates are moving at various locations.
Turn on the GPS Monuments folder to see the location of three plate
monitoring sites (Neah Bay, WA, Mission Viejo, CA, and Parkfield, CA).
Then go to the link below and scroll down to the “Plot Real
Time GPS Data” section. For each of the three stations, select the
station name from the drop down list and view the graphs of North to
South and East to West movements for each of these stations. Click the
"analyze" button to see the average velocities over the past 5 years and
record them in your field notebook. Click here to open the link.
Next, turn on the Earthquakes layer under the Gallery folder in the Layers panel and compare
the seismic activity around the three sites in Google Earth. Record your findings in your field
notebook.
Folder 3: Driving Forces
What causes the plates to move?
You already know something about the Earth's structure that is the key to making plates move.
Can you remember what it is? If not, review your notes from Module 2, Investigation 3, Folder 3:
"Voyage to the Center of the Earth".
In that investigation, you determined that the asthenosphere is a relatively soft layer between
harder sections of the upper mantle below and rigid lithosphere above. This allows
the asthenosphere to act as a kind of lubricating layer. The lithosphere, which makes up
the tectonic plates, "floats" above it.
In Investigation 2, Folder 3: "The Hypothesis", you also learned that liquid iron in the outer core
moves in large convection currents. The heat carried by these currents is generated from
radioactive decay of several chemical elements in the Earth's core, including potassium,
uranium, and thorium. This heat is extracted by the mantle, which also has convection currents
that move hot material to the surface and cool material downward. Many scientists believe that
these convection currents in the mantle act like conveyor belts, pulling the plates above them
along. The process is usually depicted in a simple diagram as shown below. However,
mathematical models of how convection might actually occur are far more complex. View the
animation here to see for yourself.
Folder 4: Plate Interactions
Living on the Edge
Plate boundaries can either be convergent (colliding), divergent (separating),
or transform (sliding past each other). To view each type of boundary, click the play button
below and hover your mouse over each button in the interactive diagram. Then, click on the
links following the diagram to view animations about each type of plate boundary. Be sure to
take notes on each situation in your field notebook and then answer the questions posed.
Folder 5: Mystery Tour
Mystery Tour
Now that you know what happens at the different types of plate boundaries, let’s see if you can
identify them “in the field”. You will visit seven locations. At each one, you must determine (1)
what types of plates are interacting, and (2) what type of interaction is occurring. Before you
begin, turn on the Volcanoes and Earthquakes layers under the Gallery folder in the Layers
panel. These layers contain important clues!
If you click on an earthquake icon, you can see the depth of the hypocenter. Also, remember
that only earthquakes that are recorded by seismographs in an international network will appear
in Google Earth.
Make sure to record your observations and interpretations in your field notebook. Good Luck!
Summary
Let's Review!
Turn on your Plate Geography folder and the Volcanoes and Earthquakes layers under the
Gallery folder in the Layers panel. Zoom in until the earthquakes and volcanoes appear, and
take your own tour of the plate boundaries. Would you say that the Earth is geologically active?
Why or why not? Do you see that volcanoes and earthquakes tend to occur along plate
boundaries?
What types of plate boundaries usually have volcanoes? Why do volcanoes form at these
boundaries?
What types of boundaries usually have earthquakes? Why do earthquakes occur at these
boundaries?
If you had to live near a plate boundary, what type of boundary would you want it to be?