Exercise 4_Plate Tectonics
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Geology
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Apr 3, 2024
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Geology for Engineers
Name _____________________________
Plate Tectonics
Plate Tectonics in Hypothetical “Geoworld”
Introduction
During this activity, we will analyze the plate tectonics of part of a mythical
planet called Geoworld; a tectonic map of the area is shown on the last page
of the activity. Geoworld is flat instead of spherical. In answering the
questions below, you may assume that the movement rates for plates (both
relative and absolute) are constant throughout the time between your past and
future reconstructions (there were, quite different at one time – see Part 1 A,
questions 6 and 7). All questions are in bold face. Please put your answers in
any blanks provided.
Part 1: Analysis of the present state of Geoworld
A.
Seafloor magnetic anomalies
Geoworld has a magnetic field aligned in a north-south direction. The
polarity of this field reverses at random intervals through time, just as the one
on Earth does. The history of polarity of the magnetic field is well known for
the past 150 million years and is shown by the adjacent magnetic polarity
time scale. Shading represents normal polarity (the magnetic field points
north) while white represents reverse polarity (the magnetic field points
south).
Detailed aerial and ship-board magnetic surveys have been conducted over
the oceans of Geoworld. Parts of the seafloor have an extra strong magnetic
field while other parts ohave an extra weak magnetic field. Just as on Earth,
the areas with anomalously strong magnetic fields are composed of rocks that
are magnetized with the same polarity as the current field while areas with
anomalously weak magnetic fields are composed of rocks that are
magnetized with the opposite polarity as the current field. Portions of the
seafloor with an anomalously strong magnetic field are shaded; portions of
the seafloor with an anomalously weak magnetic field are unshaded.
Note that the magnetic anomalies form a striped pattern on the map of
Geoworld. Note also that the pattern of the Elrond Sea is symmetrical while that in the Aragorn Ocean is asymmetrical. This is because there is an active spreading ridge in the Elrond Sea but not the Aragorn Ocean. The center of symmetry in the Elrond Sea is right along the spreading ridge.
1.
The pattern of positive and negative magnetic anomalies in the Elrond Sea can be correlated with
the pattern of normal and reversed magnetism on the magnetic polarity time scale. Based on this
correlation, how many million years ago (Ma) did seafloor spreading begin in the Elrond Sea? (10 pts)
2.
Using the magnetic anomaly pattern in the Elrond Sea, determine the full spreading rate on the Hobbit Ridge (i.e. determine how fast are the Bilbo and Frodo continents moving away from each other). Hobbit Ridge is labeled in one section – note it extends down the entire center of Elrond Sea. Give your answer in mm/yr (millimeters per year). (10 pts)
3.
Using the magnetic anomaly patterns in the Aragorn Ocean, determine when seafloor spreading started to and from the Aragorn Ocean crust (remember, spreading is not actively going on now but it was at one time). (10 pts)
4.
Give one example of each of the following types of plate boundaries / tectonic features on Geoworld. Note: the boundary between two plates can vary in type from place to place. (20 pts)
a)
Hot Spot: b)
Subduction Zone: c)
Passive Margin: d)
Continent-Continent collision (active or inactive): 5.
How many lithospheric plates are there in all? (10 pts)
6.
Provide two places on the planet of Geoworld where you would expect large numbers of earthquakes. (10 pts)
7.
When will the Bilbo continent and the Sauron trench first collide (in millions of years from now)? (15 pts)
8.
How wide will the Elrond Sea be at the time? (15 pts)
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