Nicholas DeNobrega Lab 8 Plate Tectonics
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“Geoworld” Plate Tectonics Lab 8 Geol 116 Fall 2023
Nicholas DeNobrega
Simplified from exercise © 2003 Ann Bykerk-Kauffman, Dept. of Geol. and Envi. Sci., California State Univ. Chico
Purpose:
To better understand the principles of
the plate tectonics theory by applying them
to the past, present and future analysis of a
theoretical planet.
Introduction
During this lab exercise, we will analyze the
plate tectonics of part of a mythical planet called
Geoworld; a tectonic map of the area is shown on
p. 5
Analysis of the Present State of
Geoworld
. Geoworld has a plate tectonics system just like
ours on Earth, only Geoworld is flat instead of
spherical. All questions are in bold face. Please
put your answers in any blanks provided.
A. Sea Floor 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 sea floor have an
extra strong magnetic field while other parts
have 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 sea floor
with an anomalously strong magnetic field are
shaded; portions of the sea floor 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 in 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 in the Aragorn Ocean. The center of
symmetry in the Elrond Sea is right along the
spreading ridge. 1
2
“Geoworld” Plate Tectonics Lab
“Geoworld” Plate Tectonics Lab
1.
On the map on page 5, highlight the mid-ocean ridge in the Elrond Sea and label it the Hobbit
Ridge
. 2.
Draw a diagram and explain
the origin of the striped magnetic anomaly pattern visible in the
Elrond Sea. The striped magnetic anomaly pattern in the Elrond Sea is formed due to alternating bands of
strong and weak magnetism in the ocean floor. These stripes result from field reversals when
new oceanic crust forms at mid-ocean ridges
3.
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 (m.y.) ago did sea floor spreading begin in the Elrond Sea?
Spreading Began 62 m.y. ago
4.
Using the magnetic anomaly pattern in the Elrond sea, determine the 1/2-spreading rate on the
Hobbit Ridge (i.e., determine how fast the Frodo continent is moving away from the Hobbit
ridge). Give your answer in mm/y (millimeters per year). Show your equations and explain
where each number came from
.
Useful Information:
1 cm (10 mm) on the map = 400 km on Geoworld 1 mm/y = 1 km/m.y. (kilometers per million years)
distance = rate x time, rate = distance/time , time = distance/rate\
To calculate the ½ spreading rate, we first measure the distance from the middle of Hobbit
ridge to the end, which is 32 mm. From this we can convert the distance into km by using
conversion of every 10mm = 400km. This gives us a distance of 1280km. To get the time,
we can use the normal/reversal pattern with the Magnetic Polarity Scale, which gives us
time of 62 million years. So the ½ spreading rate = distance/time = 1280km/62 m.y = 20.6
km/m.y = 20.6 mm/y
1/2 spreading rate = 20.6
mm/y
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4
“Geoworld” Plate Tectonics Lab
5.
What is the full spreading rate (in mm/y) of the Hobbit Ridge? (i.e., how fast are the Bilbo and
Frodo continents moving away from each other)? Show your equations and explain where each
number came from.
To get full spreading rate, we can just double the ½ spreading rate calculated before, giving us
a full spreading rate of 20.6 mm/y x 2 = 41.2 mm/y
Full
spreading rate = 41.2
mm/y
6.
Using the magnetic anomaly patterns in the Aragorn Ocean, determine when sea floor spreading
started to form the Aragorn Ocean crust (remember, spreading is not actively going on now but
it was at one time). Spreading Started 132 m.y. ago
7.
At the time that active sea-floor spreading was occuring in the Aragorn Ocean
, at what rate was
new sea-floor being created? (Think
: is this related to the full spreading rate or the half
spreading rate)? Show your equations and explain where each number came from
.
The rate of new sea-floor being created is related to the full spreading rate. We can use 2
known reversals of the Aragorn ocean, t and o, to find the time of the reversal. t occurs at
120 m.y ago and o occurs at 90 m.y ago. 120 m.y ago - 90 m/y ago gives us a time of 30 m.y
ago. To find the distance, we can measure the full length of the Aragorn ocean plate using
the Geoworld scale. Since this distance is 30 mm, 30mm x 40km = 1200 km. 1200km/ 30
m.y = 40 km/m.y = 40 mm/y Rate of formation of Aragorn Ocean crust = 40
mm/y
B. Plate Boundaries
1.
There are two major plate boundaries on the map. You already highlighted one of them in Part
1, A. 1. above. Highlight the other boundary also.
The other highlighted boundary is the Sauron Trench
2.
Give one example of each of the following types of plate boundaries on Geoworld (Note: the
boundary between two plates can vary in type from place to place). Divergent boundaries Hobbit Ridge
Convergent boundaries Sauron Trench
“Geoworld” Plate Tectonics Lab
Transform boundaries The boundaries located between the plates along Hobbit Ridge, these are labeled on Geoworld map with arrows
3.
How many lithospheric plates are there in all?
3
6
“Geoworld” Plate Tectonics Lab
C. Transform Faults
There are several transform faults in the Elrond Sea. The only parts of these faults that are
presently active
are the parts between the segments of the Hobbit Ridge. 1.
Using arrows, show the sense of motion on each of the active transform faults on the map.
Shown in blue arrows on map
2.
In the space below, draw a diagram and explain
why the sense of motion on a transform fault is
the way it is. The fracture zone that creates the transform plate boundaries is known as the transform fault.
The sense of motion on a transform fault is the way it is because they are the locations
where two plates slide past each other, i.e two different plates that are moving away from
the spreading center of a divergent boundary. Part 2: Analysis of the Future State of Geoworld
A. When will the Bilbo Continent and the Sauron Trench first collide? Show your equations and explain
where each number came from
. Note that, although you computed relative motion across the Hobbit
Ridge in Part 1, here you need to know the current rate of movement of the Bilbo Continent toward
the Sauron Trench; it turns out this is 50 mm/yr (computed in a part of the exercise that you’re not
doing).
To do this calculation, we need to know the shortest distance between the Bilbo Contitnet and
Sauron Trench, which we were given as 2.9cm, or 29mm = 1160km. The given rate of Biblo
Continent moving toward Sauron trench is 50mm/yr, which is equal to 50 km/m.y. Then
finally, to calculate the time it will take we can do 1160 km / (50 km/m.y) to get a time of
collision of 23.2 m.y
Time of Collision = 23.2 m.y. from now
B. How wide will the Elrond Sea be at that time? Show your equations and explain where each number
came from
.
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“Geoworld” Plate Tectonics Lab
We know that full spreading rate of Elrond Sea is 41.2 km/m.y and that the time of collision is
23.2 m.y from now. Since distance = rate x time, We can calculate how much wider Elrond Sea
will be at this time by doing 41.2 km/m.y x 23.2 m.y = 955.84 km. Then we can add this to the
original width of Elrond Sea which is 6.2 cm or 6.2mm, and multiplying it by the full
spreading rate Hobbit ridge to get an original distance of 2,5554.4 km. Adding these together
we get 3510.24 km
Width of Elrond Sea = 3510.24 km
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“Geoworld” Plate Tectonics Lab
Plate Tectonic Map of Geoworld