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ESC 101
Sea Floor Spreading and Continental Drift
Tectonics is the study of the origin and arrangement of Earth’s structural features which include not only folds and
faults but also continents, mountains, and earthquake regions. The basic idea of tectonic theory is that Earth’s lithosphere
(the crust and the rigid upper mantle) is divided into several large, thick plates that slowly move and change in size and
shape. Plate boundaries are the sites of intense geologic activity (e.g., earthquakes and volcanoes).
Plate tectonics combines two previous theories, continental drift and seafloor spreading. Continental drift is the
idea that continents move freely, changing their positions relative to one another. Seafloor spreading is the hypothesis that
seafloor forms at the crust of the mid-oceanic ridge and moves horizontally away from the ridge crest towards an oceanic
trench.
Part A: Seafloor Spreading
One test for the sea-floor-spreading hypothesis involves magnetic patterns on the seafloor. In the late 1950's,
scientists mapped the present-day magnetic field generated by rocks on the floor of the Pacific Ocean. The volcanic rocks
which make up the sea floor have magnetization because, as they cool, magnetic minerals within the rock align to Earth's
magnetic field. The intensity of the magnetic field they measured was very different from the intensity they had calculated.
Thus, the scientists detected magnetic anomalies, or differences in the magnetic field from place to place. They found
positive and negative magnetic anomalies. Positive magnetic anomalies are places where the magnetic field is stronger than
expected. Positive magnetic anomalies are induced when the rock cools and solidifies with the Earth's north magnetic pole
in the northern geographic hemisphere. The Earth's magnetic field is enhanced by the magnetic field of the rock. Negative
magnetic anomalies are magnetic anomalies that are weaker than expected. Negative magnetic anomalies are induced when
the rock cools and solidifies with the Earth's north magnetic pole in the southern geographic hemisphere. The resultant
magnetic field is less than expected because the Earth's magnetic field is reduced by the magnetic field of the rock. Figure
1 illustrates the pattern of magnetic anomalies observed in seafloor rocks along a mid-ocean ridge.
When mapped, the anomalies produce a zebra-striped pattern of parallel positive and negative bands (Figure 2). The
pattern was centered along, and symmetrical to, the mid-ocean ridge. A hypothesis was presented in 1963 by Fred Vine and
Drummond Matthews to explain this pattern. They proposed that lava erupted at different times along the rift at the crest
of the mid-ocean ridges preserved different magnetic anomalies
Figure 2
Figure 1
1.
In Figure 1 determine which rock unit on the east side of the mid-ocean ridge that matches rock unit X on the
west side. Mark the matching rock unit on the east side with an X.
2.
In Figure 1 draw arrows on the east side and west side of the ridge showing the direction the seafloor is moving.
3.
What type of magnetic anomaly would be found in igneous rocks that have
recently solidified at the ridge crest?
ESC 101: Professor Vorwald
Seafloor Spreading and Continental Drift
_______________________
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4.
According to Figure 1, how many magnetic reversals are indicated?
_______________________
5.
In which hemisphere was Earth’s north magnetic pole when the rocks
identified by the X in Figure 1 solidified?
_______________________
6.
According to Figure 2 what is the relationship between the age of the seafloor rock as distance increases east
and west of the mid-ocean ridge?
7.
What does the varying thickness and spacing of the rock units on either side of the ridge indicate about the rate
at which the seafloor is moving over time?
Part B: Ocean Bottom Profile of the North Atlantic
1.
Construct an ocean bottom profile on the graph titled “North Atlantic Ocean Bottom Profile”(Figure 3) using
the ocean depth data in the data table that follows. Note that 0 km is sea level and that negative numbers on the
vertical scale of the profile indicate depths below sea level. Positive numbers indicate elevations above sea
level (land).
Data Chart: Ocean Depths
Distance (km)
Depth (km)
Distance (km)
Depth (km)
Distance (km)
Depth (km)
0
0
2900
-2.7
4100
+0.5
120
-0.2
3000
-1.8
1025
-2.2
200
-2.7
3090
-4.0
4500
-4.6
400
-3.7
3100
-2.4
5000
-5.0
490
-3.7
3200
-2.9
5300
-4.4
620
-4.6
3500
-3.5
5800
-3.7
680
-1.8
3600
-3.7
6000
-2.7
720
-4.6
3650
-3.7
6075
-0.2
2000
-4.6
4025
-4.0
6100
0
2500
-4.0
4050
-2.7
Depths are at distances from the east coast of the United States. Data has been simplified for student use.
2.
The profile shows the seafloor topography of the North Atlantic. The vertical exaggeration of the profile
makes slopes appear steeper than they really are and some important submarine features can’t be seen.
Referring to Figure 18.5 on page 471 of your textbook, label the following features. (If a feature is found on
both sides of the ocean label them in both locations.) continental shelf, continental slope, abyssal plain, MidAtlantic Ridge, rift valley on the ridge, seamounts
3.
Draw arrows on either side of the Mid-Atlantic Ridge representing the directions in which the seafloor is
moving.
4.
What is the vertical scale of the profile? (Recall that 1 km = 100,000 cm)
_________________
5.
What is the horizontal scale of the profile?
_________________
ESC 101: Professor Vorwald
Seafloor Spreading and Continental Drift
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Figure 3
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Seafloor Spreading and Continental Drift
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6.
Calculate the vertical exaggeration of the profile. Show your work in the space
provided below. (Recall that VE = fractional vertical scale/fractional horizontal scale)
___________X
7.
If you were to construct the profile so that it has no vertical exaggeration, using the
vertical scale for the horizontal scale, how wide would your profile be?
Express your final answer in meters (100 cm = 1 m). Show work below.
_____________ m
8.
What motion of the seafloor has formed the Mid-Atlantic Ridge’s rift valley? ___________________________
9.
The rate of plate movement along portions of the Mid-Atlantic Ridge has been measured
at 3.0 cm/yr. At this rate, how long will it take the Atlantic Ocean to widen another one
kilometer. Show work in the space below.
____________ Years
Part C: Continental Drift
An ancient continent called Gondwanaland formed sometime in the Proterozoic Eon and eventually collided with
other landmasses to form the supercontinent of Pangaea by the end of the Paleozoic Era. Gondwanaland was comprised
of the modern-day continents of Africa, South America, Australia, India, and Antarctica. During the early Mesozoic Era
Pangaea began to rift apart. Africa and South America formed the western part of Gondwanaland until the middle
Jurassic Period, when they rifted apart and opened the South Atlantic Ocean between them. The former contact between
the two continents can be seen from the following evidence.
‘ The shapes of their coasts and continental shelves
‘ Late Carboniferous glacial till deposits in Africa (below 12 degrees South latitude), Uruguay, and in southern Brazil
‘ Late Carboniferous glacial-marine deposits (mostly ice-rafted gravels) in central Africa between 0 and 12 degrees
south, Argentina, Paraguay, southern Bolivia, and the eastern coast of Brazil
‘ The Atlas Mountains in Morocco and Algeria and northern the Cordillera de Merida in Venezuela (Late Permian fold
belts)
‘ Mid-Jurassic evaporite deposits along the eastern coast of Brazil between Rio de Janeiro and Recife, and the western
coasts of Cameroon, Gabon, and Angola in Africa.
Figure 4 on page 9 shows the outlines of the continents and continental margins of Africa and South America. The
positions of the equator (0o latitude) and the 12o S parallel of latitude have been estimated and drawn on each map.
1. Estimate the locations of the outcrop belts of the Paleozoic and Mesozoic sedimentary deposits and structural
features listed above and outline them on the maps. Use a different colored pencil for each one and label them for
reference.
2. Cut out the outlines and position them on the blank page (p. 6) as they were once joined in western Gondwanaland.
Glue or staple the cut-outs to the blank page.
Continued on the next page (
ESC 101: Professor Vorwald
Seafloor Spreading and Continental Drift
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3.
What do the evaporite deposits indicate about the position of the continents at the time they were deposited
during the Jurassic Period?
3.
Examine the distribution of glacial till deposits in southern Africa. What does this indicate about the location of
western Gondwanaland relative to the South Pole during the Carboniferous Period?
ESC 101: Professor Vorwald
Seafloor Spreading and Continental Drift
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Blank Page for Attaching
Reassembled Continents
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Seafloor Spreading and Continental Drift
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Reference Page
Cordillera de Merida Mountains
Venezuela
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Seafloor Spreading and Continental Drift
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Africa
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Seafloor Spreading and Continental Drift
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Cut-Out Page
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Seafloor Spreading and Continental Drift
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