Download Seafloor spreading and plate tectonics are major concepts in geology

Plate Tectonics Lab
Seafloor spreading and plate tectonics are major concepts in geology. In 1915 Alfred Wegener
suggested that todays continents had originally been part of a “supercontinent” called Pangaea
which had, at some point in geologic time, fragmented into pieces (the continents) and gradually
drifted to their present positions. Very few additional technical studies were undertaken to explain
or refine this concept of continental drift until the early 1950s. Scientists like Harry Hess, J. Tuzo
Wilson, Robert Dietz, Patrick Hurley, Fredrick Vine, and others began to synthesize the extensive
oceanic geological data on a global scale to evaluate further the viewpoint current at that time that
the oceanic crust was still closing and opening. Abundant new hydrographic and geophysical data
derived from studies of ocean ridges, ocean trenches, earth geomagnetism, earthquakes and the like
were amassed and analyzed utilizing computer synthesis. Thus, many geologists began to
reinterpret Wegener's ideas in terms of a dynamic earth on which surface plates composed of
continental and ocean-floor rock move as large units. The figure below is a composite illustration
of the modern theories of plate tectonics showing three major concepts.
The rigid upper layer of the earth is known as the lithosphere. The lithosphere includes the
continental and oceanic crust and the upper part of the mantle. Because of stresses set up,
apparently by movement of mantle material, the lithosphere is often broken by a system of fractures
which occur in specific zones called plate boundaries These plate boundaries are outlined by the
locations of volcanoes and earthquake epicenters (see Fig. 6.1). Modern plate tectonics theory
postulates that plate boundaries can be included in one of three categories.
The first type of plate boundary is the diverging boundary also known as a spreading center, and is
exemplified by the East Pacific Rise and the Mid-Atlantic Ridge. Impelled by the movement of
heated mantle material upwelling from below the lithosphere, the older lithospheric rock is spread
apart and newly-generated volcanic material fills the fracture at the spreading center
As new ocean-floor rock is created at the mid-ocean ridges, there must be a compensating
destruction of old ocean-floor crust so the earth doesn't constantly expand. This destruction of
earth's old crust appears to happen in the areas where it is subducted in to ocean trenches. A
Plate Tectonics Lab
subduction zone is an area where a cold slab of seafloor is forced back into the mantle beneath
another plate. Plates are moving toward each other at this, the second type of plate boundary,
known as a converging boundary. Geothermal heat and friction increase the temperature of the
down-going plate and create magma, which erupts at the surface as a chain of andesitic volcanic
islands or andesitic volcanoes along a continental margin (e.g., the Andes of South America).
Continental collisions produce mountain chains like the Hymalayas where the continents are
sutured together. Earthquakes occur at various depths along the subducted plate.
The third common type of plate boundary is formed in areas where lithospheric plates slip
horizontally past each other and is called a transform fault. Transform faults are formed either in
the ocean or on a continent by a system of large lithosphere firactures that occur essentially
perpendicular to a spreading center. The transform faults are developed as a consequence of
differential spreading rates caused by the curvature of the earth’s spherical surface. The San
Andreas Fault system in California is an example of a transform fault connecting the East Pacific
Rise in the Gulf of California to the Juan de Fuca Rise off the coast of Oregon and Washington.
Rates of plate movement are slow, to be sure. They are calculated in terms of centimeters per year.
Two of the exercises in this lab will allow you to calculate the rate of movement of two different
plates and compare those rates.
Exercise 1: Seafloor spreading in the South Atlantic
Alfred Wegener had plenty of evidence supporting his theory of continental drift but he could not
come up with a mechanism that explained what he saw. In this exercise you will look at the
evidence, play with the way the continents fit together and calculate the rate that the continents
have spread apart.
Procedure: Cut out the continents of South America and Africa found on the last page. Take care to
include the continental shelf area when you cut them out. Next, take the cutouts and lay them on
top of the map on the following page and slide them back and forth noting the direction and
rotation needed to move the continents from their joined position (in the center) to their present
positions. Also take note of the structural trends, tillite patterns and continental shelf material with
respect to how they line up when the continents are together.
Plate Tectonics Lab
Fig.1
1. In the 1960’s Bullard postulated that the continental shelves should be included as part of
the continent. After fitting the continent cutouts on the overlay map, decide whether
Bullard’s or Wegener’s approach to continental fit works better. Explain your answer using
the information in the lab.
2. Based on fig. 2, why should the continental shelves be considered to be part of the
continents?
Plate Tectonics Lab
Fig.2
3. If the distance between South America and Africa is 4,828 km presently, and 120 million
years ago (Early Cretaceous) they were together (see fig.1), determine the spreading rate
between the continents in cm/yr. (Show your work) Note: 1 km. = 100,000 cm.
4. The graph on the right shows the age of the
bedrock at various distances from the Mid-Atlantic
ridge. Determine the spreading rate according to
this graph (select any point along the line then
compute: distance/time)
__________________ cm/yr
5. How does your answer to question 4 compare with
your answer in question 3? Explain why this is so.
6. When you moved your continents back to the pre-drift position, was the movement parallel
to the lines of latitude? In discussing your answer, explain their motion using direction and
rotation ( east, west, northeast, clockwise, counter clockwise etc.)
7.
List all the evidence you can that supports the theory that the continents were once
connected? (at least 4 pieces)
Plate Tectonics Lab
Exercise 2: Hotspots
There are places on the earth far from plate boundaries where the internal heat in the Earth
is so great, that it pushes up magma and creates a chain of volcanic islands as the plate moves over
the hotspot. The Hawaiian Islands is such a chain. In the following exercise, you will determine the
approximate rate of movement of the Pacific plate bases on the age and position of the island’s
bedrock.
Plate Tectonics Lab
Island
Age of island
in M.Y.
Hawaii
0.0
Maui
1.0
Distance in
km.
(cm x 10-5)
xxx
Measured
distance in cm.
from map.
xxx
Distance in
centimeters
(17x106 x cm.)
xxx
Difference in
age
xxx
Hawaii- Maui
Molokai
1.6
Molokai-Maui
Oahu
2.5
Oahu-Molokai
Kauai
4.7
Kauai-Oahu
Nihoa
7.0
Nihoa- Kauai
Midway
20.0
MidwayNihoa
8.
What is the average rate of movement of the Pacific plate? ______ cm/yr.
Rate of
movement
cm/yr
xxx
Plate Tectonics Lab
9.
Between which years was the plate movement most rapid? __________
10. Between which years was the plate movement slowest? __________
11.
Which plates seem to be moving faster, the Pacific or Atlantic? ________
Exercise 3: Converging Boundaries and Subduction Zones
As stated previously, if ocean floors are spreading in areas of mid-ocean ridges, they must be
converging in other areas, if the surface area of the earth is to stay constant. Apparently, deep ocean
trenches represent zones of subduction where ocean-floor rock sinks into the mantle. One line of
evidence for this theory lies in the distribution of earthquakes of intermediate and deep focal
depths. Earthquakes occur only in rigid crustal rock material, not in plastic or molten mantle or core
material.
Most earthquakes are the
shallow focus type that
occurs within the upper 70
kilometers of the earth
(within the lithosphere). Only
near trenches do earthquakes
of intermediate (70 to 350
kilometers) focal depth occur.
This fact would indicate that
something solid and rigid is
being forced deep into the
mantle in these areas. Since
the earthquakes are occurring
within this subducted plate, it
can be profiled by locating
earthquakes and their various
focal depths relative to the
position of the trench. The
following map shows an area
in the western Pacific called
the Izu Trench, located
between the Mariana Trench
and the Japan Trench. Plotted
on this figure are the
epicenters of recent
earthquakes, with different
symbols for each of the three
types of focal depths. A line
of cross section, A-A', is
indicated.
Data
Assume that you manage a seismic monitoring station set up in the Benin Island group to the south
and east of the Izu Trench (see map). Over a week-long period, you record 24 significant
earthquakes in the Izu Trench (along cross section A-A') and calculate their focal depths and
Plate Tectonics Lab
relative distances from your recording station. These data are compiled in the following table.
12.
What is the approximate angle of descent for the sinking slab? __________
13. a. Name the over-riding (upper) plate involved in this boundary: _______________________
b. Name the subducting plate (lower) involved in the boundary: _______________________
14. Explain how you arrived at your answer to question 13.
15. Could the subducting plate be the over-riding plate? Explain your answer.
16. Compare the angle of descent of the plate at the three trenches. Which is the steepest?
Shallowest?
Plate Tectonics Lab
Plate Tectonics Lab