2 Homework Plate Tectonics
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Coastal Carolina University *
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Course
112
Subject
Geology
Date
Dec 6, 2023
Type
Pages
4
Uploaded by laceyfralix13
MSCI 112 Homework
Full Name: Lacey Fralix
Plate Tectonics: Driving Forces
Homework 2
Go to a USGS on-line publication called "This Dynamic Earth: The Story of Plate Tectonics" (in the
subsection "Some Unanswered Questions"):
http://pubs.usgs.gov/publications/text/unanswered.html
Read the first section "What Drives the Plates?" and chapter 2 in the textbook. Answer the questions
below.
(a) The motion of mantle materials associated with the tectonic plates is called convective flow. Where
are the sources of energy driving convective flow in the moving tectonic plates? (5%)
Heat(radioactive decay and residual heat) and gravity
(b) What kind of plate motion and plate boundaries are associated with convective flow of mantle
materials? (5%)
Colder denser material moves down while hotter lighter material moves up due to gravity.
(c) Briefly describe the most recent idea about the major specific mechanism that drives the moving
tectonic plates. (10%)
Ridge push propels and maintains plate movement while slab pull sinks oceanic slab and drags the plate
with it.
MSCI 112 Homework
Full Name: Lacey Fralix
Plate Tectonics: Speed of Motion
In the 1960s, geologists were seeking ways
to prove or disprove the then new idea of
moving plates. For example, geologists
observed that three linear chains of
dormant volcanoes (seamounts), volcanic
islands, and submarine volcanoes are
stretched across the Pacific Ocean Basin as
shown below. These seamounts, volcanic
islands, and underwater volcanoes do not
lie on a divergent plate boundary. Dating
of the rocks from these volcanos has
generated estimates of the ages of these
volcanoes, i.e., the years since their
formation via eruptions of lava onto the
seafloor. The very youngest of these is
Lohii which is still being formed and is
still submarine.
This seamount lies 20
miles off the south coast of the Big Island
of Hawaii. It rises 3000 m above the ocean
floor to within 1,000 m of the water
surface.
a)
Explain the process that has caused them to form as linear chains.
(5%)
This is due to active plate tectonics. This is caused by a convergent boundary between two plates
that form the mountain chains.
The basalt rock dates for the formation of the volcanoes are listed on the map.
They have been used to
reconstruct the rates of plate motion.
Let’s do this by answering the following problems.
b)
The basalt forming the most northwest island in the Hawaiian Island Chain is 43 million years old.
The island chain has a total length of 3400 km.
On the map, draw an arrow parallel to the
Hawaiian Island Chain marking the average direction of plate motion during its formation. (5%)
c)
What was the average rate of movement of the Pacific plate in cm/y during the formation of
Hawaiian Island Chain?
In in/y?
Show all your work. (15%)
The average rate was: _______
7,097
____________ cm/y or ___________________ in/y.
100,000cm in 1 km = 340,000,000cm
340,000,000cm/ 43,000,000 years = 7,097 cm/y
MSCI 112 Homework
Full Name: Lacey Fralix
d)
What does the kink between the Hawaiian Island Chain and Emperor Seamount Chain suggest
about the direction of plate movement during the time that the Emperor Seamount Chain was
forming?
(5%)
Direction of plate is moving north west, which is shown by the direction that the
chains are moving.
e)
The age of the basalt in the northern most seamount in the Emperor Chain is 72 million years.
The length of the chain is 1900 km.
What was the average rate of movement of the Pacific plate
in cm/y during the formation of the Emperor Seamount Chain?
Show all your work. (15%)
1900 / 72 = 26.389 km
26.389 * 100,000 = 2,638,900 cm/y
f)
The spreading rate has [sped up /
slowed down
] since the formation of the Emperor Seamount
chain (circle the correct answer). (5%)
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MSCI 112 Homework
Full Name: Lacey Fralix
Plate Tectonics: Speed of Motion
Plate tectonics theory states that sea floor is formed at the mid-ocean ridges and pushed away from the
mid-ocean ridge as time goes by. In the early 1960s, scientists found that if a magnetometer (a device that
measures the intensity of the earth’s magnetic field) is towed across the ocean’s surface perpendicular to a
mid-
ocean ridge, the intensity of the earth’s magnetic field varied in a regular manner. In fact, the
variation in intensity was symmetrical about the mid-ocean ridge. If a particular piece of sea floor was
formed during
normal magnetic polarity
, the measured magnetic field would be anomalously high. If a
particular piece of sea floor was formed during
reversed magnetic polarity
, the measured magnetic field
would be anomalously low. Using radiometric dating technique, it was possible to determine the age of
the various magnetic anomalies.
The figure below shows magnetic anomaly plotted against distance and age from mid-ocean ridge for
ocean X. The mid-ocean ridge is located at distance =0 km. The ages in million years (my) of some
reversals dated with radiometric dating are shown. Dark bars represent normal polarity while white bars
represent reversed polarity.
a)
Is 1 kilometer/million years equal to 1 millimeter/year? Show your work for full credit. (5%)
b)
Calculate the rate of sea-floor spreading between 0 and 8 my and between 8 and 18.7 my. Are the
spreading rates constant? (15%)
100
betweeen 0 and 8 million years it spread 25 km / 1,000,000. It spread 300km 300/10.7 =
28.04
c)
Based on answer to question 1, what is the age of the sea-floor located 1000 km from the mid-
ocean ridge? (10%)