Complete Lab 4
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Earth Sciences 1023/2123
Lab # 4
Plate Tectonics
1
1.
Purpose
The objective is to provide an overview of the structures and process of Plate Tectonics.
2.
Theory
Earth’s
Interior
Earth’s interior is divided into four major layers. At the center of Earth is a solid metallic
sphere called the
inner core
, which is 2432 km in diameter. This solid inner-most region is
composed mainly of iron (Fe), with nickel (Ni) and a minor amount of other elements
present. Surrounding the inner core is a layer of mobile liquid called the
outer core
, which
occupies the shell between radii 1216 km and 3486km (or 2270 km wide). This liquid layer is
mainly Fe (with 10%Ni) and lighter elements such as sulfur (S), oxygen (O) and/or silicon
(Si), which lowers its melting point. Above the outer core, is the
mantle
, a thick, plastic-like
layer which is capable of flow over long timescales. It is 2885 km thick. This rocky layer is
composed of silicate minerals that are rich in Si, O, magnesium (Mg) and Fe. The mantle is
overlain by a very thin, rigid outermost layer called the
crust
. The crust contains both oceanic
crust, which is covered by our planet’s oceans, and continental crust, which is the land on
which we live. The oceanic crust varies from 5-10 km in thickness, and is composed of a
dark, igneous rock called basalt. Continental crust varies on average from 30-40 km in
thickness, and it has a composition similar to the igneous rock granite. Continental rocks are
generally lighter in color and less dense than oceanic rocks. In addition to the four major
layers of Earth, there is a hot, weak zone within the upper mantle. This upper mantle zone is
called the
asthenosphere
. Above the asthenosphere is the solid, rigid outer portion of the
Earth called the
lithosphere
. The lithosphere therefore consists of all of the crust and the
uppermost mantle. The weak rock within the asthenosphere allows the Earth’s lithosphere to
move and drift around on the surface.
Continental
Drift
Our understanding of Earth’s interior supports the hypothesis known as Continental Drift.
The Continental Drift hypothesis is credited to Alfred Wegener, a German scientist who
proposed this idea in the early part of the 20th century. Wegener proposed that a
supercontinent, named Pangaea meaning “all earth”, existed over 200 million years ago.
About 180 million years ago, this supercontinent began breaking into smaller continents,
which then “drifted” to their present positions. Fig. 1 illustrates the distribution of the
continents on the globe from the time of Pangaea to today. Eventually, the continents are
expected to form another supercontinent. Many different future supercontinent scenarios are
possible, but the most accepted one is called Pangaea Proxima (future Pangaea). There are at
least four key pieces of evidence that are used to support Wegener’s hypothesis of Continental
Drift.
1
This lab is based on an interactive module created by Tasa Graphic Arts, Inc. and the activity book Dynamic Earth by Prentice Hall Science, Inc.
1
(A) Fit of the Continents
Wegener first suspected that the continents might have been joined when he noticed the
similarities between the coastlines of South America and Africa. He then proceeded to make a
crude jigsaw puzzle fit of all of the continents.
(B) Fossil Evidence
Fossils of the same ancient plants and animals are found today on continents widely separated
today by seawater oceans including; fossils of Cynognathus, a reptile that existed around 240
million years ago, in South America and Africa; fossils of Mesosaurus, a reptile that swam in
freshwater lakes and rivers about 260 million years ago, in South America and Africa; fossils
of Lystrosaurus, a reptile that roamed the land around 240 million years ago, in Africa, India,
and Antarctica; and evidence of Glossopteris, a plant which existed around 260 million years
ago, appears across all of these continents.
(C) Evidence from Rocks
When a typical jigsaw puzzle is completed by fitting all the pieces together, a complete
picture emerges and the picture must be continuous. The picture that must be continuous in
the “Continental Drift Puzzle” is represented by the rock types and mountain belts. The
Appalachian Mountains trend northeastward along the eastern flank of North America and
disappear off the coast of Newfoundland. Mountains of about the same age and structure are
found in the British Isles and Scandinavia. When these land masses are placed in their pre-
drift locations, these ancient mountain chains form a nearly continuous belt.
(D) Climatic Evidence
Earth scientists have learned that about 250 million years ago, vast ice sheets covered
extensive portions of the Southern Hemisphere’s land masses. The glaciated area on Africa
actually extends into the equatorial region. Today, scientists realize that the areas containing
these ancient glaciated landscapes were joined together in the single supercontinent that was
located far south of their present positions. Scientists have rejected the idea that Earth was
simply colder at that time because during this period, large tropical swamps existed in the
Northern Hemisphere. These swamps, with their lush vegetation, eventually became the
major coal fields of the eastern United States, Europe, and Asia.
Plate
Boundaries
During the years that followed Wegener’s proposal, great advances in technology permitted
mapping of the sea-floor. By 1968, these and other discoveries led to the unfolding of a far
more encompassing theory than Continental Drift, known as Plate Tectonics. The theory of
Plate Tectonics holds that the outer, rigid crust of Earth currently consists of about 20
segments called plates. According to the Plate Tectonic model, each plate moves as a distinct
unit. As the plates move, the distance between two cities on the same plate, New York and
Denver, for example, remains constant while the distance between New York and London is
continually changing. Fig. 2 shows the current plate boundaries.
Divergent Boundaries
Plates move apart at divergent boundaries resulting in upwelling of molten material from the
upper mantle to create new sea floor. Most divergent boundaries (also called spreading
centers) are associated with mountainous areas on the ocean floor called oceanic ridges. As
the plates spread apart, the gap is filled with molten rock that oozes up from the hot mantle.
The molten rock cools slowly to make new sections of sea floor. As these plates move apart,
new oceanic crust is created at an average of ~5 cm each year.
Convergent Boundaries
Older crust is being destroyed at convergent boundaries, where plates collide. I) When two
oceanic plates converge, one of them is bent downward and can slide beneath the other, which
produces an oceanic trench. The region where the oceanic plate descends into the
asthenosphere is called a subduction zone. As the plate plunges into the hot mantle, it begins
to melt and magma rises to the surface. A chain of volcanoes, called a volcanic arc, can form
on the surface. An example of a volcanic arc resulting from ocean-ocean convergence is the
Japanese Islands. II) During oceanic-continental place collisions, the oceanic plate, which is
the denser plate since it is composed of denser rock than continental plate, will subduct
underneath the continental plate. The Cascade mountain range, which includes Mt. St. Helens
in Washington State, is an example of this kind of volcanic arc resulting from ocean-continent
convergence. III) When two continental plates collide, neither plate can dive beneath the other
because continental crust is light and cannot sink into the mantle. The colliding plates buckle
to form huge mountain ranges. The Alps in Europe, as well as the Ural Mountains and
Himalayan Mountains in Asia, are results of continent-continent plate convergence.
Transform Fault Boundaries
Transform faults occur where sections of plates slide past one another without the production
or destruction of crust and without any vertical movement. Transform faults were first
identified where they join segments of the oceanic ridge system. Where transform faults
connect two ridge crests, the crust on one side of the fault is moving in a direction opposite of
that on the other side - the plates slide or grind past one another. Although transform faults
usually occur within the oceanic crust, some are located at the edge of a continent, such as the
San Andreas fault in California, which is part of an ancient oceanic ridge system.
Earthquakes
As rock is deformed by large tectonic forces in the lithosphere, it can bend and store elastic
energy (like a stretched rubber band). Once the rock is strained beyond its breaking point, it
ruptures, releasing the stored-up elastic energy as vibrations of an earthquake. Most
earthquakes are produced at depths of less than 70km where rocks are relatively cold and
brittle and these are called shallow focus earthquakes. Only shallow focus earthquakes occur
along the oceanic ridge system where cold oceanic crust is pushed away from the ridge. The
hot, mobile rocks of the asthenosphere and deeper parts of the upper mantle are ductile and
are not capable of storing elastic energy. Therefore, the mechanism of brittle failure cannot
produce earthquakes at these depths. Yet, earthquakes at depths of nearly 700 km are known.
Since subduction zones are the only places where cold slabs of oceanic crust reach great
depths and gradually warm up, these are the only sites where so-called deep focus
earthquakes occur. The mechanism of deep focus earthquakes
2
is however very different than
for shallow focus earthquakes and is not discussed here.
2
Officer, T. and Secco, R.A., 2020,
Detection of high P,T transformational faulting in Fe
2
SiO
4
via in-situ acoustic emission:
Relevance to deep-focus earthquakes;
Physics of the Earth and Planetary Interiors
,
https://doi.org/10.1016/j.pepi.2020.106429
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Hot
Spots
Mapping of seamounts in the Pacific basin revealed a chain of volcanic mountains that extend
for about 6000 km. This group of volcanic structures extend from the Hawaiian Islands to the
Aleutian trench. Age dating of this chain indicated that the volcanoes get older with
increasing distance from Hawaii towards the northwest. A rising plume of hot mantle material
is located beneath the island of Hawaii. Such plumes result in a volcanic zone called a hot
spot. Most plumes are believed to remain stationary during their entire existence of perhaps
100 million years. As the Pacific plate moves over the Hawaiian hot spot, successive volcanic
structures are generated. The age of each volcano indicates how long ago it was located over
the mantle plume. Thus, a chain of volcanic mountains rising from the sea-floor is called a
hot spot “track” and it can provide a method of tracing the direction and speed of plate motion
in the past. More than 50 hot spots have been identified and many others probably exist.
Approximately a dozen hot spots are located along the Mid-Atlantic Ridge. It has been
proposed that ~200 million years ago, these and other hot spots contributed to the
fragmentation of Pangaea. Recent seismological studies show that many hot spots have their
origin at the core-mantle boundary at a depth of 2890km below Earth’s surface.
The
Driving
Mechanism
of
Plate
Motions
One of the early proposals suggested that large convection currents in the mantle drive the
plates. The hot, less dense material of the lower mantle rises and spreads laterally just below
the surface. Where upwelling mantle flow (vertical) splits into two limbs and change direction
(horizontal), the underside of the lithosphere is dragged along with the flow, splitting the
continent and producing a new ridge system. Eventually, the material cools and begins to sink
back into the mantle in the process of subduction. Partly because of its cyclical simplicity, this
proposal has had a wide appeal. However, it is now understood that flow in the mantle is
more complex than simple convection cells. Other mechanisms that may contribute to plate
motion have been suggested. Ridge-push is a form of gravity sliding caused by the elevated
position of crust at ocean ridge crests where the cold oceanic crust is pushed away from the
ridge. By contrast, slab-pull occurs as oceanic lithosphere gradually cools and becomes
denser than the asthenosphere and begins to sink at a subduction zone. Slab-pull is also
known as the “table cloth mechanism” where a pulling force drags the table cloth (draping
over the side of a table) and all dishes on top with it in the same way that the gravity force
(caused by cold and dense lithosphere) pulls the oceanic plate toward the subduction zone.
Another model suggests that upward flow in the mantle is confined to hot, narrow plumes,
while the downward flow occurs where cold, dense plates subduct. Perhaps a combination of
all of these mechanisms generate plate motion.
3.
Exercises
Complete the exercises on pages 6-10, and hand in your completed lab by the end of the
laboratory session. Use the figures on page 5 as aids to some questions.
Fig. 1:
Continental drift. Illustration of the distribution of landmasses during the past 225
million years.
https://www.britannica.com/science/plate-tectonics/Development-of-tectonic-theory
Fig. 2:
Plate boundaries and tectonics diagram. Convergent boundaries are in red, with the teeth
on the overriding plate, divergent boundaries are in white, and transform boundaries are in
orange. Earthquake activity is shown by the yellow circles.
https://serc.carleton.edu/details/images/181641.html
Earth Sciences 1023/2123
Lab #4
Plate Tectonics
NAME: Carlo Fernando
Lab Section: Monday 11:30 to 1:30
Answer all questions. Questions 1-20 are valued at a total of 45 marks and question 21 is valued
at 15 marks, for a total of 60 marks.
1.
Label the 4 major layers of Earth on figure (a) and the 5 layers indicated by an arrow
or line on figure (b).
(a)
(b)
1a) Crust, Mantle, Outer Core, Inner Core
1b) Top to Bottom: Crust/Moho, Athenosphere, Lithosphere,
Upper Mantle, Mantle
2.
(i) What is the layer that is liquid?
Outer Core
(ii) What is the layer that is the thinnest?
Crust
(iii) What is the layer with the greatest thickness?
Mantle
(iv) What is the layer that is a solid metallic sphere?
Inner Core
3.
What is Earth’s solid, rigid outer layer?
(i) Lithosphere
(ii) Asthenosphere
(iii)
neither
of
the
above.
4.
What is composed only of continental crust?
(i)
Lithosphere
/60
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(ii) Asthenosphere
(iii) neither of the above.
Amasia
Pangaea Proxima
5.
What is the hot, weak zone found within the upper mantle?
(i) Lithosphere
(ii)
Asthenosphere
(iii) neither of the above.
6.
What layer allows Earth’s rigid outer shell to move?
(i) Lithosphere
(ii)
Asthenosphere
(iii) neither of the above.
7.
(i) Which region is composed primarily of basalt?
Oceanic Crust
(ii) Which region is composed of iron and magnesium rich silicate minerals?
Mantle
(iii) Which region is composed primarily of iron?
The Core
(iv) Which region is composed of material similar to the rock granite?
Continental Crust
8.
Based on Fig 2., which of these scenarios of a future supercontinent seems
most probable?
Novopangaea
9.
What was Pangaea?
(i) a fossil amphibian
(ii) the name of the continental drift hypothesis
(iii)
the
name
of
the
most
recent
supercontinent
10. When did Pangaea exist?
(i)
about
200
million
years
ago
(ii) about 600 million years ago
(iii) about 2 billion years ago
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11. Who proposed the continental drift hypothesis?
(i) Harry Hess
(ii)
Alfred
Wegener
(iii) Fred Vine
12. Label each of the six pieces of evidence of a supercontinent in the boxes in the
diagrams below. Add description below the box if needed.
Continental Fit
I cant put them into the boxes on word
Fossil Evidence
Evidence from rocks
Climatic Evidence
Recently Formed ocean ridges and trenches
Mountain ridges aligning with different mountains on another continent
13. From Fig 2., which of the plates below is/are entirely composed of oceanic crust?
(i) North American Plate
(ii)
Indo-Australian
Plate
(iii) Pacific Plate
Oceanic-Oceanic plate boundary -----> 1
Oceanic-Continental plate boundary ---- 3
Continental-Continental plate boundary ---2
1 Volcanic arc
2 Mountain
3Island arc
14. According to the plate model, select all that apply.
(i) Continental plates move in the opposite direction of oceanic plates
(ii)
Plates
move
as
units
(iii) Continental plates move faster than oceanic plates
(iv)
The
distance
between
cities
on
different
plates
is
always
changing
15. All major plate interactions occur:
(i)
Along
plate
margins
(ii) Near the center of each plate where stress is greatest
(iii) On the underside of each plate
16. Most divergent boundaries are located:
(i) In mountainous regions such as the Alps and Himalayas
(ii) In the deep-ocean trenches
(iii)
At
the
oceanic
ridges
17. When two plates converge:
(i) Only the oceanic crust can slide into the mantle
(ii) Only the continental crust can slide into the mantle
(iii)
Either
the
oceanic
or continental
crust
may
slide
into
the
mantle
18. Match the following with arrows:
19. Select all correct statements about transform faults.
(i) New crust is formed at transform faults
(ii)
Sections
of
plates
slide
past
each
other at
transform
faults
(iii) Oceanic trenches are produced along transform
faults
(iv)
Most
transform
faults
are
found
in
the
oceans
(v)
Transform
faults
only
cause
horizontal
movement.
20. Draw arrows on the map at right illustrating the
plate motion directions responsible for producing
the Emperor Ridge and Hawaiian Ridge.
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21. The following diagram illustrates the main Hawaiian Islands. The Big Island of
Hawaii currently experiences active hot spot volcanism.
(i) Identify on the diagram above, the age of each island based on this pool of answers:
4,000,000 years; 1,200,000 years; 0 years; 2,500,000 and 1,500,000 years.
Hawaii – 0 years, Kauai – 4 million years, Maui – 1.2 million years, Molokai
– 1.5 million years, Oahu – 2.5 million years
(ii) Using the scale shown on the diagram, measure the distance between the pairs of
islands (using the dot at the center of the islands) and enter your answers in the table
below.
(iii) Calculate the age differences between the pairs of islands and enter your answers in
the table below.
(iv) Using the following formula, calculate the speed at which the Pacific oceanic plate
was moving between the times that each of the islands formed.
𝑐?
𝐷𝑖𝑠𝑡𝑎𝑛𝑐? 𝑏?𝑡𝒃𝒃??𝑛 𝑡ℎ? 𝑡𝒃𝒃? 𝑖𝑠𝑙𝑎𝑛𝑑𝑠 (𝑐?)
𝑆𝑝??𝑑 ?? 𝑝𝑙𝑎𝑡? ??𝑣???𝑛𝑡 (
𝑦?
) =
𝐷𝑖?????𝑛𝑐? 𝑖𝑛 𝑎𝑔?𝑠 ?? 𝑡ℎ? 𝑡𝒃𝒃? 𝑖𝑠𝑙𝑎𝑛𝑑𝑠 (𝑦?)
(v) Calculate the average speed (cm/yr) of the Pacific plate from Kauai to Hawaii. This is
T𝐓𝐓𝐓𝐓𝐓𝐓𝐓𝐓
Distance
T𝐓𝐓𝐓𝐓𝐓𝐓𝐓𝐓
Age
Difference
,
not
the average of the four calculated speeds.
Enter your answers in the table below.
Islands
Distance
(km)
Distance (cm)
Age difference
(yr)
Speed
(cm/yr)
Average
speed
(cm/yr)
Hawaii
and Maui
100 kms
1 x 10^7
1.2 Million
8.33 per year
8.75 cm per
year
Maui and
Molokai
60
6 x 10^6
300 thousand
20 cm per
year
Molokai
and Oahu
80
8 x 10^6
1 Million
8 cm per year
Oahu and
Kauai
110
1.1 x 10^7
1.5 Million
7.33 per year