Grade 9 Academic Geography – Unit 3
Plate Tectonics
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Watch National Geographic Plate Tectonics -The Changing Shape of the Earth at
http://education.nationalgeographic.com/education/media/plate-tectonics/?ar_a=1
In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root "to
build." Putting these two words together, the term plate tectonics is derived. It refers to how the Earth's surface
is built of plates.
The theory of plate tectonics notes that the Earth's outermost
layer, the Crust, consists of about over 60 large and small
plates. The plates are moving relative to one another as they
ride atop hotter, more mobile, molten rock material in the
Earth’s Mantle.
From geological history, it is believed that the present-day
continents are the fragmented pieces of preexisting larger
landmasses ("supercontinents") called Pangaea (meaning "all
lands" in Greek).
In 1922, Alfred Wegener presented the Theory of Continental
Drift which states that parts of the Earth's crust slowly drift atop a liquid core. By the end of the Cretaceous era,
the continents were separated into land masses that look like our modern-day continents. NOTE: The drift
movement continues today (see below – Earthquakes).
Wegener's theory was based on the following evidence:
 Similar geologic structures (e.g., rocks) on the matching coastlines of North America and Europe
 Similar plant and animal fossils found on the matching coastlines of South America and Africa
 Dramatic climate changes on some continents AND the fossil remnants of the change (e.g., fossils of tropical
plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must
have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could
grow; distinctive fossil ferns (Glossopteris) discovered in now-polar regions; glacial deposits in present-day
arid Africa, such as the Vaal River valley of South Africa.
 The remarkable puzzle-like fit of the South American and African continents (….first noted by Abraham
Ortelius three centuries earlier).
Today, science has shown that the surface of the Earth is in a constant state of change. We observe and measure
mountains rising and eroding, oceans expanding and shrinking, volcanoes erupting and earthquakes striking. From
the video, you learned about the 1977 mapping of the seafloor using sonar, called the Tharp-Heezen map,
provided key data supporting plate tectonics and continental drift. In plate tectonics, the Earth’s solid outer crust,
the lithosphere, is separated into plates that move over the asthenosphere, the molten upper portion of the
mantle. The plates come together, spread apart and interact at their edges. Each type of plate boundary
generates distinct geologic processes and landforms.
 When plates diverge (i.e., move apart) and separate, a narrow rift valley in the newly exposed area forms.
Here, geysers spurt super-heated water, and magma (i.e., molten rock) rises from the mantle and solidifies
into basalt to form new crust. In the oceans where plates are diverging, oceanic crust is created. The midocean ridge, the Earth’s longest mountain range, is a 65,000 kilometers long and 1,500 kilometers wide
divergent boundary. In Iceland, one of the most geologically active locations on Earth, the divergence of the
North American and Eurasian plates along the Mid-Atlantic Ridge can be observed as the ridge rises above sea
level.
 At convergent boundaries, plates move together and collide. The force of the collision (1) buckles the edge of
one or both plates to create a mountain range or (2) forces one of the plates under the other (i.e., subduction)
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to form a deep seafloor trench. At convergent boundaries, continental crust is created as the hot rock rises
and cools, while oceanic crust is destroyed as it moves downward, melts and becomes magma. Convergent
plate movement also creates earthquakes and often forms chains of volcanoes. The highest mountain range
above sea level, the Himalayas, was formed 55 million years ago when the Eurasian and Indo-Australian
continental plates converged. The Mediterranean island of Cyprus formed at a convergent boundary between
the African and Eurasian plates. Hardened mounds of lava, called pillow lavas, were once on the bottom of the
ocean where this convergence occurred, but over long periods of time, the mounds were pushed up to
become visible as islands.
A transform boundary is formed as tectonic plates slide horizontally past each other. Parts of these plates get
stuck at the places where they touch, and stress builds in those areas as the rest of the plates continue to
move. This stress causes the rock to break or slip, suddenly lurching the plates forward and causing
earthquakes. These areas of breakage or slippage are called faults. An example is the San Andreas Fault along
the west coast of North America,
Task
 Read Pages 31-39 in Making Connections
 On Page 36 and using the Internet, try Plate Motion Calculator activity. NOTE: It may be difficult to
understand.
 On Page 39, answer Questions #1,2,5
Earthquakes
Earthquakes occur after the Earth’s plates build up pressure by
pushing into or squeezing past each other as each plate moves.
When plates cannot take the pressure anymore, they slip suddenly along
the space between the plates (…fault), and when the rocks along a fault
move quickly, they send out shock waves through the ground.
An earthquake happens where two blocks of the earth move past
one another. The surface where they slip is called the fault or fault
plane, the location below the earth’s surface where the earthquake
starts is called the hypocentre, and the location directly above it on
the surface of the earth is called the epicentre.
As shown, the
Earth has four major layers:
the inner core, outer core, mantle and crust. The crust
and the top of the mantle make up a thin skin on the
surface of Earth. Recall, the crust is not all in one piece –
it is made up of many plates. Moreover, the plates
slowly move, sliding past one another and bumping into
each other. The edges of the plates are called the plate
boundaries. The plate boundaries are made up of many
faults, and most of the earthquakes around the world
occur on these faults. Since the edges of the plates are
rough, they get stuck while the rest of the plate keeps
moving. When the plate has moved far enough, the
edges break apart on one of the faults and there is an
earthquake. The earthquake produces seismic waves which cause the earth to “shake.”
The Richter Scale measures the energy released by an earthquake (0 – 9.0).
 Weakest quakes noticed by people are 3.0
 5.0 will crack walls and objects will fall off shelves
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8.0+ means total destruction
Task
Answer the following questions.
1. A tsunami is a gigantic wave. How would an earthquake produce a tsunami? Explain with the support of a
diagram.
2. Mountains form from earthquake activity. How? Explain with the support of a diagram.
3. It is unlikely that an earthquake would occur near Kingston. Why?
4. How fast do you think tectonic plates move each year on average?
5. What is the Ring of Fire?
Volcanoes
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A geological landform where magma erupts through the surface of the earth.
Tend to occur near boundaries of continental plates.
Sometimes, there are dormant volcanoes. This means they were active in the past and could erupt again.
Majority are part of the Pacific Ring of Fire – global system of active volcanic mountains that circle the Pacific
Ocean. The Ring of Fire is a string of volcanoes and sites of earthquakes around the edges of the Pacific
Ocean. Roughly 90% of all earthquakes occur along the Ring of Fire, and the ring is dotted with 75% of all
active volcanoes on Earth. NOTE: The Ring of Fire is not quite a circular ring; rather, it is shaped more like a
40,000-kilometer horseshoe. Most of the active volcanoes on The Ring of Fire are found on its western edge,
from the Kamchatka Peninsula in Russia, through the islands of Japan and Southeast Asia, to New Zealand.
New Zealand’s Mount Ruapehu, 2,797 meters high, is one of the more active volcanoes in the Ring of Fire,
with yearly minor eruptions, and major eruptions occurring about every 50 years.
Task
 Using the Internet, watch National Geographic’s slideshow Ring of Fire - Pacific Ring of Fire, circum-Pacific belt
at http://education.nationalgeographic.com/education/encyclopedia/ring-fire/?ar_a=1
 Make five observations from the Internet information (e.g., What new information did I learn? What
surprised me?)