Unit 4 - Earth & Space Science Notes
, Plate Tectonics
Continental Drift - Plate Tectonic Theory
● Explorers in the early 1500s recognized similar shapes of coastlines. In 1912, Alfred Wegener
proposed the continental drift theory, in which he hypothesized that the supercontinent
Pangaea began to drift apart 200 million years ago. He based his theory on the jigsaw fit of
Africa and South America, fossils of the same plants and animals found on either side of the
ocean, mountains of the same age and structure formed when North America collided with
Europe and Northern Africa, and evidence that ice sheets had once covered parts of Southern
Africa, India, and Australia.
● Canada Contributes:
○ John Tuzo Wilson hypothesized that the Hawaiian islands formed over a hot spot in the
middle of the Pacific plate (a contradiction to the continental drift theory which proposes
that volcanoes only form at plate boundaries).
○ In 1963, he was instrumental in providing further support for the movement of the
continents, and he proposed the third type of boundary, transform boundary.
○ Lawrence Morley, along with British scientists Frederick Vine and Drummond
Matthews, hypothesized that the magnetic striping along the ocean floor was produced
by repeated reversals in Earth’s magnetic field. The discovery of magnetic striping
indicated that the ocean floors were moving apart.
○ Harold Williams helped to transform the notion of continental drift into the theory of
plate tectonics. In the 1970s he described how mountain belts (Appalachians)
developed from colliding continents; a sampling of the rocks he used for his analysis
are preserved in Gros Morne National Park. He has the recognition of being the most
cited Canadian geoscientist.
● As new evidence was compiled and the mechanism of movement were better understood, the
continental drift theory was changed to the plate tectonic theory in which the continental plates
moved over the denser oceanic plates.
Three Types of Plate Boundaries
● Divergent Boundaries:
○ Divergent boundaries are regions where two plates are moving apart, and as the plates
move apart, magma spews from the cracks that are formed and harden to produce new
crust. The most studied and known divergent zone is the mid-Atlantic ridge, which
extends from the Arctic Ocean to the tip of Africa. The spreading rate at the ridge is 2.5
cm/year.
○ Combined, the mid-oceanic ridges are a 50,000 km “mountain chain” wrapped around
the planet. They are 1000 km wide and the ridges range in height from 1000-3000 km
above the sea floor.
○ In east Africa a divergent zone has pulled Saudi Arabia away from the rest of Africa,
producing the Red Sea and perhaps a future ocean.
, ● Convergent Boundaries:
○ Since the Earth’s size has not changed since its birth, what happens to the new crust
that is continually being produced at divergent zones?
○ The new crust is being recycled as fast as it is being made. This occurs where two
plates are moving toward each other.
○ a) Where an oceanic and continental plate coverage, the thin, more dense oceanic
plate slips below the continental plate, completing the recycling of oceanic plates back
into the Earth. The descending plate pulls the edge of the continental plate creating
deep ocean trenches. The continental plate is lifted up and a mountain chain develops
on the overriding continental plate. The oceanic crust melts as it descends into the
Earth and some of the molten rock rises through cracks in the crust to form volcanoes.
○ The processes and features of continental-oceanic convergence can be studied along
the west coast of South America where the Nazca plate (Pacific Ocean) and the South
American plate meet. The Andes mountains, volcanic activity, and the many
earthquakes in this area are evidence of the subduction zone. A subduction zone
occurs when an oceanic plate subducts/descends under the continental plate.
○ b) Where two oceanic plates converge the same general process occurs; however,
with the formation of very deep trenches. The Marianas Trench in the Pacific Ocean
plunges 11,000 metres into the Earth’s interior. Another feature of these zones is the
volcanic island arcs.
○ c) Where two continental plates converge neither rock subducts because they are both
relatively light. Instead the rock buckles and is pushed upwards. The collision of India
into Asia 50 million years ago caused the Eurasian plate to crumple up and the Indian
plate to slide beneath (not to subduct into the Earth). Over the millions of years since,
the Himalayas and Tibetan plateau have been pushed to their present heights, the
former producing the highest mountains on Earth as high as 8,854 meters above sea
level.
, Plate Tectonics
Continental Drift - Plate Tectonic Theory
● Explorers in the early 1500s recognized similar shapes of coastlines. In 1912, Alfred Wegener
proposed the continental drift theory, in which he hypothesized that the supercontinent
Pangaea began to drift apart 200 million years ago. He based his theory on the jigsaw fit of
Africa and South America, fossils of the same plants and animals found on either side of the
ocean, mountains of the same age and structure formed when North America collided with
Europe and Northern Africa, and evidence that ice sheets had once covered parts of Southern
Africa, India, and Australia.
● Canada Contributes:
○ John Tuzo Wilson hypothesized that the Hawaiian islands formed over a hot spot in the
middle of the Pacific plate (a contradiction to the continental drift theory which proposes
that volcanoes only form at plate boundaries).
○ In 1963, he was instrumental in providing further support for the movement of the
continents, and he proposed the third type of boundary, transform boundary.
○ Lawrence Morley, along with British scientists Frederick Vine and Drummond
Matthews, hypothesized that the magnetic striping along the ocean floor was produced
by repeated reversals in Earth’s magnetic field. The discovery of magnetic striping
indicated that the ocean floors were moving apart.
○ Harold Williams helped to transform the notion of continental drift into the theory of
plate tectonics. In the 1970s he described how mountain belts (Appalachians)
developed from colliding continents; a sampling of the rocks he used for his analysis
are preserved in Gros Morne National Park. He has the recognition of being the most
cited Canadian geoscientist.
● As new evidence was compiled and the mechanism of movement were better understood, the
continental drift theory was changed to the plate tectonic theory in which the continental plates
moved over the denser oceanic plates.
Three Types of Plate Boundaries
● Divergent Boundaries:
○ Divergent boundaries are regions where two plates are moving apart, and as the plates
move apart, magma spews from the cracks that are formed and harden to produce new
crust. The most studied and known divergent zone is the mid-Atlantic ridge, which
extends from the Arctic Ocean to the tip of Africa. The spreading rate at the ridge is 2.5
cm/year.
○ Combined, the mid-oceanic ridges are a 50,000 km “mountain chain” wrapped around
the planet. They are 1000 km wide and the ridges range in height from 1000-3000 km
above the sea floor.
○ In east Africa a divergent zone has pulled Saudi Arabia away from the rest of Africa,
producing the Red Sea and perhaps a future ocean.
, ● Convergent Boundaries:
○ Since the Earth’s size has not changed since its birth, what happens to the new crust
that is continually being produced at divergent zones?
○ The new crust is being recycled as fast as it is being made. This occurs where two
plates are moving toward each other.
○ a) Where an oceanic and continental plate coverage, the thin, more dense oceanic
plate slips below the continental plate, completing the recycling of oceanic plates back
into the Earth. The descending plate pulls the edge of the continental plate creating
deep ocean trenches. The continental plate is lifted up and a mountain chain develops
on the overriding continental plate. The oceanic crust melts as it descends into the
Earth and some of the molten rock rises through cracks in the crust to form volcanoes.
○ The processes and features of continental-oceanic convergence can be studied along
the west coast of South America where the Nazca plate (Pacific Ocean) and the South
American plate meet. The Andes mountains, volcanic activity, and the many
earthquakes in this area are evidence of the subduction zone. A subduction zone
occurs when an oceanic plate subducts/descends under the continental plate.
○ b) Where two oceanic plates converge the same general process occurs; however,
with the formation of very deep trenches. The Marianas Trench in the Pacific Ocean
plunges 11,000 metres into the Earth’s interior. Another feature of these zones is the
volcanic island arcs.
○ c) Where two continental plates converge neither rock subducts because they are both
relatively light. Instead the rock buckles and is pushed upwards. The collision of India
into Asia 50 million years ago caused the Eurasian plate to crumple up and the Indian
plate to slide beneath (not to subduct into the Earth). Over the millions of years since,
the Himalayas and Tibetan plateau have been pushed to their present heights, the
former producing the highest mountains on Earth as high as 8,854 meters above sea
level.
