Download U4-T2.4-Evidence for Plate Tectonics

Earth Systems 3209
Unit: 4
The Forces Within Earth
Reference:
Chapters 4, 15, 16, 19; Appendix A & B
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Unit 4:
Topic 2.4
Evidence which Supports Plate Tectonics
Focus on . . .
 outlining and describing current evidence which
supports the theory of plate tectonics.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
 The main evidence to support the idea of plate tectonics
focuses on the different plate boundaries. The many
different features seen at these boundaries provide
overwhelming proof that the sea floor is indeed moving,
in fact, it is being recycled!
 Evidence supporting Plate Tectonics Include:
1) Earthquakes and Volcanoes
2) Paleomagnestim
A) Polar wandering
B) Magnetic Reversals and Seafloor Spreading
3) Deep Ocean Drilling
4) Hot Spots
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Text Reference:
Pages 521 – 527
and 540 - 547
Evidence For Plate Tectonics
1) Earthquakes and Volcanoes
 Earthquakes and volcanoes do not occur
randomly throughout the world, but occur
in rather limited belts. These belts mark
the location of Plate Boundaries.
 The largest active belt in the world
surrounds the Pacific Ocean and is
referred to as “The Pacific Ring of Fire.”
90% of all the world’s earthquakes occur
there. Some of the more famous
volcanoes are found surrounding the
Pacific ocean.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Mt.
Fuji
Evidence For Plate Tectonics
1) Earthquakes and Volcanoes
 These boundaries are areas where compressional
forces cause tectonic plates to move toward one
another and stress builds up. When the stress is
to great, fractures (faulting) may occur within the
tectonic plates and the plates may suddenly slip
which results I an earthquakes.
 The boundaries are also places of high heat flow,
which are characterized by molten rock rising to
the surface and forming volcanoes.
Example: Mount Saint Helens in USA.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
2) Paleomagnetism (Fossil Magnetism)
 The permanent magnetism in rocks which indicates the
direction of the magnetic field when the iron rich minerals
became magnetized.
 The most persuasive evidence to support the Plate Tectonic
Theory comes from the study of Earth’s magnetic field. The
ideas of polar wandering and magnetic reversals on the
ocean floor provide this evidence.
 Basaltic rocks contain iron-rich minerals which become
magnetized in the direction of the magnetic field at the time
when the rock solidified. If the rocks move the magnetism in
the rocks retain its original magnetic alignment and “remember”
the location of the magnetic poles at that time.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
2A) Polar Wandering
 Is the apparent movement of the
magnetic poles as outlined from
studying the magnetism fossilized in
successive basaltic lava flows ranging
in age over millions of years.
 A plot of this magnetism showed that
the magnetic pole appeared to change
position considerably over the past 500
million years.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
2A) Polar Wandering
 This was clear that either the magnetic pole
had moved with time, an idea known as
polar wandering, or the basaltic lava flows
had moved, explained by continental drift.
 Plate Tectonic theory is believed to be the
best explanation for polar wandering. If the
magnetic poles remain stationary, then their
apparent movement was caused by the
drifting of continents.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
2B) Magnetic Reversals and Seafloor Spreading
 Paleomagnetism also provided evidence for the Plate
Tectonic Theory when scientist discovered that the
magnetic field reverses polarity. Basaltic lavas solidifying
during a time of reverse polarity would display opposite
magnetism as rocks forming today (normal polarity).
 Rocks with magnetism the same as our present magnetic
field is said to have normal polarity, while rocks with
opposite polarity is said to have reverse polarity.
Normal Polarity
Convection Currents
Reverse polarity
Asthenosphere
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
2B) Magnetic Reversals and Seafloor Spreading
 This alternating magnetic polarity can be seen in:
1) successive lava flows making up a volcano;
2) the basaltic rock making up the ocean floor.
 At oceanic ridges the plates move apart and new basaltic rock is
added to each plate. The magnetism of these basaltic rocks
appears to alternate to produce identical magnetic patterns on both
sides of oceanic ridges. This proved to be the strongest evidence
to support seafloor spreading and therefore Plate Tectonics.
Normal Polarity
Convection
Currents
Reverse polarity
Asthenosphere
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
3) Ocean Drilling
 From 1968 to 1983, the Deep Sea Drilling Project
collected convincing evidence confirming the seafloor
spreading idea and the Plate Tectonic theory.
 Drill core samples of the ocean floor and sediments on
the ocean floor were collected with increasing distance
from ocean ridges.
Younger
Older
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
3) Ocean Drilling
 When the oldest sediment from each drill
site was plotted against the distance from
the ocean ridge, it was noted that the age
of the sediment increased with increasing
distance from the ridge.
 This evidence also confirmed the idea
that the ocean basins are relatively
young, because no sediment older than
180 million years was found. Continents
were dated to be 4.6 billion years.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
4) Hot Spots
 Mapping of the seafloor in the Pacific revealed a chain
of volcanoes and seamounts that extend from the
Hawaiian Islands to the Midway Islands and continue
north to the Aleutian trench of the coast of Alaska.
 Scientist proposed that a plume of magma presently
exist beneath Hawaii and the Pacific plate moved over
this stationary magma chamber.
Older
Asthenosphere
Hot
Spot
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Evidence For Plate Tectonics
4) Hot Spots
 This confirmed that the tectonic plates do move in
relation to Earth’s interior thereby supporting the theory
of Plate Tectonics.
 Radioactive age dates of the seamounts and volcanic
islands confirm that the age increases the farther away
you go from Hawaii, and the hot spot.
Older
Asthenosphere
Hot
Spot
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Example 1:
Based on the information below, which is
true of letters “X” and “Y”?
(A) different age, different polarity
(B) different age, same polarity
(C) same age, different polarity
(D) same age, same polarity
What do apparent polar wander paths obtained by
paleomagnetism tell us?
(A) the earth’s magnetic poles have moved
(B) the earth’s magnetic poles have reversed
(C) the earth’s rotational axis has moved
(D) the lithospheric plates have moved
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Example 2:
Which plate tectonic evidence is described in the information below?
As igneous rocks cool at mid oceanic ridges, the iron-rich
minerals align to the Earth’s poles.
(A) deep-ocean drilling
(C) paleomagnetism
(B) hot spots
(D) Wadati-Benioff Zone
What contributes to the reverse magnetic patterns found on the ocean
floor near ridges?
(A) alternating iron and nickel deposits
(B) alternating bands of basaltic and granitic rocks
(C) alternating magnetic polarity of Earth’s magnetic field
(D) alternating magnetic attraction of Mars and Venus on the Earth
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Your Turn . . .
Take the time and complete the following questions . . .
(Solutions to follow)
Question:
With the aid of a labelled diagram, explain how hot spots
can be used to support the Theory of Plate Tectonics.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Solutions . . .
Question:
With the aid of a labelled diagram, explain how hot spots can be
used to support the Theory of Plate Tectonics.
Answer:
The hotspot can prove the idea of plate
tectonics because we know there are areas of
hot spots in Earth’s interior. They burn their
way through the crust. If the crust didn’t move
then there would be one very large shield cone
above the hot spot. Instead, we see a chain of
volcanic islands that are increasing in age as we
move away from the hotspot. The differing ages
show the direction that the plate is moving. It
moves in the direction that the islands get older.
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador
Summary . . .
Overview of Points covered:
Evidence supporting Plate Tectonics Include:
1) Earthquakes and Volcanoes
2) Paleomagnestim
A) Polar wandering
B) Magnetic Reversals and Seafloor Spreading
3) Deep Ocean Drilling
4) Hot Spots
Copyright © 2014 All rights reserved, Government of Newfoundland and Labrador