Evidence For Continental Drift
Introduction
 As Scientific Theories go, Plate Tectonics is a relatively
new scientific concept, introduced in the 1960’s.
 It is the most revolutionary idea in Earth Sciences – it
has changed our notion of a generally quiet ,
unchanging planet (Uniformitarianism) into the
understanding that the Earth is a dynamic planet.
Plate Tectonics – the Unifying
Theory of Earth Sciences
 The theory has unified the study of the Earth by
drawing together many branches of the earth sciences,
from paleontology (the study of fossils) to seismology
(the study of earthquakes).
 It has provided explanations to questions that
scientists had speculated upon for centuries -- such as
why earthquakes and volcanic eruptions occur in very
specific areas around the world, and how and why
great mountain ranges like the Alps and Himalayas
formed.
Look At The Globe
 By examining a globe, one can
observe that many of the
continents seem to fit together
like a puzzle: the west African
coastline seems to fit nicely
into the east coast of South
America and the Caribbean
sea; and a similar fit appears
across the Pacific.
 The fit is even more striking
when the submerged
continental shelves are
In 1858, geographer Antonio Snidercompared rather than the
Pellegrini made these two maps showing
coastlines.
his version of how the American and
African continents may once have fit
together, then later separated
Alfred Wegener
 In 1912 Alfred Wegener (1880-1930) noticed
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that fossil encyclopedias listed fossils that
existed on opposite sides of the ocean - he
found this idea to be highly unusual.
Wegener then noticed that the continents
seemed to fit together.
By linking continents together, the fossil
regions would be joined together.
Wegener proposed that the continents were
once joined into a single supercontinent
which he called Pangaea (meaning "all
lands").
He believed that over time the continents
have drifted apart into their current
distribution.
Wegener is generally regarded as the father
of the idea of “Continental Drift”
Wegener was not only well
known for his theory of
Continental Drift
(Kontinentalverschiebung)
but also for his work in
Meteorology. He studied
polar air circulation in
Greenland which contributed
to the discovery of the Jet
Stream. Wegener also
collected ice drilling samples
of the Greenland Glaciers.
Wegener died while working
in Greenland at the age of 50.
Continental Drift
 Wegener believed that Pangaea
was intact until the
late Carboniferous period,
about 300 million years ago,
when it began to break up and
drift apart.
 Wegener's hypothesis lacked a
geological mechanism to
explain how the continents
could drift across the earths
surface as he proposed.
Background PowerPoint
 http://faculty.uml.edu/Nelson_Eby/87.201/Instructor
%20pdfs/Plate%20Tectonics%20Class.pdf (University
of Massachusetts at Lowell)
You Tube Video
 https://www.youtube.com/watch?v=X9_TIf2navc
(21:41 - Wegener’s Evidence)
 https://www.youtube.com/watch?v=NwqX4OLJRe4
(10:42 - Continental Drift)
 https://www.youtube.com/watch?v=0KNqUwgqbZw
(Bill Nye - 100 Greatest Discoveries - 47:00)
Continental Drift
 Wegener’s Hypothesis was not widely accepted until
the 1950’s.
 The following evidence made his hypothesis more
acceptable to the Scientific community:
1) Coastline Matching
2) Fossils collected in Africa and South America
3) Ocean Floor Exploration
4) Patterns of Seismic Activity (Earthquakes and
Volcanoes)
5) Glaciation
1) Matching Coastlines
 Continents, especially if you
include the continental margins,
fit together very clearly as seen in
the globe at right.
 The suggestion is that the
continents were joined together
and broke apart and moved into
their present positions.
 More proof was needed to add
strength to this hypothesis.
Geologists looked to match rock
formations on continents now
separated by oceans.
 Similarly, paleontologists looked
for similar trends with fossil
assemblages.
Continental Fit and Mountain Ranges
Appearance of same rock
sequences and mountains of
the same age on continents now
widely separated.
Major time of mountain
formation ended about 250 mya
(million years ago), before
Pangaea split
11
TLTH, fig. 1.15
TLTH, fig, 1.13
2) Fossil Data
 Fossils within
specific stratigraphic
sequences were
found on opposite
sides of oceans.
 When the continents
were pieced together,
the fossil sequences
merged.
Mesosaurus, a reptile
Possible Explanations for Fossils
being separated by oceans:
Possible
explanations?
3) Ocean Floor Exploration
 In the 1950’s and 1960’s, the
first scientific exploration of
the seafloor was undertaken.
Among the data collected
were depth measurements
(using precision depth
sounding), sea-floor
sampling (and subsequent
radiometric age dating),
paleomagnetic readings.
Glomar Challenger
Harry Hess - Sea-Floor Spreading
 https://www.youtube.com/watch?v=GyMLlLxbfa4&lis
t=PL8aTbGWgTg6U95JvrN-g2mCnz7QNPs9r4 (Bill
Nye - 4:01)
 Geology professor Harry Hess acted as a naval
transport captain during WWII. While travelling the
Pacific Ocean, Hess discovered that there was a
mountain range under the ocean.
 In 1953, Oceanographers discovered the mid-ocean
ridge. The ridge is as high as 2500 m above the sea
floor.
Harry Hess - Sea-Floor Spreading
 Hess followed up by studying the age of rocks and
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sediments on the ocean floor by using core samples and
sonar data.
He makes the surprising discovery that the sea floor is
youngest at the mid-ocean ridges and that the crust gets
older as you move away from the ridge.
He also learns that the mid ocean ridge is a line of
underwater volvcanoes where new crust is being formed.
Hess discovered that new crust is produced at the mid
ocean ridge and is pushed away from the ridge as new
ocean crust (basalt) is created.
Hess discovers that the Sea Floor is Spreading.
Sea Floor Spreading
 Hess reasoned that if:
a) new crust is being created at mid-ocean ridges
b) the Earth is not increasing in size
 Then crust must be destroyed somewhere else on the
planet.
 Hess suggests the idea of subduction zones at the
margins of some continents.
 Hess’ Sea-Floor Spreading resurrects Wegener waning
Hypothesis of Continental Drift and leads to the larger
idea of Plate Tectonics.
Ocean Floor Exploration
 Mapping of the ocean floor
revealed the presence of
mid-ocean ridges and
trenches. Mid-ocean ridges
constitute rift zones that
form interconnected sub-sea
mountain ranges that span
the globe.
 The Glomar Challenger was
the famous exploration craft
used to sample the ocean
floor.
Ocean Floor Exploration
 The following topographic features can be found on the
seafloor – mid-ocean ridge, continental slope, continental
shelf, trenches, abyssal plains and volcanic islands.
Ocean Floor Exploration
 A cross-section of an ocean does not show a featureless
topology. Exploration of the ocean floor showed the existence
of mid-ocean ridges, continental shelves and deep trenches.
4) Patterns of Seismic Activity
 Seismic activity such as earthquakes or volcanoes do
not occur in random locations.
 The vast majority of seismic activity occurs at the
boundaries of plates (large sections of oceanic and
continental crust).
 In fact, by plotting the location of major seismic events
Trenches are deep troughs associated with active
earthquakes zones and chains of volcanoes.
Location of Major Earthquakes
Location of Major Plates
Glaciation
 Patterns of Glaciation were matched across continents.
 When continents were joined the patterns made more
sense.
Further work
 http://www.suu.edu/faculty/colberg/hazards/platetectonic
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s/platetectonics.html
http://www.sciencecourseware.org/eec/Earthquake/
http://hyperphysics.phyastr.gsu.edu/hbase/geophys/platevid.html
http://pubs.usgs.gov/gip/dynamic/developing.html
http://www.ucmp.berkeley.edu/geology/tecmech.html
http://scign.jpl.nasa.gov/learn/plate4.htm
Sea floor spreading:
http://www.suu.edu/faculty/colberg/hazards/platetectonic
s/30_SeaFloor.html
New Evidence Emerges
1) Paleomagnetism
2) Magnetic Pole Reversals
3) Magnetic Striping
4) Earthquake Mapping
Still a mechanism is required
1) Paleomagnetism
Since the Earth has a
liquid iron-nickel core,
the planet generates a
strong magnetic field
(as seen in the
diagrams at right)
2) When igneous rocks
containing a magnetic
element (iron, nickel
and cobalt) cool from a
liquid state, the grains
of the element will
align with the
magnetic field.
1)
Paleomagnetism
3) As a result the direction
of the magnetic field at
the time of their
solidification is
preserved.
4) These rocks can be used
to pinpoint the direction
of magnetic north at the
time of solidification.
Paleomagnetism
5) Rocks that move across the globe due to continental
drifting can be traced back to their original location.
6) This allows geologists to track the movement of
continents over time.
 https://www.youtube.com/watch?v=uGcDed4xVD4
(Video showing the movement of continents over the
past 600 million years and 100 million years into the
future).
2) Magnetic Pole Reversals
 https://www.youtube.com/watch?v=BCzCmldi
aWQ (2:35)
 The direction of the Earth’s magnetic poles has
flipped many times in the past.
 We know there have been about 170 magnetic
pole reversals during the last 100 million years,
and that the last major reversal was 781,000
years ago (see chart at right - black represents
normal polarity).
2) Magnetic Pole Reversals
 These shifts leave traces in
rocks. When lava cools, metal
oxide particles within the rock
become frozen in the direction
of the prevailing magnetic field.
 It is believed that changes in
the convection of the liquid
outer core cause these flips to
occur (as well as changes in
strength of the Earth’s magnetic
field)
 It is believed that the rate of
reversals is increasing (possibly
due to the increase in the size
of the solid inner core)
The layers of (iron-rich) lava
show differing magnetic pole
directions.
3) Magnetic Striping
1) The most important
application of this property
was done during Ocean
Floor Mapping.
2) Magnetic detection devices
called magnetometers
(developed during WWII to
detect submarines)
measure the strength and
direction of the magnetic
fields found in iron-bearing
rocks such as basalt.
3) Magnetic Anomalies
3) Geologists noted a pattern of
“stripes” on the ocean floor of
normal and reverse polarity.
Most importantly, the patterns
mirrored each other on opposite
sides of a mid-ocean ridge.
4) Watch this brief YouTube video
to see how this works:
https://www.youtube.com/watc
h?v=YIAXiE8RedA
3) Magnetic Anomalies
5) The magnetic
striping leaves a
distinctive pattern
across all oceans
and can be used to
date ocean crust
and sediments.
6) The oldest ocean
crust is less than
250 million years
old.
4) Earthquake and Volcano Mapping
 The Earth’s major tectonic plates and the location of
volcanoes. Note that the vast majority are located on or
near a plate boundary.
Earthquake Depths
 This depth vs distance chart
(cross section) shows the
focus of earthquakes along
an ocean crust-continental
crust boundary.
 This evidence gives the idea
that slabs of ocean crust
descend underneath
continental crust. Crust is
“subducted” into the Earth’s
mantle.
Earthquake Depths
 The diagram shows a cross-section of the subduction zone off Japan’s
east coast. Ocean crust is subducted under more ocean crust.
 Melting of the descending slab is responsible for the chain of volcanoes
that form the Japanese islands.
 Suggest the rock type that forms these islands.
Putting the Evidence Together to
Create a Theory
 https://www.youtube.com/watch?v=X9_TIf2navc
(start at 18:00)
 https://www.youtube.com/watch?v=0KNqUwgqbZw
(Bill Nye - start at 15:25)
Mantle Convection
 Mantle Convection Video (short 0:45)
https://www.youtube.com/watch?v=p0dWF_3PYh4
 The mechanism that Wegener needed to drive
continental drift is the convection of the mantle.
 The mantle is semi-molten and convects very slowly this is the reason continents drift at an average rate of
5 cm/year.
 Rising mantle “plumes” drive sea-floor spreading and
are also responsible for mid-plate hot spots like Hawaii
and Yellowstone.
Sea Floor Spreading at Mid Ocean
Ridges - New Crust Forms
 Why is ocean crust subducted under continental
crust? (Why does continental crust “float” higher in
the mantle)
Sea Floor Spreading at Mid Ocean
Ridges - New Crust Forms
 Ocean crust (dark blue) and continental crust (grey) sit on a
semi-molten layer called the lithosphere (brown).
 https://www.youtube.com/watch?v=Kpoko_l34ZE (good video
showing mantle convection and discussing mantle layers. - 1:15)
Mantle Hot Spots - Hawaii
 Some volcanism occurs in the middle of plates.
 Rising mantle plumes are hot enough to “punch”
through the mantle and produce volcanoes that rise
from the ocean floor and in the case of Hawaii’s big
island, rise well above the surface.
Mantle Hot Spots - Hawaii
 These mantle “plumes” or hot spots remain stationary while
the oceanic plate moves over the hot spot.
 As a result, the volcano loses its source of magma and
becomes extinct and is carried (westward in the case of the
Hawaiian Islands) and a new volcano forms.
 What type of rock do we expect to find in Hawaii? What
type of volcano do we expect?
Mantle Hot Spots - Hawaii
 The older extinct volcanoes are eroded into smaller and
smaller islands (often ringed by a lagoon) and
eventually disappear beneath the surface and are called
seamounts.
Mantle Hot Spots - Hawaii
 This oceanic map shows the Hawaiian island chain and the
Emperor seamounts that were created by the same hot spot.
 The oldest seamount is 65 million years old. The oldest Hawaiian
island is 7 mya. The change in plate direction occurred 42 mya
 Notice that the plate direction changed at some point in the past.
Mantle Hot Spots
 Yellowstone happens to
be hot spot located
beneath a continent.
 What type of rock do
we expect to find at
Yellowstone? What type
of volcanism?
 https://www.youtube.c
om/watch?v=bYv6V5EJ
AKc (Great short video Hawaii’s hot spot
volcanoes - 3:05)
Subduction Zones - Crust is Subducted
and remelted into the Mantle
 When heavier basaltic ocean
crust (3.1 g/cm3) is forced
beneath lighter (and thicker)
granitic continental crust (3.1
g/cm3), the subducting slab
melts to produce volcanoes 50 300 km from the plate
boundary.
 An ocean trench is formed
where the slab begins its
descent. The deepest trench
(Marianas Trench) is almost 11
km deep.
https://www.youtube.com/watch?v=K
8J7t3KYr9c (Model using sand to
simulate mountain growth near
suduction zones - very accurate)
Subduction Zones
 The west coasts of North and
South America are these types
of subduction zones
 Volcanoes occur from Alaska
down the Rocky Mountains to
the Andes Mountains.
 This whole region is at risk
from volcanoes, earthquakes
and tsunamis. The Cascadia
Zone has been hit with
massive tsunamis in the past.
 What type of rock do we
expect? What type of
volcanism?
Subduction Zones
 Most of the largest earthquakes
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(and subsequent tsunamis) are
produced at subduction zones.
https://www.youtube.com/watch?v
=f-nIb8JkFrg (Subduction zone
earthquakes - 2:50)
https://www.youtube.com/watch?v
=Yukp0bPkQxs (45 minutes Cascadia Megaquake)
https://www.youtube.com/watch?v
=j0YOXVlPUu4 (Japanese tsunami)
https://www.youtube.com/watch?v
=w3AdFjklR50 (Japanese tsunami short video)