Steps to Plate TectonicsStep 1 – Continental Driftwww.mat.docx

Steps to Plate Tectonics:
Step 1 – Continental Drift
www.math.montana.edu / ~nmp / materials / ess / geosphere /
inter / activities / plate_calc / pangaea_map.gif
The Continental Drift hypothesis published by Alfred Wegener
in his 1915 book “ The Origin of Continents and Oceans”,
although this was partially based on the work of earlier
investigators.
Continental Drift = the continents were once connected in a
single supercontinent called Pangaea. They have since drifted
apart and are still moving today.
http://www.kidsgeo.com/geology-for-kids/0042-pangaea.php
Wegener’s hypothesis had several problems:
1) He had no power source – no way to make the
continents move.
2) He thought the continents moved through the seafloor
just like boats move through the ocean, but there was no
evidence of this (no wake)
3) He was a meteorologist so many geologists didn’t take
him seriously!
BUT Wegener had lots of evidence to show that the continents
were once connected!
1) The jigsaw puzzle-like fit of the continents.
Figure 2.3 in text
Identical fossil assemblages on now widely spaced continents!

best about 250-200 MY ago
become increasingly dissimilar the closer to today we
look!
Garrison, 2012, Essentials of Oceanography
3) Sequences of similar rock types on continents which do not
now have the same geologic environment!
http://www.geology.ohio-
state.edu/~vonfrese/gs100/lect25/index.html
4) Geologic structures (mountain ranges, faults, chains of
volcanoes) which match up on either side of oceans but can not
be found underwater.
5) Apparent polar wander – paleoclimatic evidence the
continents had very different climates 250 MY ago than they do
today.
either the continents moved or
the climate bands moved – which means the Earth’s poles
of rotation moved.
Earth’s climate zones today are arranged symmetrically around
the poles.
http://www.webquest.hawaii.edu/kahihi/sciencedictionary/C/cli
matezone.php
Paleoclimatic data from ˶300 MY ago, figure 2.5 in text

After Wegener died, his ideas were largely dismissed, until…
Post-Wegener evidence for drifting continents (and plate
tectonics)…
6) Apparent Polar Wander – Paleomagnetic evidence.
Figure 2.7 showing that the Earth has a magnetic field very
similar to that created by a bar magnet.
Directions of magnets parallel to Earth’s magnetic field lines.
Post-Wegener evidence for drifting continents (and plate
tectonics)…
Figure 2.7b showing how magnets align to the Earth’s magnetic
field when allowed to move freely.
Rocks containing the mineral magnetite (especially basalt)
record the orientation of the Earth’s magnetic field at the time
the rocks formed.
Figure 2.8a showing apparent polar wander paths for Europe
and North America.
Figure 2.8b showing alignment of polar wander curves if the
Atlantic Ocean is “closed”
The polar wander tracks for all the continents show great

variation, suggesting it is the continents that moved!
If we put the continents “back together” we can make their
paleomagnetic poles all line up at the true north pole!
When geologists studied the paleomagnetic record, they
discovered:
Old rocks showed that the north pole in the past was not where
the north pole is today (did the pole move or did the continent
move?)
Every continent recorded a different position for the north pole
If we “move” the continents back together – reassemble
Pangaea – the paleo-North Poles rearrange to all line up
together at the position of today’s north pole!
Other Important discoveries that confused scientists:
1) The presence of the Mid-Ocean Ridge, a chain of
underwater volcanoes that circle through the world’s oceans
like the seams on a baseball!
www.ceoe.udel.edu / extreme2001 / mission / location / images
/ ridgemap.gif
Other Important discoveries, cont.:
2) The presence of the Deep-Sea Trenches, a series of
deep basins on the seafloor next to continents or island chains.
Other Important discoveries:
3) No area of the seafloor is older than 200 MY, about
1/20th the age of the oldest continental crust, and

4) The seafloor in brand-new at the mid-ocean ridges, and
gets older the farther from the ridge we look. The oldest
seafloor is located next to the deep sea trenches.
Continental Drift: The continents were once connected in a
supercontinent called Pangaea, they have since moved apart and
are still moving today.
Problems: No power source and no “wake.”
Evidence for:
“Jigsaw puzzle” fit of the continents,
matching fossils, rocks, and geologic structures on now
widely separated continents
apparent polar wander
changing climates
“wandering” magnetic north pole
Other confusing discoveries
mid-ocean ridges
deep sea trenches
age distribution of the seafloor
none older than 200 my
newest at the mid-ocean ridge, gets older with
distance
REVIEW
REVIEW
Steps to Plate Tectonics: Step 2 – Seafloor Spreading
The Theory of Seafloor Spreading was first published in 1962
by Harry Hess, a geology professor at Princeton University. It
was based on work he did as a naval captain during WWII!
According to the Theory of Seafloor Spreading, the seafloor is
moving like a conveyor belt from its creation at Spreading

Centers (mid-ocean ridges) to its destruction at Subduction
Zones (deep sea trenches). This process is powered by mantle
convection.
http://rst.gsfc.nasa.gov/Sect2/Sect2_1b.html
Mantle Convection is similar to boiling water in a pot on the
stove, but much slower. In mantle convection, the rocks move
as fast as your fingernails grow.
Hotter, less dense, so rises
Cooler, more dense, so sinks
Evidence for Seafloor Spreading:
1) Age distribution of the seafloor
2) Presence of Mid-ocean Ridges
3) Presence of Deep Sea Trenches
4) Paleomagnetic Evidence – Seafloor Magnetic Stripes
http://portale.ingv.it/research-areas/the-earth/characteristics-of-
the-earth-s-magnetic-field
Studies of the paleomagnetic field as recorded in rocks
worldwide discovered that the Earth’s magnetic field has
reversed periodically.
N becomes S and S becomes N during one of these flips. Your
compass would point south during a reversed period!
Normal Field
Reversed Field

On land (rocks don’t move) magnetic reversals are recorded by
layers of basalt with normal and reversed magnetic polarity.
Figure 2.9
Hess said that, if the seafloor is spreading as he described, then
the magnetic reversals should be recorded as a series of normal
(+) and reversed (-) polarity stripes.
http://higheredbcs.wiley.com/legacy/college/levin/0471697435/
chap_tut/chaps/chapter07-08.html
Once scientists began to look for them, they found these
magnetic stripes in every ocean basin, suggesting that seafloor
spreading did indeed occur as Hess suggested!
Garrison, 2007, Oceanography
Magnetic stripes are sections of reversed and normal polarity
seafloor. These record the Earth’s magnetic field polarity shifts
in the same way as a tape recorder records a voice. Like a tape
recorder, the sea floor can only record these stripes if it is
“turned on”, i.e. if the seafloor is really spreading!
The Continental Drift Hypothesis showed that the continents
move, but couldn’t explain how or why.
The Theory of Seafloor Spreading showed that the seafloor was
moving and explained how, but it did not explain the moving
continents.
? ? ? ? ? ? ?

? ? ? ? ? ? ?
According to Plate Tectonics, the Earth is covered by a small
number of lithospheric plates that move relative to each other.
This movement is powered by mantle convection.
The oceanic portion of each plate moves as described in
seafloor spreading, carrying the continental portion of the plate
with it!
Lithosphere = the top 100 km of the Earth = cold and rigid
material.
Mantle Convection = the slow boiling of the rocks in the
mantle due to the heat released during radioactive decay.
Garrison, 2009, Oceanography, Introduction to Marine Science
The 7 major plates: North American, South American, Indo-
Australian, African, Eurasian, Pacific, and Antarctic; plus some
minor plates. Note that most plates contain both continental
and oceanic lithosphere!
Plate Boundaries are the regions where tectonic plates interact
or move relative to each other. There are three types of
boundaries:
1) divergent – New plate is formed (constructive).
2) convergent – Old plate is destroyed (destructive).
3) transform – Plate size is constant (conservative).

Garrison, 2012, Essential of Oceanography
Simple plate model showing the three types of plate boundaries
formed when plate A moves westward relative to plates B and
C.
The location of plate boundaries are marked by belts of
earthquakes and volcanoes
Plate Boundaries, arrows indicate direction of plate motion,
numbers show rate of motion.
Table 2.1
29
Divergent Plate Boundaries
(constructive) occur where two plates pull apart and new
oceanic plate is created!
Divergent Plate Boundaries (DPB) are also called
Mid-Ocean Ridges and Spreading Centers.
At Divergent Plate Boundaries, we see

1) frequent basaltic volcanic eruptions which build new
seafloor
2) small, shallow earthquakes ≤ magnitude 5, ≤ 10 km
deep
Mature DPB always occur within an ocean basin,
where they act to “grow” the ocean larger!
Examples of DPB include the Mid-Atlantic Ridge
and the East Pacific Rise.
Divergent Plate Boundaries form by continental rifting, a
process which rips continents apart.
This occurs when heat builds up under the thick continent
following the development of a mantle plume or “blow torch” of
super-heated mantle rock.
The warm rock bulges and cracks, letting magma in and
splitting the continent.
Continued eruptions grow the new ocean.
Map of the East African Rift System = one of two places where
we can see active continental rifting and a new ocean basin
being “born”. Figure 2.18

The Gulf of California is the other place where we see
continental rifting in action!
http://pictures.servegame.org/picture-of-the-gulf-of-
california.html
http://earthobservatory.nasa.gov/IOTD/view.php?id=1282
Transform Plate Boundaries
(conservative) occur where two plates slide past each other!
Plate is neither
destroyed or constructed.
Transform plate boundaries are rips in the rigid plate which
form to allow curves in the plate boundary. They always form
between two segments of a mid-ocean ridge/DPB
compression
stretching
compression
stretching
Transform Plate Boundaries have:
1) large, shallow earthquakes, ≤ 10 – 20 km deep, ≤ magnitude
7 (rarely higher)
2) no volcanoes
Examples include our very own San Andreas Fault!
http://www.calstatela.edu/dept/geology/SanAndreasFault.htm

Most transform plate boundaries are found on the seafloor,
where they offset divergent plate boundaries. Fig. 2.23
Convergent Plate Boundaries
(destructive) occur where two plates push together and old plate
is destroyed!
There are three possible situations at CPB based on what type of
plates are converging: Oceanic – Oceanic, Oceanic –
Continental, and Continental – Continental. Each behaves
differently…
At Oceanic – Oceanic and Oceanic – Continental convergent
plate boundaries
old, cold oceanic plate is subducted.
Subduction = when a lithospheric plate is somehow pushed or
pulled into the mantle beneath a second oceanic or continental
plate.
Subduction is accompanied by:
1) large to great (up to magnitude 8+) and shallow to deep
(0 to 700 km) earthquakes
2) explosive andesitic volcanoes – the most dangerous on
Earth!
3) mountain building – volcanoes on land or islands
4) accretion of new land (continent) by scraping sediments,
islands, coral reefs, etc off of the top of the subducting plate
and adding them to the overriding plate.
cdn.autoanything.com / images / products / rell / hitch /
red_chevy_pushing...

http://www.geology.um.maine.edu/geodynamics/AnalogWebsite
/UndergradProjects2005/Perry/html/index.html
Example of oceanic-continental convergence = the Peru-Chili
Trench along western South America. Garrison, 2012, Essentials
of Oceanography, see also figure 2.20
Example of oceanic-oceanic convergence = subduction at the
Japan Trench.
Garrison, 2012, Essentials of Oceanography, see also fig. 2.20
At Continental – Continental convergent plate boundaries
continental collision occurs.
Continental lithosphere is too buoyant to subduct.
http://www.los-angeles-injury-lawyer-blog.com/car_accidents/
Continent-continent collision occurs after many years of
subduction completely closes an ocean basin!
This is how supercontinents like Pangaea are constructed!
Continent – continent collision creates chains of tall mountains
as the edges of the colliding continents crumple under the
pressure. The subducting oceanic plate breaks off and
disappears into the mantle.
The Himalaya Mountains of Asia are the result of continent –
continent collision between India and Eurasia. The collision
and mountain building continues today!

Additional evidence comes from Hot Spot Tracks
= chains of volcanoes located in the middle of tectonic
plates.
The Hawaiian Island – Emperor Sea Mount chain is an
example.
Figure 2.26
Figure 2.24 showing how we think hot spot volcanoes form over
mantle plumes= rising blobs of superheated mantle.
Map showing location of active hot spots.
Hot spot track volcanoes are always
newest on one end and
get older the farther up the chain you look.
Only the most recent (end) volcano is active.
These chains are believed to form as the tectonic plate passes
over a hot spot or mantle plume.
Mantle Plumes are superheated jets of hot mantle material
that remain fixed in place in the mantle.
They may also cause continental rifting
Once we accept that hot spot chains form over stationary mantle
plumes, we can use the age and spacing of the volcanoes to
calculate how fast and in what direction the plate moved in the
past.
In the example, the plate velocity or rate is: 100 km/5 my = 20
km/my = 2 cm/yr

The direction of plate motion is always from the newest toward
the oldest volcano, so in the example, the plate is moving:
Northwest
N
Old Island = 5 MY
New Island = 0 MY
100 km
One of the many questions left in plate tectonics is how plates
move. Present rates are determined by satellite.
Notice that the islands erode flat at the surface, then get deeper
and sink as they move farther from the hot spot = get older.
Why?
Isostasy => the extinct volcanoes cool down, get more dense,
and sink deeper.
Results in guyot = tablemounts = flat topped volcanoes, eroded
at the surface, but now far below the surface.
The same thing happens at mid-ocean ridges
Coral reefs will change shape as the island sinks out from
underneath them due to isostasy.
Figure 2.28
47

Figure 2.32
48
The Wilson Cycle.
Figure 2.34
49
Supercontinent cycle – plate tectonic cycle from supercontinent,
to continental rifting, to drifting of the continents across the
Earth as oceans open and close, to eventual creation of a new
supercontinent. This cycle probably takes about 500 MY. We
think we can identify at least 3 past supercontinents. including
Pangaea.
The Earth today (left) and 200 my in the future (right) assuming
today’s motions continue.
http://www.historyoftheuniverse.com/cf200.html
Chapter 2:Read the entire chapter.
Watch the following Youtube videos from “Geoscience Videos”
https://www.youtube.com/watch?v=vxjUn8PVvmc
https://www.youtube.com/watch?v=7nxITuot-ko
Vocabulary (do not turn in):
Alfred Wegener
Continental drift

Evidence for continental drift
Jigsaw puzzle fit of continents
Matching sequences of rocks
Matching sequences of faults and mountain chains
http://carnivoraforum.com/topic/9691943/
Climatic evidence
Glacial ages
Matching fossil assemblages (groups of fossils)
Problems with continental drift
Power source / why continents move
How continent move
Harry Hess
Seafloor spreading
Mantle convection
Mid-ocean ridge = spreading center
Deep sea/ocean trench = subduction zone
Evidence for Seafloor spreading
Magnetic stripes on the seafloor
Magnetic field
Age of the seafloor
Heat Flow
http://pubs.usgs.gov/gip/dynamic/developing.html
Plate Tectonics
Evidence for plate tectonics
Hot spot tracks
Apparent polar wander
Igneous rocks
Magma
Magnetite
Basalt
Distribution of earthquakes and volcanoes
Types of plate boundaries (know examples)
Divergent plate boundaries (DPB)
Mid-ocean ridge = spreading center = DPB

Ridge vs. rise
Difference in shape, size, and earthquakes
Rifting = continental rifting
Convergent plate boundaries (CPB)
Oceanic vs. Oceanic CPB
Island arc Subduction
Continental vs. Oceanic CPB types of
volcanoes
Continental arc types of earthquakes
Continental – Continental CPB
Continental collision
redxken192jyd.blogspot.com
Types of earthquakes
Transform plate boundaries (TPB)
Oceanic transform faults
Continental transform faults
Earthquakes at TPB
Hot Spots
Mantle plumes
Hot spot tracks
Formation of guyot (table mountains)
Evidence for plate motion over time
Coral reef development and relationship to plate tectonics and
isostasy
Fringing reef
Barrier reef
Atoll
Paleogeography
Continental accretion
Predictions for the future
Wilson Cycle – life cycle of oceans

http://www.starrynighteducation.com/sntimes/2012/jan/
Name ____________
Homework:
1) Concept Check 2.1 #1 Cite the lines of evidence Alfred
Wegener used to support his idea of continental drift? Why did
scientists of the time doubt that continents had drifted?
2) T or F? Harry Hess’s Theory of Seafloor Spreading explains
only the motion of the seafloor, but not continental movement.
3) Answer the following questions about evidence for seafloor
spreading using the terms magnetic stripes, age of the seafloor,
deep sea trenches, mid-ocean ridges, or both.
a. Records the periodic switch of the Earth’s magnetic field (N
<-> S) like a VCR records a movie._________________
b. Least / smallest at the spreading center, and increases with
distance away from the spreading center. _________________

c. Also known as the spreading center. _________________
d. Locations where seafloor is believed to subduct back into the
mantle. _________________
4) What do I mean when I say that plate tectonics combines
continental drift and seafloor spreading?
5) What is the power source that is believed to move the plates?
_________________
6) About how fast do the plates move? a) centimeters per year
b) meters per year c) kilometers per year d) what is a plate?
7) T or F? Plates with both divergent and convergent plate
boundaries move faster than plates with only a divergent plate
boundary (see videos listed above for answer).
8) Concept Check 2.4 #1 How is the age distribution pattern of
the Hawaiian Island- Emperor Seamount chain explained by the
position of the Hawaiian hot-spot? What could have caused the
curious bend in the chain?
9) Complete the following table comparing the various types of
plate boundaries.

Type of Plate Boundary
Variety of that Type of Plate Boundary
Volcanoes: none, explosive, non-explosive.
Earthquakes: small/large, shallow / deep / both
Effect on plate size: grows, shrinks, stays the same
Example
Divergent Plate Boundary (DPB)
Non-explosive (basalt)
Mid-Atlantic Ridge
Convergent Plate Boundary (CPB)
Ocean-Ocean

Marianas Trench
Ocean- Continental
Continental- Continental
Transform Plate Boundaries (TPB)
Large, shallow
Stays the same
10) T or F? Subduction along a continental margin (edge of a
continent) will cause the continent to get bigger (wider) over
time.
11) T or F? Table mountains / guyot formed by wave erosion
when sea level was much deeper (>2000 m) than it is today?

Steps to Plate TectonicsStep 1 – Continental Driftwww.mat.docx

Steps to Plate TectonicsStep 1 – Continental Driftwww.mat.docx

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