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Plate Tectonics
Horizontal Movement of
Earth’s Lithosphere
Chapter 3
Additional Reading: USGS Plate Tectonics (Pdf) – class web site
Learning Objectives
Understand the processes that are continuously changing Earth’s
surface as lithospheric plates move relative to one another.
Identify the role of oceanic ridges, transform faults and deep-sea
trenches in defining the edges of lithospheric plates.
Explain the distribution of magnetic anomaly stripes, seismicity,
and volcanism in terms of the concept of global plate tectonics.
Calculate spreading rates of ocean basins.
Plate Tectonics
1. The Theory of Plate Tectonics
2. Plate Boundaries
a) Spreading Centers
b) Subduction Zones
c) Transform Faults
3. Plate Movement
The Theory of Plate Tectonics
“Continental Drift” - theory* proposed by
Alfred Wagner, a German meteorologist (1915)
Explained by:
• geologic fit
• fossils
* Not accepted by scientific community
- no mechanism to explain plate movement
Plate Tectonics
http://pubs.usgs.gov/gip/dynamic/dynamic.html
The Theory of Plate Tectonics
(cont’d.)
Plate Tectonics - evidence for theory of continental
drift by Hess, Heezen and Tharp (1960’s) found
lithospheres plate boundaries that can be 3 types:
1) ridges (spreading centers)
2) trenches (subduction zones)
3) transform faults (plates sliding past one another)
The Theory of Plate Tectonics
(cont’d.)
Lithospheric Plates
major plates:
1.
2.
3.
4.
5.
6.
7.
Pacific – 105 x106 km2
Eurasian - 70 x106 km2
Antarctic - 60 x106 km2
Australian - 45 x106 km2
S. American - 45 x106 km2
African - 80 x106 km2
N. American - 60 x106 km2
minor plates:
1.
2.
3.
4.
5.
6.
7.
8.
Cocos - 5 x106 km2
Phillipine - 6 x106 km2
Caribbean - 5 x106 km2
Nazca - 15 x106 km2
Arabian - 8 x106 km2
Indian - 10 x106 km2
Scotia - 5 x106 km2
Juan de Fuca - 2 x106 km2
Plate Boundaries
a) Spreading centers - ‘rift zones’ (cont’d.)
1) Convection cells form
•
Density differences – cool vs. hot
2) Convection cells cause frictional drag on lithosphere
3) Lithosphere stretches due to convective movement
4) Lithospheric crust weakens
Plate Boundaries
(cont’d.)
a) Spreading centers - ‘rift zones’ (cont’d.)
5) Faulting – break in overlying lithosphere
6) Magma flows upward
7) New lithospheric crust formed
Creating new ocean crust
Plate Boundaries
(cont’d.)
a) Spreading centers - ‘rift zones’ (cont’d.)
•
•
Plates split apart -‘divergent plate’ boundary
New crust formed - ‘constructive’ plate
boundary
Evolution of a mid-ocean ridge system
1. Upwarping
2. Rift valley
3. Linear sea
4. Mid-ocean ridge system
Plate Boundaries
(cont’d.)
b) Subduction zones
•
•
•
Lithospheric Plates collide - ‘convergent’ plate boundary
Crust destroyed - ‘destructive’ plate boundary
Forms trenches and mountains
Plate Boundaries
(cont’d.)
b) Subduction zones (cont’d.)
3 types of subduction zones:
1. Ocean crust into continental crust – form trenches and
mountain ranges
Ex. a): Juan de Fuca plate into the N. American plate - forms Cascade Mtn. Range
Ex. b): Nazca plate into the S. American plate - forms Peru-Chile Trench and
the Andes Mtn. Range
Plate Boundaries
(cont’d.)
b) Subduction zones (cont’d.)
2. Ocean crust into ocean crust – forms trenches and island arcs
Ex. A): Philippine plate into the Pacific plate – formed the Marianna Trench
and the Marianna Island Arc system
Ex. B): N. American plate into the Caribbean plate and then the N. American
plate into the S. American plate – formed the Isthmus of Panama
Plate Boundaries
(cont’d.)
b) Subduction zones (cont’d.)
3. Continental crust into continental crust – form mountain ranges
Ex. A): Indian plate into the Eurasian plate – formed the Himalayas
Ex. B): Eurasian plate into the African plate - closing up of the
Mediterranean sea
Plate Boundaries
(cont’d.)
c) Transform faults
•
•
Plates slide past one another
Lithospheric crust neither created nor destroyed - ‘conservative’ plate
boundary
Ex. A) Pacific plate sliding past N. American plate – forms the San Andreas Fault
Type of boundary between plates:
Constructive margins  Midocean ridges
Destructive margins
 Subduction zones
Conservative margins  Transform faults
Plate Movement
•
•
New crust is created at spreading centers at a rate of approximately
1-10cm per year
Old crust is destroyed at the same rate at subduction zones
How do we know these rates? (Rate=distance/time)
Creating new ocean crust
SUMMARY
Destructive margins
Subduction zones
Constructive margins
Midocean ridges
Driving Mechanisms for Plate Motions
Plate Movement
•
(cont’d.)
Magnetic anomalies in ocean crust...look at spreading centers
 paleomagnetism
 every so often Earth’s magnetic field flips (every 300K-500K years)

magnetic signal recorded in crust at spreading center as it’s formed, forms
bands of crust with either a weak or strong magnetic signal

determine rate of plate movement by distance of band from spreading center
divided by age of rock in band (r=d/t)
More evidence of plate moving..
Plate Movement
(cont’d.)
Hot spots
 Islands of Hawaii
 islands or sea mountains formed over hotspots
(fixed area where magma comes up)


lithosphere moves over hotspot and end up have volcanic mountain
over hotspot as well as a series of mountains in ‘front’ of hotspot
determine rate of plate movement by distance of mountain from
hotspot divided by age of rock in mountain (r=d/t)
Age of Ocean Crust
http://www.ngdc.noaa.gov/mgg/geology/geology.html
Creating new ocean crust
Oceanic crust moves
away from MOR (Mid
Oceanic Ridge) and
cools and subsides
Destructive margins
Subduction zones
Constructive margins
Midocean ridges
Driving Mechanisms for Plate Motions
the Pacific Ring of Fire IS 40,000 km long chain of volcanoes caused by "convergent tectonic plates" coming together;
Type of boundary between plates:
Constructive margins  Mid ocean ridges
Destructive margins
 Subduction zones
Conservative margins  Transform faults
Conservative margins
Transform faults
Conservative margins
Transform faults
The San Andreas fault
in southern California
Hot Spots?
• Mantle plumes originate deep within the
asthenosphere as molten rock which
rises and melts through the lithospheric
plate forming a large volcanic mass at a
“hot spot”.
Mantle Plume
Coral Reefs
Air view
Spreading rates
Geological Periods
Geological Periods
Precambrian
Cambrian
Ordovician
Silurian
Devonian
Early Carboniferous
Late Carboniferous
Permian
Triassic
Jurassic
Late Jurassic
Cretaceous
K/T extinction
Eocene
Miocene
4.6 B 514 Ma
458 Ma
425 Ma
390 Ma
356 Ma
306 Ma
255 Ma
237 Ma
195 Ma
152 Ma
94 Ma
66 Ma
50.2 Ma
14 Ma
570 Ma solidification
Gondwana, hard shell anim.
separation, coldest
Laurentia collides with Baltica
pre-Pangea, equatorial forests
Future World
Future
Future
+50 Ma N. Atlantic widens, Med. vanish
+100 Ma new subduction
+250 Ma new Pangea
western Pangea is complete
deserts, reptiles, major ext.
Life begins to rediversify,Pangea
Dinosaurs, Pangea starts to break
Pangea rifts apart, Atlantic
New oceans, India
end of dinosaurs
India collides with Asia
Modern look
Precambrian
break-up of the
supercontinent, Rodinia,
which formed 1100 million
years ago. The Late
Precambrian was an "Ice
House" World, much like the
present-day.
Source: www.scotese.com
Cambrian
Animals with hard-shells
appeared in great numbers
for the first time during the
Cambrian. The continents
were flooded by shallow
seas. The supercontinent of
Gondwana had just formed
and was located near the
South Pole.
Ordovician
During the Ordovician ancient
oceans separated the barren
continents of Laurentia,
Baltica, Siberia and
Gondwana. The end of the
Ordovician was one of the
coldest times in Earth
history. Ice covered much of
the southern region of
Gondwana.
Silurian
Laurentia collides with
Baltica closing the northen
branch of the Iapetus Ocean
and forming the "Old Red
Sandstone" continent. Coral
reefs expand and land plants
begin to colonize the barren
continents.
Devonian
By the Devonian the early
Paleozoic oceans were
closing, forming a "prePangea". Freshwater fish
were able to migrate from the
southern hemisphere
continents to North America
and Europe. Forests grew for
the first time in the
equatorial regions of Artic
Canada.
Early Carboniferous
During the Early
Carboniferous the Paleozoic
oceans between Euramerica
and Gondwana began to
close, forming the
Appalachian and Variscan
mountains. An ice cap grew
at the South Pole as fourlegged vertebrates evolved in
the coal swamps near the
Equator.
Late Carboniferous
By the Late Carboniferous
the continents that make up
modern North America and
Europe had collided with the
southern continents of
Gondwana to form the
western half of Pangea. Ice
covered much of the
southern hemisphere and
vast coal swamps formed
along the equator.
Permian
Vast deserts covered
western Pangea during the
Permian as reptiles spread
across the face of the
supercontinent.
Triassic
The supercontinent of
Pangea, mostly assembled by
the Triassic, allowed land
animals to migrate from the
South Pole to the North Pole;
and warm-water faunas
spread across Tethys. The
first mammals and dinosaurs
appeared;
Jurassic
By the Early Jurassic, southcentral Asia had
assembled. A wide Tethys
ocean separated the
northern continents from
Gondwana.
Subduction zone Rocky Mountains
Formation of the Rocky Mountains
http://wrgis.wr.usgs.gov/docs/parks/province/rockymtn.html
Late Jurassic
In the Late Jurassic the
Central Atlantic Ocean was a
narrow ocean separating
Africa from eastern North
America.
Cretaceous
During the Cretaceous the
South Atlantic Ocean
opened. India separated from
Madagascar and raced
northward on a collision
course with Eurasia. Notice
that North America was
connected to Europe, and that
Australia was still joined to
Antarctica.
Dinosaur extinction
The bull's eye marks the
location of impact site of a
10 mile wide comet caused
global climate changes that
killed the dinosaurs and
many other forms of life. By
the Late Cretaceous the
oceans had widened, and
India approached the
southern margin of Asia.
Eocene
50 - 55 million years ago
India began to collide with
Asia forming the Tibetan
plateau and Himalayas
(destroying the last of
Tethys ocean). Australia,
which was attached to
Antarctica, began to move
rapidly northward.
Collision of continental crust – formation of Himalayas
• Whereas oceanic ridges indicate tension,
continental mountains indicate compression
forces are squeezing the land together.
Sedimentary Rocks Squeezed by Compression
Miocene
20 million years ago,
Antarctica was covered by
ice and the northern
continents were cooling
rapidly. The world has taken
on a "modern" look, but
notice that Florida and parts
of Asia were flooded by the
sea. Arabia moved away
from Africa forming Gulf of
Aden and Red Sea;
Last Ice Age
When the Earth is in its "Ice
House" climate mode, there
is ice at the poles. The last
expansion of the polar ice
sheets took place about
18,000 years ago.
Modern World
If we continue present-day
plate motions the Atlantic
will widen, Africa will collide
with Europe closing the
Mediterranean, Australia will
collide with S.E. Asia, and
California will slide
northward up the coast to
Alaska.
Future +100
Earth is ~ 4.6 bill
years old –
suggested cyclic of
500 mill year
pattern of
assembling and
disassembling the
land masses;
Future +250
The
Wilson
Cycle
uses plate tectonic
processes to show
development and
creation of ocean
floor and ocean
basins;
The Wilson Cycle