Download Chapter 4

Plate Tectonics:
The Unifying Theory
Peter W. Sloss, NOAA-NESDIS-NGDC
Plate Tectonics
• Fundamental concept of
geoscience
• Integrates from many branches
• First suggested based on geology
and paleontology
• Fully embraced after evidence from
geophysics
What tectonics theory explains
• Distribution of earthquakes and
volcanoes
• Relationship of age and height of
mountain belts
• Age distribution of oceanic crust
• Magnetic information in rocks
What is Plate Tectonics Theory?
• Earth’s upper mantle (lithosphere) is broken into
rigid plates which move with respect to each
other
• Plates rest on and move in the asthenosphere
• There are 3 kinds of plate margins that are
marked by earthquakes and volcanoes
– Divergent
– Convergent
– Transform
• The movement is driven by uneven distribution
of heat within Earth and the mechanism that
drives plate movement is not well known
Mosaic of Earth’s Plates
Peter W. Sloss, NOAA-NESDIS-NGDC
Theory evolved from 2 concepts:
• Continental Drift - by Alfred Wegener
– Fit of continents - Pangaea
– Ancient climatic evidence - glacial
deposits
– Fossil evidence - Glossopteris &
Mesosaurus
– Matched mineral zones & mountain
chains
• (Problem - no acceptable method of
motion)
“Fit” of the
Continents
Anomalous Distribution of Fossils
(Mesosaurus)
AND
PALEOMAGNETIC EVIDENCE
• Polar Wandering - either
– the North Pole or the continents had
moved
– both Europe and North America had
apparently moved as a single
continent for several hundred million
years
Apparent
polar
wandering
curves
Seafloor Spreading
• “Geopoetry” of Harry Hess & Robert
Dietz
• New seafloor forms by upwelling at the
center of MOR and moves laterally
• Older crust is destroyed in the
subduction zones at the trenches
• Seafloor is younger than 200 MY
• Solved Continental Drift problem
Modern Proof of Seafloor Spreading
• Deep Sea Drilling Project - sampled
sea floor sediment & crust
– Age & thickness increased with
distance from MOR
– Ages were symmetrical about MOR
• Magnetic Anomalies - found weak &
strong signal
– Alternating normal & reversed
polarization
– Stripes || to & symmetrical about
MOR
Magnetic
Anomalies
Ocean crust
records
magnetic
reversals
Age of Seafloor Crust
R. Dietmar Muller, 1997
Fig. 20.11
More Proofs
• Hot Spots - deep, long-lived, stationary
mantle magma sources
• Expressed at the surface by:
– linear chain of volcanoes
– aged with distance from hot spot
• Over 100 identified
• Used as fixed points against which
plate motion is measured
Hawaiian-Emperor chain
Long-lived global hot spots
PUTTING IT ALL
TOGETHER
I. Plates
• Group of rocks all moving in the
same direction
• Can have both oceanic and
continental crust or just one kind
• Plate interior are relatively quiet
• Most activity is located at plate
margins
II. Types of plate boundaries
• divergent:
mid-ocean ridges
• convergent: collision zones
volcanic arcs
• strike-slip:
San Andreas Fault
Alpine Fault, N.Z.
A. Divergent plate boundaries
Usually begin within continents
as hot spot burns thru crust
and eventually grows to
become ocean basin
Two kinds of divergent margins
Mid Ocean Ridges (MOR)
Continental Rift Valleys
Features of Mid Ocean Ridges
• Central rift valley (width is inversely
proportional to the rate of
spreading)
• Region of high heat flow
Shallow-focus earthquakes
• Almost exclusively basalt
volcanism
Earthquakes Associated with
Divergent Margins
Rifting and
Seafloor
Spreading
Along the
Mid-Atlantic
Ridge
Peter W. Sloss, NOAA-NESDIS-NGDC
Rifting and Seafloor Spreading
Features of Continental Rifts
• East Africa, Rio Grande rift
• Beginning of ocean formation (may
not get that far)
• Rifting begins at a triple junction (two
spreading centers get together to
form ocean basin, one left behind).
• Rock types: basalt and sandstone
Hot Spot induced rifting
• Hot spot burns thru crust > 3 branched rift
• Divergence begins
• 2 branches are active
• 1 branch is not active
• Linear sea develops
Inception of
Rifting
Along the
East African
Rift System
Peter W. Sloss, NOAA-NESDIS-NGDC
Inception of Rifting Within a
Continent
Nile Delta
Gulf of
Suez
Gulf of
‘Aqaba
Red Sea
Earth Satellite Corp.
The Gulf of
California
Formed by
Rifting of Baja
California
from Mainland
Mexico
Worldsat International/Photo Researchers
B. Convergent boundaries
• New crust created at MOR—old crust
destroyed (recycled) at subduction
zones
• 2 kinds: subduction & collision
• Relative important densities:
continental crust ≈ 2.8 g/cm3
oceanic crust ≈ 3.2 g/cm3
asthenosphere ≈ 3.3 g/cm3
Convergent boundaries
Three types:
ocean–ocean
Philippines
ocean–continent
Andes
continent–continent
Himalaya
Ocean–Ocean
Island arcs: Chain of volcanic
islands
• Highly seismic tectonic belt of
shallow to deep earthquakes
• High heat flow arc of active
andesitic volcanoes
• Bordered by a submarine trench
Ocean–Ocean
Subduction Zone
Ocean–Continent
Continental arcs:
• Magmatic belt of active
volcanoes (andesite to rhyolite)
• Often accompanied by
compression of upper crust
which builds mountains
Bordered by a submarine trench
Ocean-Continent
Subduction Zone
Continent–Continent
• Continent–continent boundaries,
convergence is accommodated by
• Folding (shortening and
thickening)
• Strike-slip faulting
• Underthrusting (intracontinental
subduction)
Continent–Continent Convergent
Boundary
Continent-Continent
Collision
Himalayas and Tibetan Plateau
• Product of the collision between
India and Asia.
• Collision began about 45 M yr.
ago, continues today.
• Before collision, southern Asia
looked something like the Andes
do today.
Stages in the
collision of
India with
Asia
C. Transform Boundary Offsetting
Spreading Centers
III. Rates of plate motion
Mostly obtained from magnetic
anomalies on seafloor
Slow spreading: 3 cm/year
Fast spreading: 10 cm/year
Very fast spreading: 17cm/year
Relative Velocity and Direction
of Plate Movement
Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990
IV. Rock assemblages and
plate tectonics
• Each plate tectonic environment
produces a distinctive group of
rocks.
• By studying the rock record of an
area, we can understand the tectonic
history of the region.
Layer of ophiolite suite.
Precambrian Ophiolite Suite
Pillow basalt
M. St. Onge/Geological Survey of Canada
V. Exotic or Microplate Terranes
• Large blocks that contrast sharply with
surrounding area
• Wrong faulting, folding, fossils, rock
types, metamorphism, magnatism
• Thought to be fragments of continents,
seafloor, seamounts, island arcs that
rafted in & docked in new place
Approaching Arc or Microcontinent
Collision
Accreted Microplate Terrane
Microplate terranes
Added to Western
North America Over
the Past
200 Million Years
After Hutchinson, 1992-1993
VI. Driving mechanism of plate tectonics
• Thought to be convection of the
mantle.
• Friction at base of the lithosphere
transfers energy from the
asthenosphere to the lithosphere.
• Convection may have overturned
asthenosphere 4–6 times.
Other factors
• Trench pull
Plate sliding
• Ridge push
Three possible driving factors
Three possible
mechanisms for
the movement
of lithosphere
over the
asthenosphere
Fig. 17.17
VII. Tectonic reconstructions
A variety of evidence traces the motion
of continents over time:
• Paleomagnetism
• Deformational structures
• Environments of deposition
• Fossils
• Distribution of volcanoes
Assembly of Pangaea
I.W.D. Dalziel, 1995
Breakup of Pangaea
200 million years ago
After Dietz & Holden, 1970
Breakup of Pangaea
140 million years ago
After Dietz & Holden, 1970
Breakup of Pangaea
65 million years ago
After Dietz & Holden, 1970
Breakup of Pangaea
Today
After Dietz & Holden, 1970
Examining Deep-sea Drill Cores
Texas A&M University
Questions about plate tectonics
• What do we really know about
convection cells in the mantle?
• Why are some continents completely
surrounded by spreading centers?
• Why are tectonics in continental
crust and oceanic crust so different?
Cross Section of Western Canada
Fig. 20.25a
Formation
of
Magnetic
Anomalies
Fig. 20.10
Himalayas and Tibetan Plateau
Models
• Underthrusting
• Distributed shortening
•Strike-slip faulting
Wilson cycle
Plate tectonics repeats itself: rifting,
seafloor spreading, subduction, collision,
rifting, …
Plate tectonics (or something like it)
seems to have been active since the
beginning of Earth’s history.
After Hutchinson, 1992-1993
Fig. 20.22
Examples of Plate Boundaries
O-C
convergent
O-O
convergent
O-O
divergent
C-C
divergent
O-O
divergent
O-O
divergent
O-C
convergent
Fig. 20.8a,b
Volcanic and Nonmarine sediments
are deposited in rift valleys
Fig. 20.17a
Cooling and subsidence of rifted
margin allows sediments to be
deposited
Fig. 20.17b
Carbonate platform develops
Fig. 20.17c
Continental margin continues to
grow supplied from erosion of the
continent
Fig. 20.17d
Ocean–Continent Convergent
Boundaries
Opening
of the
Atlantic
by Plate
Motion
After Phillips & Forsyth, 1972
Fig. 20.13
Idealized Ophiolite Suite - Oc. Crust
Deep-sea sediments
Pillow basalt
Gabbro
Peridotite
Model for Forming Oceanic Crust
at Mid-ocean Ridges
The growth of oceanic basin 1
The growth of oceanic basin 2
The growth of oceanic basin 3
Age of the Ocean Basins
After map by Sclater & Meinke
Parts of an Ocean–Ocean
Convergent Plate Boundary
Fig. 20.18
Parts of an Ocean–Continent
Convergent Plate Boundary
Fig. 20.19
Continued Subduction
Fig. 20.20a
Continent–
Continent
Collision
Fig. 20.20b