The idea that Earth’s surface is divided into large plates that move slowly and
change in size over time.
This idea provides a model for understanding many geologic
features: faults, folds,
volcanoes, earthquakes, and mountain
Plate Tectonics is the culmination of two
1) Continental Drift- The idea that continents move freely over Earth’s surface, changing their position relative to
each other. This concept was proposed by German meteorologist Alfred Wegener in the early
2) Seafloor Spreading- The concept that new seafloor forms at mid-ocean ridges,
moves horizontally away from the ridge toward an ocean trench/subduction zone (the seafloor is a
conveyor belt). This idea was proposed by
Princeton geologist Harry Hess in 1962.
The Early Evidence
for Continental Drift
1) The continents look like they could fit together (like a
Wikipedia Commons Image.
2) Rock types are correlated from continent to continent (across
3) The extinct plant fossil,
Glossopteris, which grew in temperate
climates, is found in South America, Africa, India, Antarctica, and Australia.
4) The extinct reptile,
Mesosaurus, is found only in Brazil and South Africa. The mesosaurus lived only in freshwater, and could not
swim across oceans.
Ancient glacial deposits in South America,
Africa, India, Antarctica, and Australia suggest these continents were all
located near a polar region.
Wegener's Concept of Continental Drift: Polar Wandering
Alfred Wegener proposed that the continents were once
assembled into a supercontinent called Pangaea.
Wikipedia Commons Image.
He also proposed that Pangaea split into two parts:
1) Northern Pangaea (which includes present
day North America and Eurasia)
2) Southern Pangaea (including South America, Africa, India, Antarctica, and Australia) became Gondwanaland.
After Pangaea fragmented, Laurasia drifted northward and Gondwanaland drifted
Wegener knew that coral
reefs form near the equator, deserts form about 30° north and south of the
equator, and glaciation occurs near the poles. Based upon this information, he
determined the North and South Pole positions over geologic time. He called
the apparent movement polar wandering.
There were two possible explanations:
1) The continents remained motionless and the
poles actually moved (literal polar wandering).
2) The poles stood approximately still and the
continents moved (continental drift). Wegener preferred this explanation.
-Some scientists argued that some fossils (especially fossil plants) could have been spread from one
continent to another by wind or ocean currents.
-Land dwelling reptiles could have spread from
continent to continent by land bridges that rose up from the seafloor (this idea
was pure speculation, as no appropriate mechanism was known).
-Wegener lacked a plausible mechanism by which
continents could actually drift.
-Wegener's Polar wandering might reflect true wandering
of poles rather than drifting of continents.
New Evidence for Continental Drift
Paleomagnetism- The study of Earth’s magnetic field through time;
geologists look at the way magnetic minerals in rocks preserve the magnetic field.
When a magnetic
mineral crystallizes and cools below its Curie point, its magnetic alignment is
The direction of
the mineral alignment gives the direction of magnetic North. The dip of the
mineral alignment gives the latitudinal distance from magnetic North.
For example, Permian age rocks in North
America point to an apparent magnetic North pole in Asia whereas Permian age rocks
in Europe point to an apparent magnetic North Pole in Japan.
Today, when geologists use the term polar
wandering they are referring to an “apparent” wandering of the poles. We
know that the poles themselves did not wander (although, as we shall see, the
poles flipped or reversed many times throughout geologic history).
The continental slope- If the continental slope is taken into account,
the plates fit together extremely well.
GPS technology- Global positioning satellites allow us to actually watch and measure the drift of the continents (1-16
thought that the seafloor remained stationary whereas the continents moved.
in the 1960's Harry
Hess proposed that the seafloor was also moving.
According to Hess, oceanic crust is produced at
mid-ocean ridges and subducted at trenches. The driving force for seafloor spreading is convection
(hot mantle rises near the
mid-ocean ridges and cold mantle sinks near trenches).
The best evidence
for sea floor spreading was provided by magnetometer surveys perpendicular to
mid-ocean ridges. A zebra pattern of magnetic reversals reflects episodic
flips in Earth's North and South poles.
Image from the USGS.
of magnetic anomalies is symmetrical about the ridge crest.
The concept of
seafloor spreading explains the age of the sea floor. Near mid-ocean
ridges: sea floor is young and lacks sediment. Away from ridges: seafloor
gets older and acquires a thick blanket of pelagic sediment.
The Big Picture:
By the late 1960’s,
the hypotheses of continental drift and seafloor spreading had been combined
into a single, unified theory of plate tectonics.
Recall that a
plate is a thick, mobile slab of the Earth’s
surface made of lithosphere (crust + upper
mantle). Plates glide on the ductile asthenosphere.
There are 3 types
of tectonic plate boundaries:
1) Divergent Plate Boundaries
boundaries can occur in the middle of oceans or in the middle of a continent.
The result of a
divergent plate boundary is to create a new ocean basin.
supercontinent like Pangaea breaks up, the divergent boundary is found in the
middle of a continent, marked by a continental rift.
During the rifting
event, the continental crust is stretched and thinned, producing a normal fault.
Topographically this results in a rift valley with a central graben.
The fault provides a
pathway for basaltic magma, which rises up from the mantle to form basalt flows
and cinder cones.
continues, sea water will eventually fill the
True oceanic crust
is eventually produced at a mid ocean rift between the two diverging continents.
The trailing edge
of the continent on each side of the rift becomes a passive margin.
boundaries involve strike-slip motion of plates (a conservative boundary).
boundaries are characterized by shallow earthquakes.
The San Andreas
fault is a transform boundary. A previous plate was subducted, and after
subduction was complete a subsequent plate arrived with a strike-slip
orientation to the mainland.
boundaries also occur along the fracture zones of mid-ocean ridges.
Convergent Plate Boundaries
At convergent plate
boundaries, two plates move towards each other.
The character of
the convergent boundary depends on the types of plates that are converging:
of the oceanic plates will subduct.
results in an island chain of volcanoes called an island arc
(for example, the Phillipines).
Ocean-continent convergence- The dense oceanic crust
is subducted below the continental crust. A chain of volcanoes forms on
the continental crust as a magmatic arc.
metamorphism will occur due to the rising hot
magmas and also due to the convergent forces. On the landward side of the arc, a
fold and thrust belt will form.
Continent-continent convergence- In the case of
continental-continental convergence, neither plate
will subduct. First, ocean floor in-between them the
continents is subducted. Eventually, when
all the oceanic crust is subducted, the continents will collide. The two continents
will weld together along a suture zone.
Fold and thrust belts will form from the convergence and regional metamorphism
will occur (Himalayan Mountains).
A closer look
at subduction (for ocean-ocean or ocean-continent subduction).
A Benioff zone
defined by shallow, intermediate, and deep earthquakes will define the top of
the down-going plate.
At a depth of ~100
km, magmas will be generated in the asthenosphere overlying the down-going
plate. The magmas will rise upward, creating a chain of volcanoes on the
overlying plate that parallel the subduction zone.
What Causes Plate Motions?
Rock deep within the Earth’s interior is heated and rises whereas shallow, colder denser rock sinks.
This sets up a convection current.
cells may extend from the heat source at the core-mantle boundary all the way to
the base of the lithosphere.
assist in the movement of plates:
1) Ridge-push- Plates move apart at the midocean ridge due
to down slopes.
2) Slab-pull- Subducting slabs pull the surface
part of the plate away from the ridge.
3) Trench-suction- The subducting plate falls into
the mantle. As a result, the overlying trench and plate are pulled
horizontally, seaward, toward the subducting plate.
Mantle Plumes and Hot Spots
A modification to
the convection model suggests that the mantle transfers heat in the form of
narrow columns of hot rock called mantle plumes.
The mantle plumes
may rise from the core-mantle boundary and stay stationary.
The mantle plumes
have a wide, mushroom-shaped head and a long, narrow tail.
Mantle plumes are
proposed to produce “hot-spot” volcanic activity on the earth’s surface,
sometimes far away from any plate margins.
Hawaii volcanism is
“hot-spot” volcanism. Hawaii is located in the middle of the Pacific plate, on
volcanism is another example of “hot-spot” volcanism. Yellowstone is located on
the North American plate in the middle of continental crust.
Hot spot volcanism
fed by a mantle plume is proposed to be the reason for the breakup of Pangaea.