Introduction
Plate tectonics- 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 belts.
Plate Tectonics is the culmination of two pre-existing ideas:
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 1900’s.
2) Seafloor Spreading- The concept that new seafloor forms at mid-ocean ridges, and then 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 puzzle).
2) Rock types are correlated from continent to continent (across the oceans).
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.
5) 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) became Laurasia.
2) Southern Pangaea (including South America, Africa, India, Antarctica, and Australia) became Gondwanaland.
After Pangaea fragmented, Laurasia drifted northward and Gondwanaland drifted southward.
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.
Opposition to Continental Drift
-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
1) 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 locked in.
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.
Note: 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).
2) The continental slope- If the continental slope is taken into account, the plates fit together extremely well.
3) GPS technology- Global positioning satellites allow us to actually watch and measure the drift of the continents (1-16 cm/year).
Seafloor Spreading
Wegener thought that the seafloor remained stationary whereas the continents moved.
In contrast, 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.
The zebra pattern 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: Plate Tectonics
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
Divergent plate 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.
When a 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.
As divergence continues, sea water will eventually fill the split.
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.
2) Transform Boundaries
Transform boundaries involve strike-slip motion of plates (a conservative boundary).
Transform 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.
Transform boundaries also occur along the fracture zones of mid-ocean ridges.
3) 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:
Ocean-ocean convergence- One of the oceanic plates will subduct. The subduction 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.
Regional 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.
Huge convection cells may extend from the heat source at the core-mantle boundary all the way to the base of the lithosphere.
Several mechanisms 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 oceanic crust.
Yellowstone 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.
