Plate Tectonics

Introduction: The development of plate tectonic theory was one of the most important scientific achievements of the 20th century. The theory provides an important framework to the understanding of all naturally occurring Earth processes. It also provides an important framework to the understanding of both Earth's history and future. Importantly,  the evolution of life on Earth has been greatly affected by plate tectonic processes. This includes the historical, social, and cultural development of humankind. In short, plate tectonic theory is THE encompassing theory for all other scientific theories which explain how naturally occurring Earth processes have shaped the history of the solid Earth, the oceans, the atmosphere, and all living organisms.

An important aspect of this topic is an understanding of the important discoveries which led to the development and final acceptance of plate tectonic theory. It is a great example of how scientific theory is almost always advanced by groups of scientists working toward a common cause. The story begins in the early 20th century with the proposal of 'continental drift' by Alfred Wegener, and ends in the mid 1960's when an understanding of tectonic plate boundaries and the driving force(s) of plate tectonics is finally achieved.

I. Definitions

Lithospheric Plate - a large segment of the Earth's outermost, cool, rigid crust and underlying upper parts of the mantle. Lithospheric plates move on top of the hot, plastic underlying asthenosphere. (Fig 7.9,7.10)

Lithospheric Plate Boundary (Margin) - a fracture in the Earth's lithosphere that separates one plate from another. (Fig 7.10)

Divergent Boundary (Margin) - two lithospheric plates move away from one another. (Fig 7.10A)

Convergent Boundary (Margin) - two lithospheric plates move toward one another. (Fig 7.10B)

Transform Boundary (Margin) - two lithospheric plates slide horizontally past one another. (Fig 7.10C)

II. Important People/Events/Concepts in the Development of Plate Tectonic Theory

Alfred Wegener - German meteorologist who proposed the 'hypothesis of continental drift '(1915). Used paleontological, geological, and climatological evidence to theorize that approximately 200 million years ago all continents were joined into one supercontinent, Pangaea (Fig 7.2). Since that time, continents 'drifted' into their present-day positions. His ideas were rejected because he could not explain how the continents move through/across the solid rock underlying the ocean basins. (Box 7.1)

Discovery of the mid-ocean ridge system (Fig 1.18, 7.11)

Paleomagnetism - The study and measurement of the Earth's magnetic field from ancient rocks. (Fig 7.23)

Apparent Polar Wander - Magnetized rock samples of different ages point to different directions of magnetic north, suggesting that the Earth's magnetic pole positions have 'wandered' over time. Magnetized rock samples of the same age from different continents point to different locations for the same magnetic pole, suggesting that if the continents were fixed the Earth had several north and south poles at the same time. (Fig 7.24)

Click on this link for paleomagnetism diagrams

Cox and Dallrymple (Stanford University, U.S. Geological Survey) - Showed that magnetized rock samples of different ages from different continents all recorded changes in the Earth's magnetic field at the same time. Confirmed the 'theory of magnetic reversals '- A change in the Earth's magnetic field, in which the north magnetic pole becomes the south magnetic pole, and vice versa. Rocks of 'normal polarity' point toward the present position of the Earth's north magnetic pole. Rocks of 'reversed polarity' point toward the present position of the Earth's south magnetic pole. (Fig 7.25)

Sea-Floor Stripes - Magnetic anomaly patterns observed in sea-floor basalts. 'Stripes' of normal and reverse polarity are observed on either side of sub-marine mountain chains (mid-ocean ridges) (Fig 7.26, 7.27)

Diagram Courtesy of USGS

Harry Hess (Harvard University) - First proponent of the 'theory of sea-floor spreading'. Along mid-ocean ridges, magma is erupted from the underlying mantle. This causes the oceanic lithosphere on either side of the ridge to slide away in opposite directions, riding on the underlying plastic asthenosphere. (Fig 7.18)

Vine and Matthews (Cambridge University) - Showed that sea-floor stripes can be accounted for by the theory of sea-floor spreading. (Fig 7.26,7.27)

Benioff Zones - An inclined zone of earthquake foci which occurs along deep ocean trenches - narrow, curved depressions in the ocean floor that can reach depths over 11 km, and can extend for thousands of km along the ocean floor. Benioff zones mark regions in which the oceanic lithosphere violently descends back into the Earth's interior.  

III. Plate Boundaries

1. Convergent Plate Boundaries:

Oceanic Lithosphere - Continental Lithosphere convergent Plate Boundary - Results in subduction of the oceanic lithospheric plate beneath the continental lithospheric plate along a deep ocean trench. Partial melting of mantle rock occurs due to addition of water to the mantle wedge from the subducted oceanic lithosphere, lowering the melting temperature of the ultramafic mantle rock. Magmas are erupted along the Earth's surface, forming a continental margin volcanic arc (Examples: Cascade Range, Andean Range). (Fig 7.15A)

Oceanic Lithosphere - Oceanic Lithosphere Convergence Plate Boundary - Results in subduction of one oceanic lithospheric plate beneath the other along a deep ocean trench. Resulting magmas are erupted along the Earth's surface, forming an island volcanic arc  (Example: Lesser Antilles, western Aleutians). (Fig 7.15B)

Continental Lithosphere - Continental Lithosphere Convergence Plate Boundary - Continental lithosphere is too buoyant to be subducted into the Earth's interior, so continental rocks are pushed upwards forming Earth's highest mountain chains. (Examples: Himalayas, Alps, Appalachians). (Fig 7.15C, 7.16)

2. Divergent Plate Boundaries:

Oceanic Lithosphere - Oceanic Lithosphere Divergent Plate Boundary - Results in partial melting of underlying ultramafic mantle rock due to decrease in pressure as rock ascends beneath a mid-ocean ridge. (Example: Mid-Atlantic Ridge, East Pacific Rise). (Fig 7.18)

Continental Lithosphere - Continental Lithosphere Divergent Plate Boundary - Results in stretching, fracturing and sinking of continental crust, forming a continental rift. The low lying area along the rift is called a rift valley. Characterized by volcanoes and low-lying lakes along the rift valley. (Example: East African Rift).  The Red Sea and the Gulf of Aden are linear seas. Within each is a mid-ocean ridge which has successfully rifted the Arabian Peninsula away from the continent of Africa. (7.12, 7.13)

Map Courtesy  of USGS

3. Transform Plate Boundaries:

Continental Lithosphere - Continental Lithosphere Transform Plate Boundary - Continental lithospheric plates slide laterally past one another. Characterized by frequent earthquake activity (Example: San Andreas Fault System). (Fig 7.19)

Map Courtesy of USGS

Note: Be familiar with the various types of tectonic plate boundaries along or near the western margin of North America (Fig 7.19):

1. The Gulf of California represents a linear sea, within which lies the northern terminus of the East Pacific Rise mid-ocean ridge (divergent plate margin).
2. The San Andreas Fault is a transform fault, connecting the northern end of the East Pacific Rise with the southern end of the Juan de Fuca Ridge.  (A transform fault connects two segments of the mid-ocean ridge system). Because the San Andreas Fault lies along a lithospheric plate boundary, it is also a transform plate margin.
3. The Juan de Fuca Ridge is a mid-ocean spreading ridge (divergent plate margin).
4. Oceanic lithosphere created along the Juan de Fuca ridge is subducted beneath the North American continental lithosphere along the Cascadia subduction zone (convergent plate margin). This results in formation of the Cascade Range continental margin volcanic arc.

Note: Be sure to know the difference between the terms 'terrain' and 'terrane'. Be sure to know what types of geologic terranes have been accreted to the western margin of North America.

Terrain - A stretch of land, with regard to its natural features; the 'lay of the land'.

Tectonic Terrane - A crustal fragment whose geologic history is distinct from that of the adjoining terranes. (Fig 10.18)

Accreted Terrane - A small crustal fragment which collided with and accreted to a continental margin (Fig 10.17,10.18).

IV. Mantle Plumes

Mantle Plume  - A stationary rising column of hot, plastic mantle rock which undergoes pressure-release melting. Magmas are erupted at the Earth's surface, forming a volcanic hot spot. The mantle plume remains fixed in place as the lithospheric plate moves above it, leaving a 'trail' of volcanoes on the Earth's surface (Example: Hawaiian Islands - Emperor Seamount Chain). (Fig 7.21). Mantle plumes are not restricted to mantle beneath oceanic lithosphere. A large mantle plume is today located directly beneath Yellowstone National Park, explaining why the park is characterized by many volcanic landforms and features even though it is not located along an active tectonic plate margin.

V. The Driving Force of Plate Tectonics

Mantle Convection - The convective flow of solid rock in the mantle. Old, cold subducted lithospheric plates sink downward. Hot mantle rock rises toward the Earth's surface. Convection (heat transport by movements of currents) is driven by heat from the core, and by heat produced by decay of radioactive isotopes in mantle rock. Mantle convection cells rise beneath mid-ocean ridges, and sink beneath subduction zones. (Fig 7.30A)

Slab-Pull/Slab-Push-  Gravity acts to 'pull' oceanic lithosphere away from mid-ocean ridges, causing it to slide over the underlying plastic asthenosphere. Gravity also acts to 'pull' dense oceanic lithosphere downward into the mantle beneath subduction zones. Along mid-ocean ridges, the oceanic lithosphere is 'pushed' away from the high elevation ridge axis (Fig 7.29)

Hot Plumes - All upward convection of mantle rock is associated with rising mantle plumes. Downward convection occurs where mantle rock is being forced downward along subducted oceanic lithospheric plates. (Fig 7.30B)

VI - The Breakup of Pangaea (200 million years ago to the present) (Box 7.1)

1. 200 million to 150 million years ago (7.10)  - Two major intra-continental rifts develop. One is located within the northern landmass (Laurasia) between North America and Africa, forming the North Atlantic Ocean. The second forms within the southern landmass (Gondwanaland) and results in the separation of India, and South America-Africa from Australia-Antarctica. A major subduction zone develops along the southern margin of Laurasia.

2. 100 million years ago - Intra-continental rifting separates Africa and South America, forming the South Atlantic Ocean. The North Atlantic Ocean continues to widen. India continues to move northward toward the southern margin of Eurasia.

3. 50 million years ago - Madagascar is separated from Africa by rifting. The South and North Atlantic Oceans continue to widen. India moves northward toward the southern margin of Eurasia.

4. The Present - India has converged with the southern margin of Eurasia, forming the Himalayas and the Tibetan Plateau. Greenland has been separated by rifting from Eurasia. The Baja Peninsula has completely separated by rifting from the Central America mainland, forming the Gulf of California.

Study Questions - Plate Tectonics