Plate tectonics

Geology 101

Fall 2013

Why does the Earth have mountains and basins?

• Basic question asked by many: erosion is evident everywhere, so why haven’t all mountains eroded and filled in all basins?

• Alfred Wegener proposed continental drift in 1912.

Evidence for continental drift

• Continental “fit” – South America and Africa separated by the Atlantic

• Similar rocks separated by the Atlantic

• Similar climate markers (glacial striations) separated by the Atlantic

• Similar fossils separated by the Atlantic

Unifying theory of geology

• Extension of Wegener’s continental drift theory from the early 20th century

• Needed not only information about rocks but also breakthroughs in geochronology and geophysics

• Continental drift failed to provide a sufficient mechanism (tides and wind were not enough); plate tectonics does

Mechanism(s) of tectonics

• First, and foremost, the second law of thermodynamics says that heat is transferred from hot bodies to cold bodies (e.g., Earth’s core to space)

Mechanism(s) of tectonics

• Heat transfers in three ways: radiation, conduction and convection. It is the last of these that occurs within the mantle.

• Convection (and internal heating) seems to keep the mantle hot and plastic enough for plates to sink into it -- may not be enough to move plates

Mechanism(s) of tectonics• Slab pull seems to be the major “drag”,

ridge push and plate sliding are more minor forces

Tenets of plate tectonics

• Plates exist

Plate materials• Remember: lithosphere, not crust

• Continental plates are made of granite

• Oceanic plates are made of basalt

• Granite and basalt are types of igneous rocks

Tenets of plate tectonics

• Plates move

Hotspots

• Focused convection plumes from mid-mantle

Hotspots show plate movement

Problems with motion

• Hotspots may not be fixed in position and they are certainly not permanent

• GPS does a better job of measuring annual rates, though over the short-term only

Tenets of plate tectonics

• Plate boundaries are areas of geological phenomena such as earthquakes, volcanoes and mountain building

Plate interactions

• Convergent

Types of convergent boundaries

• Continental plate - continental plate convergence results in non-volcanic mountain ranges

Types of convergent boundaries

• Oceanic plate - oceanic plate convergence results in a volcanic oceanic island arc

• Subduction occurs

Types of convergent boundaries

• Continental plate - oceanic plate convergence results in a volcanic continental arc

Plate interactions

• Divergent

Types of divergent boundaries

• Between two oceanic plates

Sea floor spreading

• Oceanic divergent boundaries provided evidence for plate tectonics. Paleomagnetism (the measurement of “stripes” of different polarity minerals in ocean floor rock of different ages) allowed Hess and others (1950s) to postulate the movement of the sea floor itself.

Types of divergent boundaries

• Between two continental plates

Plate interactions

• Transform

Tenets of plate tectonics

• Plate interiors are geologically quiet, except for hotspots

Hotspots are areas of magma upwelling from the mantle

They can occur in the middle of a tectonic plate (e.g., Hawaii) or at a plate boundary (e.g., Iceland)

As shown below for Hawaii, the plate onto which the hotspot magma erupts continues to move, leaving a “trail” of extinct volcanoes

Hotspots remain more or less stationary during their lifetime, though there is some controversy about this.

Grander synthesis: the Wilson (supercontinent) cycle

• J. Tuzo Wilson (1970) suggested that plate tectonics allow supercontinents to rift apart and reform over and over on a roughly half billion year cycle

• Evidence for this: evidence of pre-Pangea supercontinents, such as Rodinia

Summary of plate tectonic theory

• Plates exist

• Plates move

• Plates interact and generate geologic phenomena

• Interiors of plates are geologically quiet*

Synthesis

• All three types

Nearby!