GEO 102: Plate Tectonics

The Prune Effect

• Earth starts molten then cools and contracts; Crust wrinkles and crumbles, mountains form 

Problems with this idea: 

• Not all mountains are made of lava 

Expanding Earth 20th Century

Scientists

• Henri Becquerel 

• Marie Curie 

Theory:

The pulling-apart of the crust would account for rift valleys and for the apparent fit between the coastlines of South America and Africa, which would once have been joined together on the surface of a smaller planet

• Radioactive decay 

• Heat causes Earth to expand 

• Rips apart continents 

Problems: 

• Does not explain mountains, If the earth is constentaly expanding, how do mountains form? 

Alfred Wegener (1915) & Pangea

• Polar explorer 

• Geophysicist 

• Astronomer 

• Proposed the idea of a super-continent, Pangea

Evidence

• Matching coastlines 

• Matching geology: rocks in non-connecting areas match up 

• Matching climate belts 

• Glacial deposits 

• Fossil Evidence 

Glacial Striations 

• Can tell us where the glacier came from and where they moved to 

• Need a higher ice center, dome, to spread 

Pangea: Fossil Evidence 

• Many fossils are found across non-connected continents

• Organisms cannot evolve to be exactly the same in different environments, they must have been connected 

Paleomagnetism

• If rocks contain ions, they are pulled in the direction of the poles 

• Poles don’t move, so the continents/crust must have

• The pattern of stripes on one side of a ridge was a mirror image of the pattern on the other side

• Geologists soon realized that the stripes were recording the periodic reversals of Earth's magnetic field

Harry Hess 

• Noticed earthquakes and high heat flow along mid-ocean ridge 

• Sediments thicken as you move away from continents 

• Earthquakes at edge of continents 

• Deep ocean trenches may be places where ocean crust is recycled (subduction) 

• Ocean is shallower in the middle 

• Deepest parts of the ocean are near continents 

Marie Tharp 

Created the first map of the ocean 

Parts of the Crust 

• Lithosphere: solid crust 

• Asthenosphere: moves and flows (but not liquid) 

Important factors 

Density: ocean crust is denser than continental 

Heat: hotter crust in the middle 

 As the rock cools below a temperature Curie point the magnetic alignment becomes frozen into place 

Convergent: Ocean- Continental

As the slab sinks, water trapped in its minerals is released into the overlying mantle wedge, lowering its melting point (flux melting) and triggering the formation of magma. The magma rises and erupts at the surface, producing a parallel chain of explosive volcanoes on the overriding continent

• Andes Mountains

• Dense ocean crust subducts 

• Volcanos form inland 

• Can cause tsunamis 

• 39 mm per year 

Convergent: Ocean- Ocean

Subduction produces a deep trench and a parallel arc of volcanoes built on the overriding plate, but in this case the volcanic arc rises directly from the sea floor rather than from a continent

• Japan, Philippines, Aleutian Islands 

• Forms island arcs

• Older/colder ocean crust subducts 

• Can cause tsunamis 

Convergent: Continental- Continental

The crust crumples, thickens, and is pushed upward into towering mountain ranges

• Himalaya Mountains

• Why mount Everest isn’t a volcano 

• Neither crust subducts 

• Forms mountain belts 

Divergent Plates

• Continental: rift valleys 

• East African Rift System prime example 

• When a continent begins to rift apart, the crust thins, faults develop, and a long, narrow valley opens between the two pulling-apart blocks

Transform Fault

• Two plates slide horizontally past each other without creating or destroying lithosphere

• The relative motion is parallel to the boundary, either left-lateral (sinistral) or right-lateral (dextral), depending on the direction of slip

• A small amount of vertical motion may occur where the boundary bends, but the dominant motion is sideways

• Usually associated with frequent, shallow earthquakes

• Short segments that link other types of boundary; they are particularly common as offsets along mid-ocean ridges, where the ridge crest is broken into many short segments connected by transform faults at right angles

Mantle Drag

• The mantle beneath the plates is in slow motion. Heated from below, hot rock rises, spreads beneath the lithosphere, cools, and eventually sinks again, completing a circuit known as a convection cell

• The earliest physical models of plate tectonics imagined that these convection currents drove the plates directly: where the moving mantle met the base of the rigid lithosphere, friction along that contact was thought to drag the plates along, much as a moving current carries a boat on its surface

Slab Pull

• Most powerful driving force

• As an oceanic plate ages, it cools and becomes denser

• At a subduction zone, this cold, dense slab sinks into the warmer mantle below

• As it descends, it pulls the rest of the plate along behind it, much as a heavy chain dropped over the edge of a table pulls the rest of the chain after it

Ridge Push

• More modest but still important force

• The lithosphere at a mid-ocean ridge is hot, thin, and elevated above the surrounding sea floor

• As the lithosphere moves away from the ridge, it cools, becomes denser, and sinks slightly, sliding down the gentle slope of the ridge flank under its own weight

• The push comes from the difference in gravitational potential energy between the high ridge crest and the lower, older sea floor on either side