Global Tectonics II-Origin of an Idea

• Divergent Plate Boundaries– Plates moving apart

• Convergent Plate Boundaries– Plates moving toward one another

• Transform Plate Boundaries– Plates move laterally past each other

Cascadia Suduction Zone-local Plate Tectonics

Continental Drift

• Predecessor to modern plate tectonics

• Shape and “fit” of the continents was the initial evidence– Snider-Pelligrini (1858)

– Taylor (1908)

– Wegner (1915)

Continental drift maps by Wegner (1915)

Continental Drift

• Alfred Wegener (1880-1930)– Proposed all of the continents were once

part of a large supercontinent - Pangaea

– Based on:• Similarities in shorelines • Distinctive rock and fossil groups found in

Africa & South America

Evidence for Continental Drift

• Paleontological– Similarity of fossils on opposite sides of

the Atlantic and Indian Oceans

Paleontological evidence

Evidence for Continental Drift

• Rock type & structures– Distinct rock type and geologic structures

on both sides of the Atlantic Ocean

Rock type & structure evidence

Evidence for Continental Drift

• Glaciation– Late Paleozoic glaciation

• Covered large portions of the southern continents

• Distinct glacial deposit

Reconstruction from glacial deposits

Evidence for Continental Drift

• Paleoclimate– Evidence of extreme changes in climate

as compared to the present

Paleoclimate evidence

Development of Plate Tectonic Theory

• Original evidence for continental drift was from continental rocks

• Technological advances in the 1950’s and 1960’s allowed investigation of the sea floor

• Geophysics & paleomagnetism provided new data

Geology of the Ocean Floor

• Topography of the ocean basins– Basins are divided by a large ridge

system

– Ridge system continuous around the entire globe

– Central rift valley within the ridge

• Paleomagnetism– Fe rich minerals magnetized by Earth’s

magnetic field present at time of formation.

• Polar wandering– Earth’s north magnetic pole has

(apparently) moved through time

Reconstruction from paleomagnetic data

• Magnetic reversals– Earth’s magnetic field polarity has

reversed through time

• Magnetic polarity stripes in ocean crust parallel ridges– Symmetrical on either side of the ridge

– Polarity chrons give age of seafloor• Increases away from ridge• Rates of plate motion may be calculated

Patterns of magnetic reversals

Age of the sea floor

Global Distribution of Earthquakes

Major tectonic boundaries

• Divergent Plate Boundaries– Plates moving apart

• Convergent Plate Boundaries– Plates moving toward one another

• Transform Plate Boundaries– Plates move laterally past each other

Divergent plate margins

Convergent plate boundaries

Rates of Plate Motion• Two ways to look at plate motion

– Relative velocity – the movement of one plate relative to another

– Absolute velocity – compares plate movement to a fixed position

• Rates vary from 1 to 20 cm/yr

Rates of plate motion around the world

Tectonic Mechanisms

• Convection of heat from the core and mantle drives tectonics– Convection cells bring new material to

the surface

– Old crust is pushed away from ridges

– Subduction carries cool crust back into the mantle

Models of plate tectonic motion