Plate Tectonics: Overview

• Earth’s crust: broken into about 12 major plates

• Plates move several cm each year

• Asthenosphere drives the plate – Slow moving layer– Convection currents

• Plates can be either– Oceanic: thinner but

MORE dense (sink)– Continental: thicker

but LESS dense (rise)

Table of Contents:Types of Plate Boundaries

Convergent boundaries : Compression forces push plates together

Divergent boundaries: Tension forces move plates apart

Transform Plate boundaries: Shearing forces cause plates to slide against each other (opposite directions or same direction @ different rates)

Convergent Boundary Type 1

Continental—oceanic: denser oceanic crust subducts under continental crust.

• Oceanic plate melts as it is driven down, less dense material punches through continental plate to create volcanoes and/or mountains

• Forms a subduction zone and oceanic trench

• Earthquakes occur as plate is subducted.

Ex: Andes Mtns and Peru-Chile trench

animation of the processes at a destructive plate boundary

Convergent Boundary Type 2

Oceanic—oceanic: one subducts creating a VERY deep trench and an island arc

Ex. Kuril Islands and Marianas Trench: plunges deeper into the Earth's interior (~11,000 m) than Mt Everest, the world's tallest mountain, rises above sea level (~8,854 m).

Island arcs (archipelagos) are created along plate boundaries!

Hot spots (like Hawaii) are located within the margins of plates, @ center, at plate boundaries (everywhere) magma pushes to surface

Do NOT confuse the creation of islands at convergent boundaries with islands created at hot spots!

Hotspot animation

Convergent Boundary Type 3

Continental—Continental: Both resist subduction• Land buckles/folds upward to create fold

mountains • No volcanic activity but major earthquakes often

occur due to collision.

Ex: Himalayas: Indo-Australian and Eurasian PlateThe Making of: the Himalayas

Divergent Plate boundaries

Processes: Hot magma rises to fill the 'gap' creating new crust and mountain ranges to form the mid-ocean ridge.

Both earthquakes and volcanoes occur at this type of boundary.

Rising magma can build up above ocean surface to create a volcanic island (for example Surtsey, Iceland).

(ex. North American and Eurasian Plate form the Mid-Atlantic Ridge)animation

Transform Plate boundaries• Movement: two plates move past each

other crust is neither created or destroyed

Processes:– Shearing forces build btw. the rock– Pressure builds b/c of friction between the

plates, slipping releases energy = earthquake.

Example: San Andreas Fault (N. American + Pacific Plates)

Divergent Convergent TransformContinental crust rift valley

Oceanic crust mid-ocean ridge

2 continental plates mountain range

Plates move against each other

Stress builds up

Stress is released earthquake

2 oceanic plates orOceanic/continental

subduction

Lets put it all together…

Review

• Name the type of plate boundaries• Name the plates: continental or oceanic• Name resulting features or earthquakes

Wegener:Early Observations

• German Scientist Alfred Wegener (1912) – proposed the idea of

moving continents– scientific hypothesis

based on fossil evidence, rock & mineral formations

Evidence from Rock Formations

• Similar rock types on opposite sides of the Atlantic Ocean.– Appalachian

Mountains in the US identical to rock in Greenland and Europe.

Fossil Evidence: Reptiles

• Lystrosaurus:– Early Triassic Period

(250 mya)– Land Animal– Antarctica, India, S Africa

• Cynognathus:– Late Triassic Period– “Mammal-like reptile”– Land Animal– S Africa, South America,

China, Antarctica

Evidence from Fossils

Climate Evidence: Ferns & Coal

– Certain climates support diff fern types

– Land connections were proved by same fossils on different continents

– Coal in Northern US originated in an equatorial climate but drifted North over millions of years

Why wasn’t it accepted?

• Problems with his theory:– Couldn’t explain how

large land masses were pushed such great distances

– Didn’t explain how continents moved through the solid crust

Accepted Today because:

• Know that asthenosphere has convection currents

• Currents push magma which then pushes plates

• Plates move at different rates• Not all pieces of plate move at same rate

or same direction as the plate in general!

Seafloor Spreading

Age of Rock• Rock NEAR ocean ridge= YOUNGER• Rock FAR from ocean ridge= OLDER• Oceanic rocks are relatively young compared to

continental rocks—>3.8 billion years old but oceanic is recycled @ subduction zones

• Thickness increases with distance from ridge

Magnetism & Seafloor• Earth’s magnetic field generated by flow of

molten iron in outer core (Fe)compass • Flow changes magnetic reversal compass

point S• Oceanic crust basaltic rock-contains large

amts of Fe-bearing minerals MAGNETIC!• Ocean floor shows record of past reversals

using magnetometers under water “strips” along seafloor– “strips” are mirror images on either side of

ridge symmetrical!! WOW!

• When iron cools it is magnetized by the magnetic field of the earth. • So the magnetic field of the sea

floor at any time is aligned with the Earth's magnetic field.

• But the Earth's magnetic field reverses every several thousand years and so when that happens there is new material on the sea floor which is magnetically orientated differently than the older material.

• This discovery led to the understanding that we can study rock samples far from ridge and understand what core was like/magnetic polarity millions of years ago.

Magnetic Seafloor Symmetry

Isochron

• Isochron imaginary line on map showing points that have same age (formed @ same time)

Seafloor Spreading

• New oceanic crust – formed @ ocean ridges– destroyed @ deep-sea trenches

• Magma forced to crust along ridge b/c hotter and less dense than mantle– Fills gap of spreading ridge– Magma solidifies ocean floor

• Continents like passengers ride along while ocean crust slowly moves