9/12/2014

1

�Thin rigid plates on surface Earth

�Plates move� Toward each other

� Away from each other

� Slide past each other

�Plate motions create major geological features

Fig. 3-14

9/12/2014

2

�Alfred Wegener (1921)� Puzzle-like fit of

continents� Edward Bullard

(1960) fit continents at continental slope

Fig. 3-39b

� Matching rock structures and ages across different continents

� Extensions of mountain belts from one continent to the other

Fig. 3-39c

�Glaciated rocks and glacial deposits� Direction of glacial movement

� Glacial ice in what is now temperate regions

�Other climate evidence� Different climates, different latitudes

�Distribution of fossils� Land fossils in separate continents without

“land bridges”

9/12/2014

3

�Continents cannot “plow” through ocean basin rocks

�Proposed energy source not likely

�Not gravity and tides

�Earth’s magnetic field and paleomagnetism� Magnetic minerals

� Magnetic field at time cooled below Curie point (600oC)

� Magnetic dip (inclination)

�Latitude

�Apparent polar wandering

Fig. 3-7a

9/12/2014

4

�Normal and reversed�Ages of changes known�Magnetic anomalies across seafloor�Symmetric about axis of mid-ocean ridge�Stripe patterns trend north-south�Henry Hess (1960)

� Bathymetry of seafloor: mid-ocean ridge and deep ocean trenches

� Ocean crust created at mid-ocean ridge

� Ocean crust destroyed at ocean trench

Vine and Matthews (1963)� Magnetic stripes

represent changes in Earth’s magnetic field

�Polarity reversals

�Symmetric pattern caused

by spreading at mid-ocean ridge

9/12/2014

5

Age of seafloor by magnetic stripes

Fig. 3-12

�Radiometric age dating of sea floor reveals sea floor youngest at mid-ocean ridge, oldest toward continents

�Heat flow is higher at mid-ocean ridge�Most earthquakes occur at plate

boundaries� Different types of earthquakes at different

plate margins

�Chemical composition� Crust: surface to 30 km (average), less

dense, mainly silicates

� Mantle: 30 km to 2900 km, more dense, mainly Fe and Mg silicates

� Core: 2900 km to 6370 km, densest, Fe and Ni

9/12/2014

6

�Physical properties� Lithosphere: surface to 100 km, rigid

and brittle, cooler temperature, less pressure

� Asthenosphere: 100 km to 700 km, plastic, partially molten; high temperature

� Mesosphere: 700 km to 2900 km, both plastic and rigid behavior; high pressure

� Outer core: liquid� Inner core: rigid

Fig. 3-18

�Gravity� Density of layers

�Seismology� Earthquakes produce seismic waves

�P and S

�Seismic tomography

9/12/2014

7

Fig. 3-16 a, b

Oceanic crust Continental crustBasalt Granite

Avg thickness 8 km Avg thickness 35 km

�Highly viscous material�Flows slowly�Oceanic crust and continental crust

“float” on asthenosphere� Oceanic denser, floats lower� Continental less dense, floats higher

� Isostatic adjustments

9/12/2014

8

Fig. 3-10

Fig. 3-20

�Plates move away from each other

�Mid-ocean ridge

�Rift filled with upwelling magma

�Early separation produces narrow, linear sea� Red Sea, e.g.

�Later separation wider ocean

9/12/2014

9

�Faster spreading rates

�Broad, gentle mid-ocean ridge with less intense earthquakes

�Slower spreading rates

�Narrow, steep mid-ocean ridge with more intense earthquakes

�Plates move towards each other

�Ocean floor destroyed by subduction into mantle at ocean trench

�Earthquake zone may extend from near surface to 670 km

�Ocean plate subducted beneath continental plate

�Continental arc

�Andesite volcanic rocks

�Explosive volcanic activity

9/12/2014

10

Fig. 3-28b

�Denser oceanic plate subducted beneath other oceanic plate

� Island arc

�Basaltic volcanic rock

�Two continental plates meet

�Neither is subducted

�High mountain ranges

�Deformed rocks

9/12/2014

11

�Plates slide past each other

�Between segments of mid-ocean ridge

�Oceanic transform faults are perpendicular to mid-ocean ridge

�Continental transform faults cut across continent

� Intraplate volcanism centered on columnar areas of magma

�Volcanoes on sea floor occur in sequence

� Island chains record plate motion�Mantle plumes may extend to mantle-

core boundary�Few occur near mid-ocean ridge

Fig. 3-32

9/12/2014

12

�Seamounts=conical underwater volcanoes

�Tablemounts=flat-topped underwater volcanoes

�Formation and erosion

�Moved away from mid-ocean ridge

�Sink farther from sea level

�Charles Darwin, coral reefs, and subsiding volcanic islands

�Large calcium carbonate structures

�Fringing reef

�Barrier reef

�Atoll

Fig. 3-34