Chapter 2: Plate Tectonics (The Unifying Theory)

 The Earth’s outermost rigid layer (lithosphere) is broken up into a many large fragments called plates.  They’re traced by distribution of earthquake epicenters and volcanism  Lithosphere = Rigid, mechanically strong outermost layer of the Earth of km thickness  Asthenosphere = Soft, weak layer below the lithosphere identified by a decrease in seismic wave velocities between ~ km depth.  The Earth’s crust comes from a range of composition with the mantle (chemistry, mineralogy, rock type).  The concept of the lithosphere is based on differences in mechanical strength with the underlying asthenosphere.  Lithosphere = Crust (continental and oceanic) + Upper Mantle  Think snow,ice,water on lake

They are traced by the distribution of earthquake epicenters (shown above) and volcanism.

 Earth’s surface is a mosaic of 13 major plates of lithosphere and some smaller plates that move slowly over the asthenosphere  There are some places (between NA plate and Pacific plate) where the plates are only moving by one another and not destoying/creating plates there.

Divergent Boundaries = Oceanic Plate separation  Mid-atlantic ridge (north american plate and eurasian plate)  Volcanoes and earthquakes are concentrated at the ridge. Divergent Boundaries = Continental Plate Separation  East African Rift Valley ( African plate and Somali Subplate)  Parallel valleys (volcanoes and eathquakes)

Divergent Boundaries Oceanic Plate Separation Mid-Atlantic Ridge North American Plate North American Plate Eurasian Plate Eurasian Plate Volcanoes and earthquakes concentrate

Divergent Boundaries Continental Plate Separation East African Rift Valley Somali Subplate African Plate Parallel valleys; volcanoes and earthquakes

 Tomographic image of oceanic slab penetrating the mantle based on seismic tomography  The chunks of rock does pass the 700km mark but only occurs only between some plates.  The chunks usually pile up in one way or another and then when they get old they drop down under the 700km mark.

Convergent Boundaries Ocean-Ocean Convergence Mariana Islands Marianas Trench Pacific Plate Philippine Plate Philippine Plate Deep-sea trench; volcanic island arc.

Convergent Boundaries Ocean-Continent Convergence Nazca Plate Andes Mountains South American Plate South American Plate Peru-Chile Trench A volcanic belt of mountains forms.

Convergent Boundaries Continent-Continent Convergence Himalaya Main thrust fault Tibetan Plateau Indian-Australian Plate Eurasian Plate Eurasian Plate Crust crumbles, creating high mountains and a wide plateau.

The Rockies near Great Falls Montana, created by satellite radar interferometry

Whole-mantle convection Upper mantle Lower mantle 700 km 2900 km Outer core Mantle Outer core Inner core

Whole-mantle convection Upper mantle Lower mantle 700 km 2900 km Outer core Mantle Outer core Inner core Stratified convection Boundary near 700 km separates the two convection systems.

Sam butler usask therm_histnoph (sab248) homepage.usask.ca/~sab248/therm_histnoph.wmv

Transform-Fault Boundaries Mid-Ocean Ridge Transform Fault North American Plate Eurasian Plate Spreading centers offset.

Transform-Fault Boundaries Continental Transform Fault North American Plate Pacific Plate Offset continental crust.

San Andreas Fault

As plates move past each other...

As plates move past each other... …creek beds are offset

As plates move past each other... …creek beds are offset San Francisco Los Angeles San Andreas fault

Convection in the outer core creates the Earth’s magnetic field Somehow—changes in convection regime cause polarity reversals of the Earth’s magnetic field, so that the SOUTH pole becomes the NORTH pole. The polarity today is called NORMAL, whereas the opposite mode is called REVERSED

The polarity of the Earth’s magnetic field is recorded in rocks (thermal remnant magnetization)

Magnetic mapping of the sea floor near Iceland An oceanic survey over the Reykjanes Ridge, part of the Mid- Atlantic Ridge southwest of Iceland, showed an oscillating pattern of magnetic field strength. This figure illustrates how scientists worked out the explanation of this pattern. Mid-Atlantic Ridge High intensity Low intensity

Magnetic mapping can measure the rate of seafloor spreading An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic Ridge southwest of Iceland, showed an oscillating pattern of magnetic field strength. This figure illustrates how scientists worked out the explanation of this pattern. Mid-Atlantic Ridge High intensity Low intensity A sensitive magnetometer records magnetic anomalies,…

Magnetic mapping can measure the rate of seafloor spreading An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic Ridge southwest of Iceland, showed an oscillating pattern of magnetic field strength. This figure illustrates how scientists worked out the explanation of this pattern. Mid-Atlantic Ridge High intensity Low intensity A sensitive magnetometer records magnetic anomalies,… Iceland Mid- Atlantic Ridge

Magnetic mapping can measure the rate of seafloor spreading An oceanic survey over the Reykjanes Ridge, part of the Mid-Atlantic Ridge southwest of Iceland, showed an oscillating pattern of magnetic field strength. This figure illustrates how scientists worked out the explanation of this pattern. Mid-Atlantic Ridge High intensity Low intensity A sensitive magnetometer records magnetic anomalies,… Iceland Mid- Atlantic Ridge …alternating bands of high and low magnetism. Symmetrical bands on both sides. Why?

Seafloor magnetic stripes

Mid-ocean ridge Ocean crust today Million years ago (Ma) 5.0 million years old

40 K- 40 Ar dating of basaltic lava flows allows for the creation of the magnetic polarity reversal time scale because the sea floor pattern of magnetic stripes can be dated.

Oceans are spreading 5-10cm per year

Hawaiian Islands have formed over a stationary hot spot Hotspots are like a jet (goes right up in a plump and not spreading horizontally). The hotspot isn’t moving but the lithosphere is moving and the volcanoes are created (hotspot blows) and islands formed. All tectonic plate activities are connected and what caused the change of momentum (change of direction) between Hawaiian chain and Emperor Seamount chain must have been because of some event on the other side of the world.

Pangea ~250 million years ago Alfred Wegener and the Continental Drifters

"Scientists still do not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter can only be reached by combining all this evidence... It is only by combining the information furnished by all the earth sciences that we can hope to determine 'truth' here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability. Further, we have to be prepared always for the possibility that each new discovery, no matter what science furnishes it, may modify the conclusions we draw.” From the “Origin of the Continents.” 1915 Alfred Wegener and the Continental Drifters

The evidence for drift—the mirror image coastlines, the similarities between sedimentary rock successions, fossil types, and climate indicators like evaporates and glacial deposits

 Type here

ASSEMBLY OF PANGAEA RODINIA Late Proterozoic, 750 Ma

ASSEMBLY OF PANGAEA RODINIA Late Proterozoic, 750 Ma Formed about 1.1 billion years ago; began to break up about 750 million years ago.

ASSEMBLY OF PANGAEA Late Proterozoic, 650 Ma

ASSEMBLY OF PANGAEA Late Proterozoic, 650 Ma The pre-Pangean pattern of continental drift.

ASSEMBLY OF PANGAEA Middle Ordovician, 458 Ma The pre-Pangean pattern of continental drift.

ASSEMBLY OF PANGAEA Early Devonian, 390 Ma The pre-Pangean pattern of continental drift.

ASSEMBLY OF PANGAEA PANGAEA (a) Early Triassic, 237 Ma

ASSEMBLY OF PANGAEA PANGAEA (a) Early Triassic, 237 Ma Assembled by 237 Ma.

BREAKUP OF PANGAEA (b) Early Jurassic, 195 Ma

BREAKUP OF PANGAEA (b) Early Jurassic, 195 Ma Signaled by the opening of rifts from which lava poured; relics can be found today in volcanic rocks from Nova Scotia to North Carolina.

BREAKUP OF PANGAEA (c) Late Jurassic, 152 Ma

BREAKUP OF PANGAEA (d) Late Cretaceous, Early Tertiary, 66 Ma

THE PRESENT-DAY AND FUTURE WORLD (e) PRESENT-DAY WORLD

THE PRESENT-DAY AND FUTURE WORLD (e) PRESENT-DAY WORLD The modern world has been produced over the past 65 million years.

THE PRESENT-DAY AND FUTURE WORLD (f) 50 million years in the future