The Plate Tectonics Revolution A paradigm shift in the Earth Sciences

Topics: ! Review of last class ! Why isn’t the Earth expanding? ! Plate boundaries, earthquakes and volcanoes ! What causes plate tectonics? ! How fast do plates move? ! Earth evolution: the supercontinent cycle ! The Snowball Earth hypothesis

Alfred Wegener Key evidence for continental drift: ! coastline fit ! fossil record ! similarity of rock assemblages on different continents Development of Plate Tectonic Theory

Harry Hess Key evidence for “seafloor spreading” ! Young age of oceans relative to continents ! Mid-ocean ridges Development of Plate Tectonic Theory

The Vine-Matthews-Morley Hypothesis Oceanic Magnetic Stripes = Seafloor Spreading + Magnetic Field Reversals Development of Plate Tectonic Theory Key evidence: ! Discovery of magnetic field reversals ! Magnetic mapping of oceans

J. Tuzo Wilson 1. Hotspot hypothesis Linear volcanic island/seamount chains 2. Transform faults Earthquake activity on oceanic fracture systems 3. Wilson Cycle Geology of mountain belts Development of Plate Tectonic Theory

Why isn’t the Earth expanding?

Theory of Plate Tectonics 1. Earth’s surface is divided into rigid lithospheric plates 2. New oceanic lithosphere is created at mid-ocean ridges 3. Oceanic lithosphere is destroyed at deep-sea trenches (subduction zones) 4. This process largely explains the global distribution of earthquakes and volcanoes Development of Plate Tectonic Theory

Tectonic plates

Types of plate boundaries

Example: Iceland Characteristic Faulting Fissure eruptions at mid-oceanic ridges Type of Volcanic Activity

Ocean-Ocean (Phillipines)Ocean-Continent (Chile) Types of plate boundaries Characteristic Faulting Continent-Continent (Tibet) Type of Volcanic Activity Stratovolcanoes

Types of plate boundaries Characteristic Faulting Example: California San Andreas Fault

What causes plate tectonics? ! Heat from deep within the Earth causes mantle convection ! Decompression melting produces mid- ocean spreading ! Plate motion is assisted by slab pull

How fast do plates move? Fastest: East Pacific Rise (15 cm/year) Slowest: Arctic Ridge (2.5 cm/year)

How do plates change with time? California has changed from a convergent boundary to a transform boundary in the last 30 million years

Mount Everest India has been colliding with Asia for the past 50 million years

The plate-tectonic rewind button: Age of oceanic crust

! About 225 million years ago, a single supercontinent existed: Pangea (all lands) ! The ancient world ocean is called Panthalessa (all seas) Earth evolution: The supercontinent cycle

! About 200 million years ago, Pangea split into Laurasia and Gondwanaland ! The ocean created by the split is called the Tethys Sea. This is now the Mediterranean Sea Earth evolution: The supercontinent cycle

! About 180 million years ago, the Atlantic ocean began to form ! Continued spreading has produced the current configuration of continents Earth evolution: The supercontinent cycle

! The formation and breakup of super- continents seems to happen on a ~ 500 million year cycle ! The previous supercontinent (~ 700 million years ago) is called Rhodinia ! Another supercontinent will most likely form in Earth’s distant future

The Snowball Earth hypothesis ! Some scientists believe that million years ago, Earth experienced a series of global ice ages ! This occurred at a time when Rhodinia was at a high latitude

1. Ice caps extend toward equator 2. Snowball Earth: H 2 0 and carbon cycles shut off 3. Super- greenhouse effect from volcanoes 4. Greenhouse Earth

The Snowball Earth hypothesis ! These climate shocks appear to have triggered the evolution of multicellular animal life ! Snowball Earth is challenging long-held assumptions regarding the limits of global change.