Plate Tectonics

The Structure of the Earth A thin crust km thick A mantle – has the properties of a solid but it can also flow A core – made of molten nickel and iron. Outer part is liquid and inner part is solid The average density of the Earth is much higher than the crust, so the inner core must be very dense

The Crust Sedimentary rocks settle in layers. The oldest rock is at the bottom. Layers of sedimentary rock can be examined to discover how they were formed. They are often found folded or fractured:

Tectonic theory People once thought that the oceans and the continents were formed by shrinkage from when the Earth cooled down after being formed. Alfred Wegener proposed something different. Consider Africa and South America: These continents look like they “fit” together. They also have similar rock patterns and fossil records. These two pieces of evidence led me to believe that there was once a single land mass. This is my TECTONIC THEORY.

Forming mountains The formation of mountain ranges can be explained by tectonic theory. Consider the Himalayas at the top of India: This is where India is now This is where India was millions of years ago The intense heat and pressure from this process causes the rocks to change structure into metamorphic rocks.

The Answer: 1)Scientists discovered 50 years later that the Earth generates massive amounts of heat through radioactive decay in the core. This heat generated convection currents in the mantle causing the crust to move 2)We also now know that the sea floor is spreading outwards from plate boundaries Tectonic theory The Evidence: 1)Some continents look like they used to “fit” together 2)Similar rock patterns and fossil records The Problems: Wegener couldn't explain how continental drift happened so nobody believed him Conclusion – scientists now believe Wegener’s Tectonic Theory

The Earth’s crust in motion

Convection currents Convection currents beneath the plates move the crustal plates in different directions The source of heat driving the convection currents is radioactivity deep in the Earths mantle Convection currents carry heat from the hot inner mantle to the cooler outer mantle.

Constructive Margin

Mid-Atlantic Ridge

How is it formed? Convection currents cause the plates to move apart As the plates move apart the magma wells up from the mantle to form new basaltic oceanic crust (new plate area is formed) The Earth’s surface area increases due to the formation of new oceanic crust – sea floor spreading

Mid-Atlantic Ridge The Mid-Atlantic Ridge passing through Iceland

Destructive Margin

What happens at the destructive margin? The thicker, denser oceanic plate slides beneath the continental plate (subduction) The oceanic plate melts as it reaches the hot mantle Folding of the continental plate forms mountains As the plates slide past each other, friction stores energy in rocks – energy is released in earthquakes

Subduction

The Andes The Andes are found at a destructive margin along the western coast of South America The Andes (shown in white)

Collision plate margins

The Collision Zone When continental plates collide head on with other continental plates, the result is upheaval! Similar densities – neither plate is subducted Gradual forward movement of each plate creates extreme pressures Over time, the rock strata become folded, and rise to create mountain ranges

The Alps and the Himalayas The Alps The Himalayas

Conservative Margins

What happens at conservative margins? As the plates slide horizontally past each other pressure builds up in the rocks either side of the fault Fault surface often rough – friction creates large strains along the faults An earthquake happens when the built up pressure and energy are released in a sudden, jerky movement

San Andreas Fault The San Andreas Fault is the border between two tectonic plates — the North American Plate and Pacific (Nazca) Plate