1. Theory of plate tectonics

2. The Theory of Plate tectonics
States that pieces of the Earth’s lithosphere are in constant slow motion driven by convection currents in the mantle caused by radioactive decay in the core.

3. The theory of plate tectonics
Earth’s outer shell is broken into thin, curved plates
Radioactive decay in the Earth’s core provides the heat that drives convection currents in the mantle
This convection causes the plates to move across the lithosphere
Most earthquakes and volcanic eruptions occur at plate boundaries
Theory of plate tectonics was not fully complete until 1970, parts of it are still hotly debated in the geological community.

4. Tools used to measure plate tectonics
Sonar Mapping
Seismometers
Deep Sea Drilling
Submersibles

5. Sonar mapping
The Navy mapped with seafloor with echo sounding (sonar) to find and hide submarines during WWII and the Cold War
Maps Showed:
Oceanic Ridges: submerged mountain ranges
Fracture Zones: cracks perpendicular to ridges
Trenches: narrow, deep gashes
Abyssal Plains: vast flat areas
Seamounts: drowned undersea islands
Rocks found at the seafloor included only basalt, gabbro, and serpentinite
No continental materials found

6. Cascade Trench

7. seismometers Measure earthquakes along tectonic plates
Developed during WWII to monitor Soviet nuclear tests
Recorded every moderate to large earthquake on the planet
Seismologists found that activity happens in narrow bands
By 1950, seismometers had been deployed in over 40 allied countries and were recording data 24/7/365.

8. Bands of Seismicity – mostly at trenches and ridges

9. submersibles
In the 1960s, scientists began using deep sea submersibles to take pictures of underwater volcanoes.

10. Deep Sea Drilling
In 1968, The Glomar Challenger drilled holes in the Atlantic ocean floor and brought back rock samples
These samples showed that rocks get older as you move away from the center of the spreading center

11. Seafloor spreading
Molten material rises from the mantle and erupts and spreads out pushing older rock to both sides
Occurs at oceanic divergent boundaries
Occurs mostly at the Mid Oceanic Ridge
Plates are moving away from each other
Magma rises along a rift zone and spreads out at the surface
This surface contains new sea floor and undersea volcanoes
Age of the rocks increases as you move away from the rift zone

13. Evidence for tectonic plates: magnetic stripes
As magma cools at divergent boundaries on the seafloor, crystals “record” the direction of the current magnetic field.
These magnetic reversals are recorded in “stripe” patterns on the seafloor.
These magnetic stripes show that the seafloor has been spreading apart over time.
The magnetic field has reversed several times in Earth’s history.

16. subduction
Process by which the ocean floor sinks beneath a deep ocean trench and back into the mantle
Occurs at convergent boundaries
One plate slides beneath another
Seafloor crust is “recycled” back into the mantle
Occurs in relation to spreading so that the Earth is not growing or shrinking

18. Tectonic plates on modern earth

19. The theory of plate tectonics
States that pieces of Earth’s lithosphere are in constant motion driven by convection currents in the mantle caused by heat from the core.
Earth’s outer shell is broken into thin, curved plates
Radioactive decay in the Earth’s core provides the heat that drives convection currents in the mantle
This convection causes the plates to move across the lithosphere
Most earthquakes and volcanic eruptions occur at plate boundaries

20. Three types of plate motions
Divergent: plates move away from each other
Convergent: plates move toward each other, one goes underneath
Transform: plates slide next to each other in opposite directions