Sea Floor Spreading: The Evidence… 1968, scientists used a research ship to drill cores of rock from the bottom of the ocean floor the rock was dated by measuring traces of radioactive isotopes left in the rock when it was created; discovered the rock was the youngest at the mid-ocean rift and oldest at the shores of the continents measured the direction of the magnetic field that existed when the rock hardened. discovered that the Earth’s magnetic field is constantly shifting, leaving alternating bands of rock with reversed magnetic polarity Based on collected data, Atlantic Ocean spreads about 1.25 cm a year, on average—about the same rate that your fingernail grows

Sea Floor Spreading: the How… Occurs at MID-OCEAN RIDGES Hot, less dense magma rises to surface, creating NEW ocean crust here Flows sideways, due to CONVECTION in the mantle/asthenosphere Ocean lithosphere (crust) is youngest NEAR the mid-ocean ridge Mid Ocean Ridge Convection Currents Oceanic Crust (lithosphere) Rising Magma

Sea Floor Spreading: The How… Convection current: Boiling-like current of magma in the mantle (the layer below the crust). This current causes the plates to gradually move. Seafloor Spreading: Magma below earth’s crust rises to the ocean surface through cracks and causes the ocean floor to slowly widen (example: mid-Atlantic ridge)

The Plate Tectonic Theory Where are we today? The Plate Tectonic Theory

The Plate Tectonics Theory Plate Tectonics Theory combines the continental drift and seafloor spreading hypotheses Earth’s crust and solid upper mantle, called the lithosphere, floats on top of the denser, liquid rock in the mantle called the asthenosphere. The lithosphere is broken into large, continent sized sections called tectonic plates. There are about 30 tectonic plates on the Earth’s surface These tectonic plates move very slowly (about 1 to 8 cm per year). This movement is the “continental drift” that Alfred Wegener discovered in 1912.

The Plate Tectonics Theory New plate area is added by rising magma at divergent boundaries, creating the mid-ocean ridges About an equal amount of plate area is destroyed at the subduction zones of convergent boundaries

Plate Tectonics: Boundaries They Move!

Plate Boundary Movements… 3 types of plate boundary movements Convergent: Boundary between two plates are pushing together. Divergent: Boundary between two plates are moving apart. Transform: Boundary between two plates are sliding past one another.

When Boundaries Collide: Convergence… Continental vs. oceanic More dense oceanic plate dives under less dense; called subduction dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere forming a trench Crust heats and melts as it is forced downward below the continental crust Hot gases and magma forced upward creating a volcanic mountain range on the continent Example: Juan de Fuca dives under North American plate, forming the Cascade Mt. Range (Mt. Rainer, Mt. McKinley, Mt. St. Helens…)

When Boundaries Collide: Convergence… Oceanic vs. Oceanic the farther a plate gets from the mid-ocean ridge that created it, the colder and denser it gets when two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink (subduct) a subduction zone forms & a curved volcanic mountain chain (volcanic island arcs as the volcanoes rise out of the ocean) forms above the subducting plate Aleutian Peninsula of Alaska is an example of a very volcanically-active island arc

When Boundaries Collide: Convergence… Continental vs. Continental when two huge masses of continental lithosphere meet head-on, neither one can sink because both plates are too buoyant at these boundaries solid rock is crumpled and faulted huge slivers of rock many kilometers wide are thrust on top of one another, forming a towering mountain range Himalayan mountain range is the perfect example as this is how the highest mountains in the world grow (see next slide for image)

Himalayan Mtns. Taken by Satellite, 2004… Tibet (\Eurasian Continental Plate) Himalayan Mountain Range Nepal India (Indian Continental Plate)

When Boundaries Collide: Divergence… hot blobs of mantle begin to move slowly upward, forming conveyor belt-like convection currents within the asthenosphere convection currents diverge where they approach the surface exerting a weak tension or "pull" on the plate above it two sides move away in opposite directions, creating a divergent plate boundary weaknesses between the diverging plates fill with molten rock which cools and quickly solidifies, forming new oceanic lithosphere. continuous process builds a chain of volcanoes and rift valleys called a mid-ocean ridge as each batch of molten rock erupts at the mid-ocean ridge, the newly created oceanic plate moves away from the ridge where it was created

When Boundaries Collide: Divergence… Rift valley is a valley in a mountain chain created by plate tectonics at divergent boundaries Mid-ocean ridge is where two plates move apart allowing magma to surface forming a mountain chain over time. Seafloor Spreading is the process where new sea floor is created as the seafloor moves apart at a boundary creating new oceanic seafloor.

When Boundaries Collide: Transform… At transform plate boundaries plates grind past each other side by side. This type of boundary separates the North American plate from the Pacific plate along the San Andreas fault, a famous transform plate boundary that’s responsible for many of California’s earthquakes

The World as it Turns In Closing…

Final Thoughts… We can measure how fast tectonic plates are moving today, but how do scientists know what the rates of plate movement have been over geologic time? The oceans hold one of the key pieces to the puzzle. Because of the ocean-floor magnetic striping records the flip-flops in the Earth's magnetic field & knowing the approximate duration of the reversal, we can calculate the average rate of plate movement during a given time span. These average rates of plate separations can range widely. The slowest rates are less than 2.5 cm/yr; the fastest rates are more than 15 cm/yr Current plate movement can be tracked directly by means of ground-based or space-based geodetic measurements; geodesy is the science of the size and shape of the Earth.

Review… As plates move, the structure of the earth changes, creating volcanoes and building mountains