Continental Drift Theory & Plate Tectonic Theory

Alfred Wegener in the early 1900s proposed the hypothesis that continents were once joined together in a single large land mass he called Pangea (meaning all land in Greek). He proposed that Pangea had split apart and the continents had moved gradually to their present positions - a process that became known as continental drift. CONTINENTAL DRIFT

According to the hypothesis of continental drift, continents have moved slowly to their current locations.

Pangaea about 200 million years ago, before it began breaking up. Wegener named the southern portion of Pangaea Gondwana, and the northern portion Laurasia.

The continents about 70 million years ago. Notice that the breakup of Pangea formed the Atlantic Ocean. Indias eventual collision with Eurasia would form the Himalayan Mountains.

The position of the continents today. The continents are still slowly moving, at about the speed your fingernails grow. Satellite measurements have confirmed that every year the Atlantic Ocean gets a few inches wider!

Continents fit together like a puzzle….e.g. the Atlantic coastlines of Africa and South America. The Best fit includes the continental shelves (the continental edges under water.) Wegeners Evidence for Continental Drift Picture from

Wegeners Evidence for Continental Drift Fossils of plants and animals of the same species found on different continents. Picture from /vwdocs/vwlessons/pl ate_tectonics/part3.h tml /vwdocs/vwlessons/pl ate_tectonics/part3.h tml

Wegeners Evidence for Continental Drift Rock sequences (meaning he looked at the order of rock layers) in South America, Africa, India, Antarctica, and Australia show remarkable similarities. Wegener showed that the same three layers occur at each of these places. Picture from art4.html art4.html

Wegeners Evidence for Continental Drift The same three layers are in the same order in areas now separated by oceans. Wegener proposed that the rock layers were made when all the continents were part of Pangaea. He proposed that they formed in a smaller small joined land mass that was later broken and drifted apart. Picture from art4.html art4.html

Seafloor Spreading Everyone agreed that Wegeners evidence was compelling. But wouldnt we feel the movement? Also, wouldnt there be evidence to show that the continents were still moving today? Wegener was a meteorologist and his theory was not well accepted.

Seafloor Spreading One reason scientists had a hard time with Wegeners theory is that there was no mechanism for the continents motion.

Seafloor Spreading In the 1960s, a scientist named Henry Hess made a discovery that would vindicate Wegner. Using new technology, radar, he discovered that the seafloor has both trenches and mid-ocean ridges. Henry Hess proposed the sea- floor spreading theory. Picture from USGS

Seafloor Spreading Hess proposed that hot, less dense material below Earths crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions. Picture from

Seafloor Spreading As the seafloor spreads apart at a mid- ocean ridge, new seafloor is created. The older seafloor moves away from the ridge in opposite directions. This helped explain how the crust could movesomething that the continental drift hypothesis could not do. Picture from

In 1968, scientists aboard the research ship Glomar Challenger began gathering information about the rocks on the seafloor. Scientists found that the youngest rocks are located at the mid-ocean ridges. Evidence for Spreading

Mechanism for Plate Tectonics Seafloor Spreading provided insight to the mechanism for how the continents moved. The magma which pushes up at the mid-ocean ridge provides the new land pushing the plates, and the subduction zones gobble up the land on the the other side of the plates. The mechanism was convection currents! Picture from

Plate Tectonic Theory Both Hesss discovery and Wegners continental drift theory combined into what scientists now call the Plate Tectonic Theory. Theory of plate tectonics: The Earths crust and part of the upper mantle are broken into sections, called plates which move on a plastic-like layer of the mantle

Plate Tectonic Theory Plate Tectonics explains –Earthquakes –Mountains –Volcanoes

Structure of the Earth The Earth is made up of 3 main layers: –Core –Mantle –Crust Inner core Outer core Mantle Crust

The Crust This is where we live! The Earths crust is made of: Continental Crust - thick (10-70km) - buoyant (less dense than oceanic crust) - mostly old Oceanic Crust - thin (~7 km) - dense (sinks under continental crust) - young

Plate Tectonics The Earths crust is divided into 12 major plates which are moved in various directions. This plate motion causes them to collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic set of Earth structures or tectonic features. The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

What are tectonic plates made of? Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle.

What lies beneath the tectonic plates? Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.

Plate Movement Plates of lithosphere are moved around by the underlying hot mantle convection cells

Divergent Convergent Transform Three types of plate boundary

Spreading ridges –As plates move apart new material is erupted to fill the gap Divergent Boundaries

Iceland has a divergent plate boundary running through its middle Iceland: An example of continental rifting

There are three styles of convergent plate boundaries –Continent-continent collision –Continent-oceanic crust collision –Ocean-ocean collision Convergent Boundaries

Forms mountains, e.g. European Alps, Himalayas Continent-Continent Collision

Himalayas

Called SUBDUCTION Continent-Oceanic Crust Collision

Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes Subduction

When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches. –E.g. The Mariana Trench is 11 km deep! Ocean-Ocean Plate Collision

Where plates slide past each other Transform Boundaries Above: View of the San Andreas transform fault

Volcanism is mostly focused at plate margins Pacific Ring of Fire

- Subduction - Rifting - Hotspots Volcanoes are formed by:

Pacific Ring of Fire Hotspot volcanoes

Hot mantle plumes breaching the surface in the middle of a tectonic plate What are Hotspot Volcanoes? Photo: Tom Pfeiffer / The Hawaiian island chain are examples of hotspot volcanoes.

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other.

As with volcanoes, earthquakes are not randomly distributed over the globe At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Figure showing the distribution of earthquakes around the globe

Where do earthquakes form? Figure showing the tectonic setting of earthquakes

Plate Tectonics Summary The Earth is made up of 3 main layers (core, mantle, crust) On the surface of the Earth are tectonic plates that slowly move around the globe Plates are made of crust and upper mantle (lithosphere) There are 2 types of plate There are 3 types of plate boundaries Volcanoes and Earthquakes are closely linked to the margins of the tectonic plates