plate tectonics. what is plate tectonics? earth’s surface is made of rigid slabs of rock called...
TRANSCRIPT
What is Plate Tectonics?
Earth’s surface is made of rigid slabs of rock called plates that move with respect to each
other.
Alfred WegenerNearly 100 years ago, a German scientist named
Alfred Wegener proposed a radical idea. He proposed to his colleagues that the continents
were not fixed to the earth’s surface, but rather they were in constant motion which he called
Continental Drift.
Wegener’s Proposal cont.
In addition, Wegener also proposed that millions of years ago, all the continents were
part of a supercontinent which he named Pangaea
Evidence
As you may guess, no one believed him. He need proof. So he began his investigation to
prove his theory. He researched three different areas: Climate, Rocks, and Fossils.
Climate
Wegener suggested, at the time of Pangaea, parts of South America, Africa, India, and
Australia were located closer to Antarctica and would have been much colder and possibly
covered by glaciers.
Rocks
Wegener analyzed sediment deposits and glacial grooves cut into the rock from each
continent. He found similarities in sediment deposits and grooves carved by glaciers that
helped support his theory.
Rock EvidenceThe Caledonian and Appalachian Mountains appear to
be a single mountain chain composed of the same rock type, age, and structure.
Fossil Evidence
GlossopterisAn ancient plant that lived in a warm wet climate. The fossils have been found in
South America, Africa, India, and Australia.Today, these continents are separated by vast
oceans and it would impossible for seeds to travel that far.
Separation of the Supercontinent
120 million years ago Pangaea split into two giant land masses, Gondwanaland and
Laurasia
Gondwanaland
Gondwanaland remained in the southern hemisphere and consists of Africa, South
America, Arabia, India, Australia, and Antarctica.
Lithosphere
The cold, rigid outermost layer of the Earth is known as the lithosphere. The lithosphere is thicker below the continents and thinnest at the mid-ocean ridges. The lithosphere forms
the floating plates. The largest plate is the Pacific plate and the smallest is the Juan de
Fuca plate.
Asthenosphere
The layer below the lithosphere is the asthenosphere. This layer is so hot that it behaves like a plastic material that flows.
Plate Boundaries
Plate boundaries are located anywhere that two or more plates interact. There are three types
of boundaries: Divergent, Convergent, and Transform.
Rift Valley
A rift valley is a divergent plate boundary that occurs in the middle of a continent.
Seafloor Spreading
Seafloor spreading is the process by which new oceanic crust forms along a mid-ocean ridge. As new crust is formed, older oceanic crust moves
away for the ridge.
Convergent Plate Boundaries
Convergent Plate boundaries form where two plates collide. The denser plate sinks below the
more buoyant plate in a process called subduction. The are three main types of
convergent boundaries: Ocean-to-Continent,Ocean-to-Ocean, and Continent-to-Continent
Ocean-to-Continent
Mount Rainier in Washington State
The Marianas Trench
The Marianas Trench is the deepest location on the planet at 36,200 feet, (6.9 miles), deep. At
that depth the pressure on a body is8 tons or 16,000 lbs!
Mariana Trench
Continent-to-Continent
The Himalaya mountains are created by the collision of the Eurasian and Indian Plates
Convection Currents
Convection current is the circulation of material caused by differences in temperature and
density. Convection currents in the mantle are created by the decay of radioactive elements
such as uranium, thorium, and potassium heating the surrounding rock.
Convection Currents cont.
The heated rock becomes less dense, rises to the crust, cools and sinks back down to be
heated again. These convection currents cause three forces to act on the crust: Basal Drag,
Ridge Push, and Slab Pull
Basal Drag
Convection currents in the asthenosphere circulate and drag the lithosphere similar to a
conveyor belt.
Ridge Push
Mid-oceanic ridges have a greater elevation than the surrounding seafloor. Because the
mid- ocean ridges are higher, gravity pulls the surrounding rock down and away from the
ridge. The rising mantle material at the mid-ocean ridge creates the potential for the plates to move away from the ridge with a force called
ridge push.
Ridge Push cont.
Ridge push moves the lithosphere in opposite directions away from the mid-ocean ridge.
Slab Pull
When tectonic plates collide, the denser plate will sink into the mantle along a subduction
zone. This plate is called a slab. Because this slab is old and cold, it sinks, pulling on the rest
of the plate with a force called slab pull.