Plate Boundaries and Motions

Biblical Reference There was a violent earthquake, for an angel of the Lord came down from heaven and, going to the tomb, rolled back the stone and sat on it. Matthew 28:2

Plate Tectonics Geologists use the word tectonic to describe the forces that shape Earth’s surface and the rock structures that form as a result. The cold and rigid outermost rock layer of the Earth is called the lithosphere.lithosphere –The lithosphere is made up of the crust and the solid, uppermost mantle. –The lithosphere is thin below mid-ocean ridges and thick below continents.

Plate Tectonics Earth’s tectonic plates are large pieces of the lithosphere that fit together like the pieces of a giant jigsaw puzzle. The layer of Earth below the lithosphere, called the asthenosphere, is so hot that it behaves like a plastic material and enables Earth’s plates to move.

Earth’s Tectonic Plates

Plate Tectonics Theory The theory of plate tectonics, proposed in the late 1960s, states that Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other.plate tectonics Plate tectonics suggests that Earth’s surface is divided into large plates of rigid rock and each plate moves over Earth’s hot and semiplastic mantle.

Plate Boundaries There are three types of Plate Boundaries on Earth: –Divergent Plate Boundaries –Convergent Plate Boundaries –Transform Plate Boundaries

Divergent Plate Boundaries A divergent plate boundary forms where two plates separate.divergent plate boundary When the seafloor spreads at a mid-ocean ridge, lava erupts, cools, and forms new oceanic crust. –Divergent plate boundaries can also exist in the middle of a continent, where they pull continents apart and form rift valleys.

Divergent Plate Boundaries

Convergent Boundaries Convergent plate boundaries form where two plates collide.Convergent plate boundaries The denser plate sinks below the more buoyant plate in a process called subduction. subduction –The area where a denser plate descends into Earth along a convergent plate boundary is called a subduction zone. –Subduction comes from the Latin word, subductus, which means “to lead under, removal”.

Convergent Plate Boundaries When an oceanic plate and a continental plate collide, the denser oceanic plate subducts under the edge of the continent, creating a deep ocean trench and a line of volcanoes above the subducting plate on the edge of the continent. When two continental plates collide, neither plate is subducted, and mountains form from uplifted rock.

Convergent Plate Boundaries

Transform Plate Boundaries A transform plate boundary forms where two plates slide past each other.transform plate boundary As they move past one another, the plates can get stuck and stop moving. Stress builds up where the plates are stuck until they eventually break and suddenly move apart, resulting in a rapid release of energy as earthquakes.

Transform Plate Boundaries

Evidence for Plate Tectonics Scientists can measure how fast continents move using a network of satellites called the Global Positioning System (GPS). Because plates are rigid, tectonic activity occurs where plates meet.

Evidence for Plate Tectonics Volcanoes form where plates separate along a mid-ocean ridge or continental rift or collide along a subduction zone. Mountains can form where two continents collide.

Volcanoes & Plate Tectonics Notice the relationship between earthquake epicenters, volcanoes, and plate boundaries.

Plate Motion Earth’s plates move because the asthenosphere moves underneath the lithosphere. Convection is the circulation of material caused by differences in density.Convection –Hot mantle material rises upward and comes in contact with Earth’s crust. –As the mantle cools, it becomes denser and then sinks, forming a convection current. –Convection currents in the asthenosphere act like a conveyor belt moving the lithosphere above it.

Convection

Plate Motion - Forces There are three forces that interact to cause plate motion: –Basal Drag –Ridge Push –Slab Pull.

Basal Drag Basal drag refers to how convection currents in the asthenosphere circulate and drag the lithosphere like a conveyor belt.

Ridge Push Rising mantle material at mid-ocean ridges creates the potential for plates to move away from the ridge with a force called ridge push. ridge push

Slab Pull As a slab, or dense plate, sinks, it pulls on the rest of the plate with a force called slab pull. slab pull

A Theory in Progress Plate tectonics has become the unifying theory of geology, but several unanswered questions remain. –Why is Earth the only planet in the solar system that has plate tectonic activity? –Why do some earthquakes and volcanoes occur far away from plate boundaries? –What forces dominate plate motion?

What will Scientists Study Next? This is a 3-D image of seismic wave velocities from a new technique called anisotropy.

Mount St. Helens – Back from the Dead 0:15 – 0:0:15 – 0:30

A.semiplastic B.lithosphere C.tectonic D.mantle What word do geologists use to describe the forces that shape Earth’s surface? Pop Quiz

Which of these is made up of the crust and the solid, uppermost mantle? Pop Quiz A.seafloor B.lithosphere C.biosphere D.hydrosphere

A.convergent plate boundary B.transform plate boundary C.new plate boundary D.divergent plate boundary What type of boundary forms where two plates separate? Pop Quiz