Lumberton HighSci Vis IIV205.03

The surface of the Earth is broken into large pieces of earth called plates. These plates float over an inner molten layer. The size and position of these plates change over

time.

Example - the idea of a super continent called Pangaea that existed millions of years ago.

Evidence suggests that the separate continents we know today migrated from this one large land mass.

The edges of these plates are sites of intense geologic activity.

Earthquakes, volcanoes, and mountain building can occur when they move against each other.

Plate tectonics are a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the

Earth's surface and their change in position relative to each other.

Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges.

Many changes and movements in the earth’s crust originate along Lithospheric plate boundaries.

These boundaries are not always easy to identify.

The familiar outlines of the continents and oceans depicted on maps may not resemble the outlines made by the plate boundaries.

Plate boundaries can be in the middle of the ocean floor, around the edges of continents, or within continents.

There are several types of plate boundaries, each of which is associated with a characteristic type of geologic activity.

Divergent boundaries are two plates moving apart from each other.

Also known as spreading boundary, a divergent boundary occurs where two plates move apart, allowing magma, or molten rock, to rise from the Earth's interior to fill in the gap.

The two plates move away from each other like two conveyor belts moving in opposite directions.

The process by which the plates move apart can also be referred to as sea floor spreading.

At this type of boundary, new oceanic crust is formed in the gap between two diverging plates.

Plate area is increased as the plates move apart.

Plate movement takes place laterally away from the plate boundary, which is normally marked by a rise or a ridge. The ridge or rise may be offset by a transform fault.

Presently, most divergent margins occur along the central zone of the world’s major ocean basins. The Mid-Atlantic Ridge and East Pacific Rise provide good examples of this type of plate margin.

Convergent boundaries are the direct collision of one plate with another.

As seafloor spreading pulls plates apart at one boundary, those plates push into neighboring plates at other boundaries.

The direct collision of one plate with another type of plate is called a convergent boundary.

Three types of collisions can occur at convergent boundaries.

Oceanic-continental convergence –

One type occurs when a plate with oceanic crust at its leading edge collides with a plate with continental crust at its edge.

oceanic crust is denser, it is subducted, or forced under the less dense continental crust.

Scientist refer to the region along a plate boundary where one plate moves under another plate as a subduction zone.

A deep oceanic trench generally forms along a subduction zone.

As the oceanic plate moves down into a subduction zone, it melts and becomes part of the mantle material.

Some of the magma formed rises to the surface through the continental crust and produces volcanic mountains.

Continent-continent convergence -- A second type of collision occurs when two plates with continental crust at their leading edges come together.

During this type of collision, neither plate is conducted because they both have the same density.

Instead, the colliding edges are crumpled and uplifted, producing large mountain ranges.

Scientist are convinced that the Himalayas were formed by this type of collision

Oceanic-oceanic convergent The third type of collision along

convergent boundaries occurs between oceanic crust and oceanic crust.

A deep ocean trench also forms when one of these plates is subducted.

Part of the subducted plate melts, and the resulting molten rock rise to the surface along the trench to form a chain of volcanic islands called an island arc.

Transform fault boundaries occur were two plates are grinding

past each other. Transform boundaries neither create

nor consume crust. Rather, two plates move against

each other building up tension and then releasing the tension in a sudden and often violent jerk.

This sudden jerk creates an earthquake.

The San Andreas Fault is undoubtedly the most famous transform boundary in the world.

To the west of the fault is the Pacific plate, which is moving northwest.

To the east is the North American Plate, which is moving southeast.

Los Angeles, located on the Pacific plate, is now 340 miles south of San Francisco, located on the North American plate.

In 16 million years, the plates will have moved so much that Los Angeles will be north of San Francisco!

Global Positioning System (GPS) is a constellation of 24 satellites which are used for navigation and precise geodetic position measurements.

They can also be used to plot global velocities.

Satellite Laser Ranging (SLR) determines round-trip time of light from ground-based lasers at widely separated points on the earth to mirrors on satellites.

Very Long Baseline Interferometry (VLBI) uses the difference in time at widely separated places of received quasar-emitted radio signals and can also be used to obtain plate movement.

Volcanic and earthquake activity can also be measured to help determine plate movement.

The “Ring of Fire” (area of volcanic activity) in the Pacific Ocean helped map out certain oceanic plates.