Lumberton High Sci Vis II V205.03

 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.VLBI

 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.