I Divergent Plate Raft Assignment

In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which eventually become rift valleys. Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges. Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill.

Current research indicates that complex convection within the Earth's mantle allows material to rise to the base of the lithosphere beneath each divergent plate boundary.[1] This supplies the area with vast amounts of heat and a reduction in pressure that melts rock from the asthenosphere (or upper mantle) beneath the rift area forming large flood basalt or lava flows. Each eruption occurs in only a part of the plate boundary at any one time, but when it does occur, it fills in the opening gap as the two opposing plates move away from each other.

Over millions of years, tectonic plates may move many hundreds of kilometers away from both sides of a divergent plate boundary. Because of this, rocks closest to a boundary are younger than rocks further away on the same plate.

Description[edit]

At divergent boundaries, two plates move apart from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.

Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper... (from thermal contraction and subsidence).[citation needed]

It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the seafloor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made by Lawrence W. Morley, Frederick John Vine and Drummond Hoyle Matthews in the Morley–Vine–Matthews hypothesis. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

Examples[edit]

Other plate boundary types[edit]

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Continental-continental divergent/constructive boundary
Bridge across the Álfagjárift valley in southwest Iceland, that is part of the boundary between the Eurasian and North American continental tectonic plates.

Divergent Boundary

Introduction
A divergent boundary is an area where two crustal plates are separating. Most of these tectonic plate boundaries are located on the floor of the oceans. The separating plates form rift valleys on the ocean floor where there are weaknesses in the crust.

New oceanic crust
The weaknesses in the crust allow molten lava to move toward the Earth's surface on the ocean floors. The lava flowing out of vents on the ocean floor form the top of a long mountain range where the tectonic plates are separating.

Mid-Atlantic Ridge running through Iceland, USGS

Global mid ocean ridge
The mountain range that the rift valley forms circles the Earth like seems on a baseball. In the upper mantle where the plates are separating molten rock forms in magma chambers. Molten lava oozes up out of the magma chambers onto the ocean floor creating pillow basalt.

New lava forms pillow basalt
Pillow basalt covers all the ocean floors. As new lava forms in the rift valley the tectonic plates continue to separate forming the global mid ocean ridge system that is the longest mountain range on Earth.

Magnetite points to magnetic north
Scientists have found the magnetite in basalt rocks recently erupted lines up with magnetic north , which is called normal polarity. They have found that in past eruptions the magnetite lined up with the south pole, which is called reverse polarity.

Suns reversals of its magnetic poles
Just like clockworks every 11 years the Sun's magnetic poles reverse. Scientists have been documenting the reversals on the Sun for a number of years. The last time the poles switched was in late 2013.

Reversal of Earth's magnetic poles
The crystals in magnetite when it erupts onto the seafloor lines up at the present time with magnetic north. When the poles on Earth reverse the magnetite crystals in the rocks line up with magnetic south.

Past reversals on Earth
Magnetite in basalt rocks shows there have been many magnetic reversals in the past. The magnetite in the basalt rocks forms a zebra like pattern on the ocean floor. The magnetic reversals show the same stripes on both sides of the rift valley indicating the lava formed at the same time.

Iceland's divergent boundary and hot spot 
Iceland sits on a divergent boundary between the North American Plate and the Eurasian Plate that has formed the Atlantic Ocean. The island also sits over a hot spot. The frequent volcanic eruptions are a result of hot spot activity and separating plates. You can visit Iceland and stand in a rift valley similar to those found in ocean basins worldwide where the two plates are pulling apart.

More Plate Tectonic Links

Crustal Plates There are large and small plates that cover the Earth. Subduction zones form where these plates are recycled.

Mid Ocean Ridge The global mid-ocean-ridge system is the longest chain of mountains on Earth and was not discovered until the 20th Century.

Earthquake Epicenter The epicenter of an earthquake is a point on the Earth's surface, not where the earthquake originates.

Caribbean Plate This plate is small but very complex. It has a subduction zone, transform fault, and triple junction.


What is an Earthquake Find out what causes the Earth to shake, rattle and roll during an earthquake.

Divergent Boundary These boundaries form the global mid-ocean ridge system which is the largest mountain chain on Earth.

Plate Tectonics Find out lots of fascinating facts and interesting trivia on plate tectonics.

Kids Fun Science The links on our home page include information about volcanoes, science activities, plate tectonics, the rock cycle and much more


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