Constructive Margins
Divergent boundaries within continents initially produce rifts which produce 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. Over millions of years, tectonic plates may move many hundreds of kilometres
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. 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.
Source: http://en.wikipedia.org/wiki/Divergent_boundary
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. Over millions of years, tectonic plates may move many hundreds of kilometres
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. 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.
Source: http://en.wikipedia.org/wiki/Divergent_boundary
Example: Mid Atlantic Ridge
The Mid-Atlantic Ridge (MAR) is a mid-ocean ridge, a divergent tectonic plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South American Plate in the South Atlantic. The Ridge extends from a junction with the Gakkel Ridge (Mid-Arctic Ridge) northeast of Greenland southward to the Bouvet Triple Junction in the South Atlantic. Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough elevation to extend above sea level. The section of the ridge which includes the island of Iceland is also known as the Reykjanes Ridge. The average spreading rate for the ridge is about 2.5 cm per year.
The Mid-Atlantic Ridge (MAR) is a mid-ocean ridge, a divergent tectonic plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South American Plate in the South Atlantic. The Ridge extends from a junction with the Gakkel Ridge (Mid-Arctic Ridge) northeast of Greenland southward to the Bouvet Triple Junction in the South Atlantic. Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough elevation to extend above sea level. The section of the ridge which includes the island of Iceland is also known as the Reykjanes Ridge. The average spreading rate for the ridge is about 2.5 cm per year.
Reason for Mid Ocean Ridges
That ridges such as the mid Atlantic ridge form on constructive margins where one might have expected a trench as the two continents are moving apart is a surprise to some people . A ridge forms because the gap between the continents allows magma to come to the surface an as heat rises this builds up vertically, eventually creating a ridge.
However the size of the ridge depends on a variety of factors including water temperature, currents and pealeomagnetism but most importantly the speed at which the continents are moving apart. The faster they are diverging the lower the ridge will be as the magma that comes to the surface will be hotter and therefore more viscous or "runny" and will prevent the ridge from building up as fast. This is shown in the mid Atlantic ridge which at various parts is moving 1-11 cm a year with the corresponding rates of growth.
However the size of the ridge depends on a variety of factors including water temperature, currents and pealeomagnetism but most importantly the speed at which the continents are moving apart. The faster they are diverging the lower the ridge will be as the magma that comes to the surface will be hotter and therefore more viscous or "runny" and will prevent the ridge from building up as fast. This is shown in the mid Atlantic ridge which at various parts is moving 1-11 cm a year with the corresponding rates of growth.
Constructive MArgin: Continental (rift Valleys)
Constructive margins can also occur on land where instead of making a ridge they make a valley. This is seen in the Great East African Rift Valley in East Africa (who woulda thunk it) where the African plate is tearing itself apart. A rift valley is often very long- in this case 4000 km and wide- 50 km. There is sometimes a central block (hoist) and fractures in the plate near the edges which allow magma to the surface and become volcanoes such as Mt Kilimanjaro and Mt Kenya. The drop in altitude in the valley floor often gives rise to great lakes such as Lake Victoria and Lake Malawi.
Later Stages of a Rift Valley
Last three sections created in the free before the due lesson