Why is oceanic plate doesn't have granite layer, and continental - has?

Question

Difficult question...
A lot of theories explaining it...

Answer 1

My understanding is the only real difference between bassalt (ocean floor) and granite is crystal size. They are both formed by cooling magma and they are both classified as igneous rocks.

When molten rock upwells out of the trenches that line our ocean floors, they hit extremely cold water right away causing them to solidify as quickly as it can well out of the crack. The crystals don't have time to develop and are virtually invisible in bassalt. Granite is full of large visible crystals (they're what give it that speckled look) but in order for them to form, the rock needs too cool slowly giving the crystals time to form and grow. The bigger the crystals, the slower the magma that created it cooled.

The same happens on land--the air is tremendously colder than lava. Crystals can't start to form until the lava has stopped flowing. The air either cools it too quickly or it falls into the ocean. Granite can only form underground where it is insulated by "parent" rock.

Answer 2

okay, check it out. Oceanic crust is made through a completely different process than the contiental crust, but the two are related! It's big process. Let's start the story!

Okay, so the oceanic crust is basaltic in composition, yes? We can't really argue that. Since the terminology you used is granite, we must first specify that granite is pretty much rhyolitic or dacitic in composition (this lets us compare the relative densities, because basalt is high Fe+Mg and low Si, so it is more dense, this is why the oceanic crust is thinner, okay?) Okay, so the basaltic oceanic crust is generated by decompression melting of the mantle. This is a result of the slab pull and ridge push being applied to the oceanic crust, subsequently inducing decompression melting.

Moving along, we also know that the mantle reservoir that is undergoing the decompression melting is depleted with respect to incompatible element. So subsequently the rocks that are formed at the divergent margins are more mafic, as we observe.

Okay, so now we also know that there is a high thermal gradient near the spreading centres of the ridge. Because of intense tectonism in these areas, there's abundant fracturing of the crust. With high fracturing and high thermal gradients, you're going to get ocean water circulation through the crust. The seawater will react with the wall rock. One of the most important of these reactions is hydration - in particular chloritic alteration (chloritic alteration is essentially the hydration of mafic minerals such as pyroxenes turning them into chlorite). This hydration goes many kilometers deep into the crust - something which was revealed by deep ocean drilling.

Okay, so I'm going somewhere with this, so bear with me...

So, the chloritic alteration is pervasive and goes deep into the oceanic crust. As the oceanic crust slowly moves away from the ridge, it will cool, and circulation of hot fluids through the crust will slow down. Eventually the hydrated oceanic crust will reach the convergent plate margin, where it will begin to subduct underneath. Okay, so, now we've got what we call a mantle wedge created, which is what we call the three-dimensional area which roughly creates a triangle or wedge-shape in front of the subducting slab, and underneath the overriding plate.

As the plate subducts, it stays cool - it doesn't heat up to a great degree because of the thermal inertia relative to the rate of descent. However, as the slab descends, it will undergo a phase change at a given depth. This is because the minerals in the crust must assume a more dense form. The most significant of these is the release of the water trapped in the chlorite that we were talking about earlier. When the water is released, it will escape into the overriding mantle wedge.

When the water enters the mantle wedge, it induces partial melting of the mantle. Basically, putting water into the mantle will reduce the melting point (solidus), and the incompatible elements will escape into the melt phase.

Because of the enrichment in incompatible elements and water, the melt will be less dense than the surrounding rock and will rise upwards by positive buyancy. It will eventually reach a point where it reach neutral buoyancy and its rise will stall. This is usually at the base of the lower crust. Here the melt will stall. When this happens, mafic minerals will beging to precipitate out, and the melt will become more and more felsic (Silica rich), until it again reaches positive buoyancy. When the remaining melt is felsic enough, it will then again continue to rise into the upper crust, where it may erupt as intermediate or felsic volcanic rocks, or intrude as granitic rocks. It is in this way that the continental granitic crust was formed.

Okay, so that was a long explanation, but that's the current thinking as far as I've researched.

Answer 3

this is because of the composition of the magma under the ocean is rich in silica and magnesium which when react form a basalt .....

Sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in silicates and magnesium minerals. Typically the sima when it comes to the surface is basalt, so sometimes this layer is called the 'basalt layer' of the crust. The sima layer is also called the 'basal crust' or 'basal layer' because it is the lowest layer of the crust. Because the ocean floors are mainly sima, it is also sometimes called the 'oceanic crust'.

The name 'sima' was taken from the first two letters of silica and of magnesium. Comparable is the name 'Sial' which is the name for the upper layer of the Earth's crust.

whereas at continental crust the composition is rich in aluminium and silica.....

Sial or SiAl is the name for the upper layer of the Earth's crust, which is also known as the continental crust because it is absent in the wide oceanic basins. This layer is made of rocks rich in silicates and aluminium minerals. When the sial comes to the surface it is typically granitic, so sometimes this layer is called the 'granitic layer' of the crust. American geologists often refer to the rocks in this layer as felsic. On the continental plates the sial runs between 5 km and 70 km deep. Because in a real sense the sial floats on the sima, mountains extend down as well as up, much like icebergs on the ocean, hence the great variation in depth. See Isostasy.

The name 'sial' was taken from the first two letters of silica and of aluminium. Comparable is the name 'sima' which is the name for the lower layer of the Earth's crust, which is exposed in the ocean basins


sial is a lighter layer than that of sima layer this is the reason that continent is above oceans

Answer 4

Oceanic plates are created by upwelling mantle, which has the mineralogy of basalt. Continents have much more diverse methods of formation and tend to concentrate lighter elements. The density of basalt is about 3.3 gm/cm^2 whereas the density of granite is about 2.8 gm/cm^2. A simple way of looking at it is that continental crust "floats" on oceanic crust. If any granite were to be produced in an oceanic plate, isostacy would dictate that it rise to the top of the plate, and would eventually become accreted to a continent.

Answer 5

Basalts are formed by magma extracted directly from the upwelling mantle, commonly in the Mid Ocean Ridges (and in other settings as well).
Granites are produced by melting pre-existing sedimentary or high grade metamorphic rocks by the addition of heat, pressure, and more importantly, water. This granitic magma can be mixed with basaltic magma to form rocks of intermediate composition, that are common in the continents.
The oceanic crust by crystal fractionation, can produce more evolved magmas, but it rarely reach granite compositions, and in very small volumes, negligibly. Most of the oceanic crust is made of basalt and gabbro (the plutonic rock with similar composition than basalt).