The oceanic crust is the section of the lithosphere that rises from the ocean basis and comprises mainly of sima or mafic stones. It is slimmer than continental crusts, but it is weightier. The continental crust is the plane of the igneous, sedimentary and metamorphic rocks, which comprises of the surface planes of the various continents in the world.
The continental crust is sometimes referred to as the sial due to its chemical composition of materials rich in aluminum and silicates. It is less weighty when drawn in comparison to the oceanic crust, which has a higher chemical composition of magnesium silicate. Now fluctuations in the velocity of the waves have proven that at a particular depth, there comes a robust peculiar contrast between the upper and lower continental crust.
We move on the continental crust, plant our crops, and do our digging on it. Even if some edges may appear to be rough or uneven, the significant parts of the crust comprise of hard rocks. The vast land areas in our continents have their roots from sedimentary, igneous, and metamorphic rocks. The shield rock is frequently regarded as the oldest rock as it is said to have existed for more than two billion years. Various planes of the earth have a variety of materials, all having peculiar attributes attached to them.
Factually, density is one of the primary reasons for the formation of the different planes in the earth. Mathematically, density is known as the mass over volume. So, in essence, thickness could be seen as how weighty a substance is.
Its planes are crust, which is lighter in density levitating on the aircraft, which are weightier, such as the mantle. Different layers of the earth are of materials with different physical properties. Both the Oceanic Crust and the Continental Crust are lighter in weight in comparison to the mantle. It is one of the reasons why the continents situated on planes are with a higher range than the regular ocean floor.
However, continental crust is much lighter in comparison to the oceanic crust, and this makes it possible for it to move higher on the planes. The oceanic and continental crusts, due to their different chemical compositions, have different densities. The continental crust comprises mainly of rocks that are granite in nature or similar to it, while the oceanic crust consists primarily of basalt.
We know that partial melting occurs in the formation of these three substances mantle, oceanic and continental crusts , and this is the reason they all vary in composition. Now, scientific studies have shown that in the process of melting a rock, some elements will remain in the fold, while others tend to move into the compressed part.
Resulting from this, the foundation formed during the partial melting is less in weight than the original stone. Then, if you also decide to subject that formed rock to the process of partial melting, you get a less dense rock. The oceanic crust is formed in the mid-ocean ridges when the partial melting takes place in the mantle. While continental crust forms when the rocks are subject to a multiple of partial melting processes, over time, thus, it results in the stones constantly getting depleted in density.
In the ocean, wherever ridges are, tectonic plates tend to drift away, and this causes molten magma to rise. It, in turn, leads to the formation of new oceanic crust. But then as this crust tends to drift away with time, it starts to age, and its temperature reduces.
I'm aware that the difference in density can be attributed to the plates differing compositions, but what I'm interested in is why these plates have different composition in the first place giving rise to their relative difference in densities.
The top levels have been proven with boreholes, whilst the lower levels have been inferred from transform fault sampling and comparisons with ophiolites. This sequence is produced by partial melting of mantle peridotite at a fairly controlled rate. In contrast, continent lithosphere is more complex and tends to be of a 'granitic' composition. This includes granites but can also include a lot of metamorphic rocks eg.
Sediments are lower density anyway high pore space , but so are quartz-rich rocks such as granites. The various processes that build continents tend to favour silica rich compositions, resulting in this bulk "granitic" composition.
For example, limited partial melting will initially produce high silica, high alkali melts. Erosion will tend to break down most common minerals before quartz - leaving quartz-rich sediments hence sandstone is primarily quartz. Migmatites are partially melted - and the melted bits are essentially granite. Basalt is denser than granite. On gravity surveys, basalts and gabbros will appear as positive anomalies, whilst granites and sedimentary basins will appear as negative anomalies.
Sign up to join this community. The best answers are voted up and rise to the top. During the s and s, geophysicists and geologists strove to understand how basaltic lava forms beneath spreading ridges. They theorized that because the oceanic plates pull apart at the surface, new material must rise to fill the gap. As the material rises, the pressure that helps keep it solid decreases. This allows hot mantle rocks to partially melt and produce basaltic liquid. However, this theory raises as many questions as it answers.
From lava compositions, we know that from an enormous volume of mantle rock, only small amounts of rock partially melt to create oceanic crust. Melt forms in micron-size pores along the boundaries of innumerable crystal grains across a mantle region that is to kilometers wide and kilometers deep. From this vast region, however, the melt somehow is focused into only a 5-kilometer-wide zone at the spreading ridge.
How is lava channeled from tiny pores in a broad region of melting into a narrow region where it forms new oceanic crust topped by massive lava flows? Our research has been funded by the U. We have shown that melt travels through the mantle in porous channels, similar to channels filled with gravel that provide permeable pathways through clay-rich soil.
Melt rising through the hot mantle can partially dissolve minerals around them and gradually enlarge the pores along the boundaries between individual crystal grains. This, in turn, creates a favorable pathway through which more melt can flow—in a positive feedback loop that spontaneously creates channels that focus the flow.
Small channels formed in this fashion coalesce to form larger channels, in a network analogous to a river drainage system.
The number and size of melt flow channels we observe in the mantle section of ophiolites supports these theories. New questions arose. If melt flows through the mantle in micron-scale pores along the boundaries of crystal grains, where does it accumulate to form massive lava flows at spreading ridges?
And, if porous flow is a continuous, gradual process, what causes the periodic bursts of molten rock that create new dikes? Once again, the Oman ophiolite provided clues. Embedded in the shallowest mantle rocks, Nicolas and Boudier found small formations of gabbro, called sills. Chemical analyses of these sills indicated that they crystallized from the same melt that formed gabbro, sheeted dikes, and lava flows in the crust. In addition, the gabbro, dikes, and lava flows all had an identical, distinctive pattern of alternating bands of dark and light minerals.
Why would melt lenses first appear in the uppermost mantle, immediately beneath the base of the crust? We propose that such lenses form where melt, approaching the seafloor, begins to cool.
Melt rising through the hot mantle can dissolve minerals surrounding it to create pore spaces, but cooling melt will begin to crystallize and clog pores. Two scenarios are possible: When the supply of melt from below is low, conduits become narrower. The melt is forced outward around impermeable barriers, migrating via diffuse porous flow along crystal grain boundaries throughout surrounding rock.
But when melt supply is large, as it is immediately beneath a spreading ridge, buoyant melt accumulates beneath impermeable barriers and creates excess pressure. Eventually, the melt bursts through the barriers and creates a melt-filled fracture that intrudes the overlying crust. If the fracture propagated high enough in the crust, it would form a sheeted dike, and if it reached even higher, it would spill out onto the seafloor and feed a lava flow.
In this cycle of buildup and release, minerals alternately crystallize and melt under conditions of higher and lower pressure. At relatively high pressure, much less of the light-colored mineral plagioclase is formed, compared to darker-colored minerals. At lower pressure, the proportion of plagioclase is larger. Thus, periodic pressure changes result in the light-and-dark banding observed in ophiolite gabbros.
Working from geological evidence in ophiolites, together with physical and chemical theory, we hypothesize that there are two distinct ways to transport melt that forms oceanic crust. Within the melting region in the mantle, melt can dissolve minerals and create additional pore space. As a result, continuous, high-porosity conduits form a coalescing drainage network that focuses melt transport to the spreading ridge. At shallow levels beneath the ridge, cooling melt begins to crystallize, clogging pore space along crystal grain boundaries.
As a result, flow becomes diffuse, melt accumulates beneath impermeable barriers. Pressure builds up until the melt periodically bursts through overlying barriers, and melt-filled fractures are injected into overlying rocks to feed dikes and lava flows. Together, these processes form a highly organized system that consistently produces new oceanic crust with a regular structure along spreading ridges. In our ongoing research, we are more rigorously testing theories about how porous conduits form in the mantle.
We seek to understand in more detail how melt lenses form beneath spreading ridges. And we want to figure out the factors that determine why and when diking and eruption events occur. Consider water flowing over a sandy surface. Where the slope is steep enough but not too steep , water begins to move sand grains downward and form channels.
As the channels grow, water flows faster, leading to more vigorous erosion of sand at the leading edge of the flow.
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