Question

Почему под океаном нет гранитного слоя, а только осадочные породы (песок) ?

Answer 1

Why is there no granite layer under the ocean, only sedimentary rocks (sand)?

The absence of a granite layer under the ocean and the presence of sedimentary rocks, such as sand, can be explained by the process of plate tectonics and the formation of oceanic crust.

Plate Tectonics and Oceanic Crust Formation: The Earth's lithosphere is divided into several large plates that float on the semi-fluid asthenosphere beneath them. These plates are constantly moving due to the convective currents in the mantle. Where two plates meet, different types of interactions occur, such as convergence, divergence, or transform boundaries.

Convergence Boundaries: At convergent boundaries, where two plates collide, one plate usually subducts beneath the other. This process occurs at subduction zones, where oceanic crust is forced beneath continental crust or another oceanic crust. As the oceanic crust descends into the mantle, it undergoes intense heat and pressure, causing the release of water and other volatile substances. This process leads to the partial melting of the oceanic crust, forming magma.

Formation of Granite: Granite is an igneous rock that forms from the slow cooling and solidification of magma deep within the Earth's crust. It is typically found in continental crust and is composed mainly of quartz, feldspar, and mica minerals.

Oceanic Crust Composition: The oceanic crust is primarily composed of basalt, which is an igneous rock that forms from the rapid cooling of magma at the Earth's surface. Basalt is rich in iron and magnesium minerals and has a different composition compared to granite.

Subduction and Recycling of Oceanic Crust: When oceanic crust subducts beneath another plate, it sinks into the mantle and undergoes partial melting. The melted portion of the oceanic crust can then rise to the surface through volcanic activity, forming new igneous rocks. This process is known as the subduction and recycling of oceanic crust.

Sedimentary Rocks in the Ocean: As the oceanic crust is continuously subducted and recycled, the process of erosion and weathering breaks down the exposed rocks on the continents. The eroded material, such as sand, silt, and clay, is transported by rivers and deposited in the ocean. Over time, these sediments accumulate and undergo compaction and cementation, forming sedimentary rocks.

Conclusion: In summary, the absence of a granite layer under the ocean can be attributed to the process of plate tectonics and the formation of oceanic crust. The oceanic crust is primarily composed of basalt, which forms through volcanic activity at mid-ocean ridges. When oceanic crust subducts beneath another plate, it undergoes partial melting and is recycled, leading to the formation of new igneous rocks. The sediments eroded from the continents are transported to the ocean and eventually form sedimentary rocks, such as sand.