The lithospheric plates float over the viscous layer of the asthenosphere due to the initiation of convection current in the mantle. These currents are formed because of the heat that is radiated from the core of the earth.
In a divergent plate boundary, the two plates move away from one another. Due to this, seafloor spreading takes place and leads to the eruption of magma forming a ridge which is commonly known as the Mid-Oceanic ridge. Here, new materials are added to the crust.
When the continental plates are stretched and pulled apart due to the extensional force, it results in the formation of a rift, and gradually with time, it forms a rift valley. This rift valley later give rise to the formation of seas and oceans due to the tectonic activities.
The oceanic crust is made up of denser basaltic rocks containing a high amount of magnesium, calcium, iron, and aluminum, whereas the continental crust is comprised of less dense granitic rocks containing silicate minerals, feldspar. So the oceanic crust is denser than the continental crust.
The thickness of the continental and oceanic crust is about 35-45 km and 5 to 10 km respectively. Hence, the continental crust is very thick.
The hotspots are the regions that are comprised of mantle plumes, where the magma is so intense that it rises up to the surface forming volcanoes or volcanic landforms.
The ice floating over water is similar to that of the moving of the crusts over the mantle. The thickness and density, both play a significant role in keeping the crust at the top of mantle maintaining the state of equilibrium. This is known as isostasy.
