Beneath our feet is a thick lithosphere, and beneath it is the asthenosphere. It is speculated that the temperature of the asthenosphere is about 1300 ° C, the pressure is 30,000 atmospheres, and the high temperature and high pressure environment make the material here in a molten state, becoming the main source of magma.
We often use eggs as a metaphor for the earth's internal structure, but in fact, the outermost lithosphere is not as complete as an eggshell, but is divided into six large plates and some small plates, which float above the slowly flowing asthenosphere, making opposite and divergent movements, shaping the basic shape of the earth. At the junction of the plates, the magma will rush outwards out of the surface along the fissure, forming a volcano.
As for why magma must gush outward, there are two main reasons: First, the density of magma is less than that of the outer lithosphere, so buoyancy is the main driving force for the rise of magma, that is, as long as the thermal energy generated by the decay of radioactive elements in the Earth's interior causes magma to form, the magma will rise. Secondly, during the ascent of magma, some components will gradually melt out, forming bubbles, generating expansion and extrusion force, so that the magma will erupt outward from the earth's interior to the surface.
In addition, the centrifugal force generated by the earth's orbit around the sun forms the earth's extrusion force from the inside to the outside, which further promotes the outward movement of the earth's internal material.
When the volcano erupts, hot material suddenly emerges and then accumulates around the crater, forming a cone-shaped mountain, which becomes a depression in the top of the cone.