$(1)$ The earth's hot crust and its shape: Although the earth's crust is solid, its center is still warm. The earth's crust also stays warm due to the heat generated by the chemical decomposition of radioactive minerals such as uranium, radium, and thorium in the interior.
$(2)$ Emergence of magma: In weak regions, for whatever reason, the pressure of the upper rock layers decreases and due to the heat of the subsoil, these rocks turn into magma. This magma in the subsoil of the earth is also responsible for volcanic eruptions.
$(3)$ Emergence of gases and steam: Gases are formed due to the expansion of the size of the magma and the heat contained in it. The cracks in the rock layers that convert seawater and groundwater into the subsoil are also converted into steam.
$(4)$ Lifting of magma towards the surface: Steam occupies more space than water. The vapor in the earth tries to get out very fast as it does not have enough space and space. It also pushes the magma fluid towards the earth's surface. The result is shallow rock layers breaking out or rushing out quietly.