The property of a substance that describes how easily or reluctantly it flows is called viscosity . Cooking analogies are useful when thinking about geology. If you leave it there longer - before it burns, it starts to bubble. Next, we heat up some soup, gently at first. If you keep heating the soup it begins to bubble and then rise up in the pan, finally overflowing the pan.
Planets are hotter deeper down.
Volcanism can be defined as hot, liquid rock coming to the surface of a planet. For volcanism to occur there needs to be hot underlying rock and cracks or holes in the crust where the lava can get through to the surface. The diagrams below show cross-sections of various types of volcanoes. One rule of thumb that can be applied to volcano types is that the more viscous the lava, the steeper the sides of the volcano. Often the entrance to the vent is quite large, a caldera.