Volcanoes form landforms when the lava that flows out of the peak solidifies into rock. Magma is molten rock from the Earth's mantle pushed up by the action of plate tectonics. When the magma flows or explodes out of the top of the volcano, it is called lava. Over geological timescales, this lava keeps piling on top of successive strata of cooled rock, making several different kinds of landforms.
There are four types of landforms that result from volcanic eruptions. A volcanic peak forms when the magma flowing to the surface has high viscosity due to the presence of silica. This viscosity results in slow-moving lava that does not get very far from the point of origin before solidifying. As a result, the lava keeps building up higher and higher into a peak. A lava flow volcano occurs when the viscosity is low and the molten rock travels until it cools and hardens.
A caldera forms when the existing volcano explodes and releases all of the magma in the underlying chamber. The heavy volcanic rock on top is no longer being supported by the magma, which results in a collapse that forms a circular crevice in place of the peak.
A volcanic neck is an old volcanic peak. The magma left in the magma chamber cools over time, if there isn't enough pressure to cause an explosion. The outside part of the volcano erodes and leaves behind this hardened rock.