Final answer:
The landform is made up of igneous rock, specifically the type that forms when magma cools slowly beneath the Earth's surface, potentially leading to an intrusive igneous rock landform.
Step-by-step explanation:
The landform left at Earth's surface after magma inside a volcano cools into rock and the surrounding weaker rock is eroded away is made of igneous rock. This igneous rock could be either extrusive or intrusive, depending on where and how the magma cooled. If the magma cools slowly beneath the Earth's surface, it forms coarse-grained igneous rock known as plutonic or intrusive igneous rock (granite is an example). On the other hand, if the magma erupts and cools quickly on the surface, it forms fine-grained igneous rock, which is known as extrusive or volcanic igneous rock (basalt is an example).
Since the question describes magma cooling into rock within a volcano, it implies that the rock formed is likely an intrusive igneous rock, as it has cooled slowly deep within the Earth. After the erosion of the weaker rock around it, this hard igneous rock remains, contributing to the landform's composition.