Why do igneous rocks form long lasting landforms?

Answer 1

Igneous rocks can be either intrusive, like granite or extrusive (erupt and then solidify at the surface) like basalt. Igneous rocks are usually homogeneous large bodies without planes of weakness, such as those found in sedimentary or metamorphic rocks (like bedding or foliation).
Chemical and physical weathering plus erosion at the surface of the earth are the processes which form most land forms. Water and/or biota are usually required by these processes. Because an igneous rock is usually a solid mass of inter grown minerals, with little or no pore space (except for pumice or other volatile rich eruptions) they are usually hard for water or biota to penetrate. For this reason they are more resistant than other rock types to weathering and therefore form prominent features in the landscape.

A good example of this is terrain inversion. If a basalt flow erupts it flows to the lowest pointy in the land scape, like a valley. Is this is in a sedimentary area, where the surrounding rocks are more easily eroded, the basalt will be the more resistant rock type. As the rocks are eroded around it, the basally which was at the lowest point in the landscape becomes the highest point, hence the term terrain inversion.