To classify a landform as a mountain a geographer considers it as what

The height the landform reaches compare to the surrounding environment.  

Explanation:

Geographer classify landform base on how far the landform rises above its surrounding environment.  Geographers uses different height level as a yard stick to classify mountain . Some say it should be at least 1000 feets  above the surrounding environment.   But one general rule about mountain is it high elevation in relation to it surrounding environment.

Mountains are formed due to tectonic forces or volcanic activities. Volcanic eruption can cause mountain formation, as two different plates collides in a convergent boundary. The denser plates slides downward causing an introduction of molten magma.  This magma pile up to form mountain belts. Example of this mountain that are formed through volcanic eruption is mountain Fuji in Japan.

Mountains can also be formed by collision of similar plates like the continental plates collision.  Example of this kind of mountain is the mountain Everest . Due to the less dense nature of the continental plates, there is active motion between the two similar plates as they converge.