Question

Why igneous rocks are hard?

Answer 1

Because the hardness of minerals in igneous rocks tends to be quite high.

Explanation:

Igneous rocks, like all rocks, consist of various kinds of minerals. Hardness in minerals is a function of the strength of their chemical bonds. In geology, the Mohs hardness classification was developed as a semi-quantitive way of the determining the relative hardness of minerals.

Diamond is the hardest mineral (`10`) and the mineral "talc" the softest (`1`) . Minerals like quartz and feldspar are moderately hard at hardness `6` and are the main minerals in felsic granites and rhyolites.

Hornblende (5-6) and Pyroxene minerals (5-6) are the main minerals in mafic basalts and gabbros.

So, the minerals in igneous rocks have a moderately high hardness and so igneous rocks themselves tend to be quite hard. They get even hard if they become a metamorphic rock.