Question

give reason the metamorphic rock are the hardest and the most compact rocks​

Answer 1

Answer:

Metamorphic rocks are almost always harder than sedimentary rocks. They are generally as hard and sometimes harder than igneous rocks. They form the roots of many mountain chains and are exposed to the surface after the softer outer layers of rocks are eroded away.

Answer 2

Answer:

I suspect you mean to indicate that metamorphic rocks are ‘Tough’, meaning that they are difficult to break, not that they are ‘Hard’, meaning that they would be difficult to penetrate with a sharp tool. For instance, diamonds are Hard, but for the most part they are not particularly Tough and can be broken when struck.

That fine point aside, most of the rocks comprising the Earth’s crust are either granular (sandstone, conglomerate, most volcanic rocks, etc.) or crystalline (granites, gabbros, most intrusive rocks). These rocks are composed of grains or crystals and will fracture along their boundaries when stressed. They tend to break and crumble fairly easily. The individual grains/crystals might be hard, but the rocks which they comprise are not particularly tough.

To address your question more directly, the metamorphic rock which most people encounter is the decorative stone sold for building sidings and floors and household use such as counter tops. They have to be quite tough or they would fracture under their intended use. When granular/crystalline rocks described above are buried at depth they are saturated with water which fills the space between the grains or crystals. When these rocks are metamorphosed by being heated under pressure, the interstitial water causes the grains/crystals to grow together into a single tough mass. This process proceeds more rapidly and thoroughly if the water is alkaline and contains significant dissolved CO2.

The metamorphic rocks which you describe as ‘Hard’ have been formed by metamorphosis into a solid, tough mass of rock with no grain/crystal boundaries, so that it is difficult for fractures to propagate through them.