Why are some intrusive rocks be left behind when rocks are worn away?

Answer 1

Intrusive rocks come into other rocks in a molten state and when they cool they are frequently very hard with interlocking cemented crystal structures. When erosion occurs it is easier for wind, ice and rain to wear down the rocks into which the igneous rocks were intruded than the hard igneous rocks themselves. This leaves the igneous rocks sticking up (like King Arther's Seat in Edinburgh or the Tours of Dartmoore) as prominent features.

Answer 2

Intrusive rocks are more resistant to weathering and erosion compared to surrounding rocks because they are typically harder and more compact. Therefore, they tend to be left behind as erosive forces wear away the softer surrounding rocks, leading to them being exposed at the Earth's surface.