how is Intrusive structure related to Volcanic activity?

Answer 1

Sills and dikes are examples of intrusive structures. They're formed when magma "intrudes" into a pre-existing rock formation, then cools. If you have a horizontal intrusion, a "sill" is formed; and if the intrusion is vertical, it will be a "dike".

Often, the igneous rock resulting from these intrusions is harder -- more erosion-resistant -- than the surrounding "country" rock. When natural processes such as weathering erodes away the country rock, the igneous rock remains and becomes exposed, sometimes in spectacular fashion. One example I've seen several times is the "Pinnacles" dike formation at Trona, California, on the edge of the Searles Dry Lake in the Mojave Desert.

Intrusive structure is therefore related to volcanic activity because magma is involved.