difference between intrusive igneous rocks and extrusive igneous rocks
Answers
The most clear answer is that extrusive rocks form from material ejected from volcanoes or spreading ridges, e.g. rocks made from lava, tuff, pyroclastic flows, etc. whereas intrusive rocks are formed from molten rock in situ underground (often relatively deep), e.g. granite, gabbro, peridotite, dolerite, etc.
Extrusive rocks cool quickly, which means they only have time to form very small crystals (basalt), or none at all (obsidian, pumice).
If you find basalt with large crystals, those are crystals that formed over time in a magma chamber, sill or dike.
Intrusive rocks take a lot longer to cool, permitting the growth of larger crystals. Sometimes these crystals can be massive, as seen in the pegmatites of Norway.
Deep, very slow-cooking magma chambers tend to produce granites and gabbros. (Granites are a huge family of rocks that I don't want to go into here!).
The pathways taken by magma as it moves from magma chamber toward the surface eventually cool and form rock formations called dikes. (If these pathways changed direction and went horizontal the resulting rock formations are called sills). Rocks formed by magma cooled in these environments have smaller crystals than granite or gabbro but (usually) much larger than basalts.
I’ll finish by saying that extruded igneous rocks usually last much longer in the destructive environment at the earth’s surface, because they formed there! The larger minerals of intruded rocks, once exposed to the elements (weather), tend to break down relatively quickly.