Intrusive rocks form as intrusions into country rock, whereas extrusive rocks form on the surface.

Temperature and pressure are crucial in determining the rate of solidification, meaning the crystallization of a rock mass. Intrusions are formed inside country rock, which prevents the magma (=molten rock) intrusion from cooling quickly. Within country rock, the conditions allow crystals sometimes to grow quite large before solidification occurs. Typically intrusions will have solidified much slower than extrusive rocks, which will solidify very quickly. As a result of the rapid cooling crystals hardly have time to form in extrusive , so the crystalline structure will be very fine.