The last two structures are similar and can sometimes be confused. Laccoliths are igneous intrusions that generally form a mushroom-shape. They are relatively small in width, only one to eight kilometers in diameter, but can be up to a thousand meters in thickness. They occur at shallow depths. They form as magma rises through the country rock and meets a more resistant rock, forcing the magma to spread laterally and forming a dome in the overlying strata. Most laccoliths are silicic or intermediate in composition (see my article on Igneous Rocks for more information on the composition of igneous rocks) and as such may not spread as far as less viscous magmas. This may account for the different between laccoliths and sills.
A Lopolith is generally a large structure, lenticular (lens-shaped) and has a centrally sunken basin or funnel-shaped intrusions. Their size ranges from tens to hundreds of kilometers in diameter, and their thickness varies but are upwards of thousands of meters in thickness or more. Lopoliths are usually composed of mafic (or basic) and ultramafic rocks. (See my article on Igneous Rocks for more information on the composition of igneous rocks.)
There are five basic types of igneous intrusions. Each is formed as magma from the earth’s interior rises through the earths crust, exploiting fractures, joints, and faults that are pre-existing in the country rock. Their shape is a reflection of the composition of the igneous rock being intruded, and the geology of the country rock. They can be relatively small, on the scale of a few kilometers, or very large, covering hundreds of kilometers. Understanding how large plutons are emplaced into the crust will give a geologist a better understanding into the nature of how the different types of igneous rocks are formed.
