Magma may cool and crystallize to become solid igneous rock when pressure increases as rock sinks into the mantle. more slowly in the presence of circulating groundwater. very slowly (in centuries to a million years) when it forms a deep pluton. when its volatile content increases.

These are all true statements. When the igneous rocks are deep in the ground they crystallize slowly. The bigger the temperature and pressure, the slower the process. This type of igneous rocks are called intrusive rocks. Because of the slow cooling and higher temperatures and pressure, the crystals are big and very well shaped, and by their size and structure you can easily tell at what depth and in what conditions the rock has formed. 

I believe July 16, 1790