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How come some dikes have smaller crystals on their edges?

As we know from another discussion, the amount of time that it takes for a magma to cool directly relates to how large the individual minerals become. This is called an igneous rock's texture, and it can range from glassy, when cooling takes place almost instantly, to very coarse, with cooling times possibly measured in millions of years.

Chill margins are generally associated with igneous dikes - planer intrusive bodies, like veins, which cut through existing rock. If the dike is wide enough, it is common to have variations in the amount of time it took to cool. Along the edges, where the dike rapidly lost heat to the surrounding rock, cooling is relatively quick and the resulting texture is correspondingly fine grained. This fine grained edge, called a chill margin, then acts as an insulator, allowing the central core of the dike to cool much slower, resulting in a coarser texture.

But chill margins aren't alone. The surrounding rock (called country rock) is also affected by the heat transfer which takes place as the dike cools. As the country rock heats up, metamorphic changes can take place, resulting in a metamorphic aureole extending into the surrounding rock. We call this metamorphic halo the baked zone, and they can range in thickness from microscopic to 10's of meters when surrounding huge granitic intrusives.

The term chill margin is also loosely applied to pillow basalt, which can form when mafic magma erupts on the seafloor. In this case, a thin membrane of crystallized basalt forms, and as the eruption continues it kind of blows up like a balloon. As is the case with chill margins and dikes, the central cores of individual pillows cool much more slowly, resulting in variations in texture. The glassy chill margins associated with pillows are very brittle, and often shatter and break off, supplying a large volume of broken material to fill in around the remaining pillow cores. Click here for more information on basalt and pillow basalt associated with the Josephine Ophiolite.


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