Hornfels Why are hornfels massive and durable?

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The answer in number 2 is pretty thorough.  I would add that anything that is pressurized is going to be stronger.  When a heat source is applied, and things melt together and then are under pressure, they bond.  This also explains why they are massive, because they essentially meld together.

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Hornfels are a fine grained metamorphic rock that forms under exposure to high heat. Because it tends to form in contact with large hot structures such as magma domes, it is usually found in large amounts, or massifs. Its hardness is due to the fineness of grain and the fact that it is heated like a ceramic when it is formed.

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