Metamorphic rocks - definition: Rocks that have, in a solid state, undergone changes in minerology, texture, or chemical composition as a result of heat or pressure.


Sources of heat and pressure:

Temperature, pressure, and depth: All other things being equal, standard relationships exist between temperature, pressure, and depth in the lithosphere:

The role of fluids: The chemical transformation of metamorphic rocks is accelerated by pore fluids which act as medium for chemical reactions and avenue for migration of ions.

The role of time: The chemical reactions resulting in metamorphism progress slowly. As a result, the highest grades of metamorphism are reached in rocks that were subjected to extreme conditions for the longest time. Indeed, rocks subjected to extreme conditions for a short time may only show a low metamorphic grade.

Complex histories: In all metamorphism, rocks are reequilibrating with the conditions of their surroundings. In fact, it is possible for a high-grade metamorphic rock to reequilibrate to lower temp and pressure conditions by re-metamorphosing into low grade metamorphic rocks. This is retrograde metamorphism.

The kyanite ghosts seen on last year's Great Falls field trip are examples. The rocks of Great Falls originated as sediments deposited on the surface, were submerged to great depth and subjected to terrific differential pressures, causing the formation of the high-grade metamorphic mineral kyanite, then tey were exhumed as erosion removed overlying rock. As pressure diminished during exhumation, the kyanite recrystallized into minerals characteristic of lower grades of metamorphism, leaving "ghosts," - arrays of the new, lower-grade minerals clumped together in the shape of kyanite crystals.
Kinds of metamorphism: Your book discusses seven.

  • Subduction Zone:

    Note: Blueschist and eclogite are much denser than basalt. The "slab-pull" resulting from this density is thought to help drive plate tectonics.

  • Impact:

    Key concepts and vocabulary: