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Metamorphism is: changes in rocks beneath the earth's surface when subjected to conditions different from those at which they formed.

Evidence for these changes:

minerals not seen in rocks that crystallized from magma nor due to chemical reactions at earth's surface
structures cannot be explained by crystallization from a melt nor by deposition and accumulation by surface processes

Minerals -

- Every mineral records physical conditions under which it originated
- Changes involve T, P, chemistry (metasomatism)
- The conditions at which the minerals formed can be determined by experiment or by thermodynamic calculation.

Structural changes -

Reflected in the arrangement of the minerals. Particularly obvious for minerals with platy (sheet) or elongated shapes.

Types of metamorphism - subdivided based on general nature of agents controlling mineral changes
- contact
- burial
- regional
- impact (shock)
- dynamic - deformation only

In the field one can often trace transformation of rocks from unaltered through various stages of recrystallization in response to the agents of metamorphism (Temperature, Pressure and stress).

- One can draw lines of equal mineral change = isograds
- due to the nature of metamorphic reactions these are often (but not always) approximately = lines of constant T
- P in earth is a function primarily of the weight of the overburden. You can calculate pressure using the formula

P = rock density * g * depth.

g = force of gravity ~ 980 cm/sec2

From metamorphism we can learn something further about heat distribution (geothermal gradient) in the earth. Do this by calculating the temperature and depth (from pressure). This tells what the temperature was at the depth at which metamorphism occurred.

When this is done we find the geothermal gradient (rate of increase of temperature with depth) varies in different parts of the Earth.
Low geothermal gradients appear to correspond to subduction zones of old cold ocean crust
High geothermal gradients correspond to areas of abundant magmatic activity where melt rising from subducting slab

Chemical changes also occur during metamorphism - one place this happens is in rocks of the ocean floor. During ocean floor metamorphism,
basalt (plagioclase and pyroxene) are changed chemically. This occurs by adding about 5% water and also K and Na - from sea water, and losing Ca.

- When this chemically metamorphosed (metasomatised) ocean floor subducts - water, Na, and K are subducted.

if create you 10 km³ per yr of new ocean floor, add 5% water, and subduct as much ocean floor as is created, then 1.5x1015gm/yr of water are subducted. This would subduct the whole ocean in about 1 billion years.
So why is there any water left? It returns to the surface as result of dehydration during metamorphism and in magmas that result from melting subducted material.

Want to know more? Check out this introduction to metamorphism and metamorphic rocks.