CHAPTER 7 - METAMORPHISM AND METAMORPHIC ROCKS (p. 165-186).
METAMORPHISM: the changes in mineral composition and texture of a rock
subjected to high temperature, pressure, and/or chemically active fluids deep within
Two kinds of Metamorphism
- REGIONAL METAMORPHIS--extenesive alteration, such as below mountain
ranges and in areas where mountain-building pressures are occurring because of Plate tectonics subduction
process or continetal collision.
where regional metamorphism is actively occuring include:
1) the base of the Mississippi Delta--over the past 100,000,000 years sediments
have piled up to a thickness of nearly nine miles beneath the Gulf of Mexico (The sediments at the bottom of this pile are undergoing regional
2) Where continental plates are colliding along "convergent boundaries" regional
metamorphism is occuring, such as in the Himalayan Mountain region. Deeply
buried rocks get "pushed up" toward the surface in mountain ranges and exposed by erosion: this
folding process raises metamorphic rocks to the surface, such as in the Appalachian and the Rocky
- CONTACT METAMORPHISM - occurs adjacent to an igneous intrusion where heat changes the rock.
Similar to toasting bread. Toast is the contact metamorphic equivalent of bread.
An unusual type of contact metamorpism is
ASTEROID IMPACTS--the heat and pressure of a large chunck of rock
shatters the rocks beneath the surface and, in an instant, can melt or even vaporize
large quantities of rock.
AGENTS OF METAMORPHISM:
These agents cause rocks to recrystallize (before melting). Most common is the conversion of
CLAY to Mica. Microscopic crystals of clay become larger with
increasing metamorphism and eventually visible mica flakes are formed in the rock schist.
- PRESSURE (associated with tectonism: forces that move rocks)
- HYDROTHERMAL FLUIDS--hot fluids (water and gases) migrating through the
subsurface under great pressure.
Plant material undergoes an easily recognizable series of changes as it undergoes
metamorphism. In the process of converting plant material to coal, volitle
compounds, such as H2O, -OH, CH4, SO2, and many organic acids are expelled.
The result is an increase in the relative abundance of carbon in the rock. Coal with
a high carbon content burns hotter, and therefore, is more valuable.
- peat - dead plant material that accumulates in swamps.
- lignite--"devolitilized" wood and plant debris (brown to dark gray)
- bitumenous--common coal (contains fossils)
- anthracite--hard, high energy coal (lacks fossils)
- graphite--metamorphic, may not have originated as coal...(?)
- diamond?--sorry, that's Hollywood; diamonds are of igneous origin!
Tectonism distorts rocks
- "Shallow depths in the Earth"--rocks "BREAK" because of the brittle nature of
solid rock, as a result faults, fissures, breccia, and joints form.
- "Deep"--rocks "BEND" or flow, like plastic, because of high confining
pressure forming FOLDS: (anticlines [upward pointing folds, "A" shaped] and synclines [downward pointing folds, "U" shaped])
TEXTURE OF METAMORPHIC ROCKS
- ROCK CLEAVAGE (SLATY CLEAVAGE)--growth of microscopic mica
crystals in metamorphic rocks causes them to break easily along mica cleavage
- SCHISTOSITY--visible recrystillized (mica) gives metamorphic rock a banded
appearrance (after the rock type: schist) (same as FOLIATION).
- NONFOLIATED: - metamorphic rocks (such as marble or quartzite) that may
not display banding. Pure minerals tend to recrystallize and flow under great
EXAMPLES OF METAMORPHIC ROCKS
Original Rock ---------------- Metamorphic Rock
sandstone --------------------- quartzite
limestone ---------------------- marble
FOLIATED (displays SLATY CLEAVAGE or SCHISTOSITY)
shale ----- slate ------ phyllite ------ schist ------- gneiss
(increasing metamorphism to the right)
granite ----------------------- gneiss
basalt ------------------------ serpentine
A metamorphic rocks ceases being a metamorphic rock when it: 1) melts and turns to magma, one metamorphic rock
that is almost remelted to magma is called Migmatite, or 2) is
weathered at the earth's surface and becomes a sediment.
Back to the Index