Module 2.2: Rocks (Metamorphic)

Metamorphism

It is the change undergone by an existing rock, in the solid state, to another rock.

Metamorphic rocks

Rocks that resulted from mineralogical and structural adjustments of solid rocks to changes in the physical and chemical conditions

Heat, pressure, and chemically active fluids

The three agents of metamorphism

Heat

Provides the energy to drive chemical reactions, resulting in recrystallization of minerals

Geothermal gradient and large bodies of molten rock or intrusive bodies

Two main sources of heat for rock metamorphism

Geothermal gradient

Rate at which temperature increases with depth beneath the Earth’s surface

Confining pressure

• Type of pressure where compression is happening in all directions

• Leads to smaller, denser packets

Directed pressure

• Type of pressure where compression is only one direction

• Leads to foliation

Water trapped in pore spaces and from magmatic bodies (hydrothermal fluids)

Two main sources of chemically active fluids for rock metamorphism

1. Shock metamorphism

2. Burial metamorphism

3. Regional metamorphism

4. High strain (cataclastic) metamorphism

5. Contact metamorphism

6. Ocean ridge (hydrothermal) metamorphism

Types of metamorphism

Contact metamorphism

• Type of metamorphism wherein the dominant agent is heat as magma intrudes pre-existing rocks

• Takes place at shallow depths and low pressure

• Produces non-foliated, fine-grained rocks

Contact metamorphism

Type of metamorphism that forms marble and quartzite

Aureole/halo

The zone of alteration where rocks form in contact metamorphism

Hydrothermal metamorphism

• Chemical alteration at high temperatures and moderate pressures by hot, ion-rich (hydrothermal) fluids that circulate through fissures and cracks

• Common in basaltic rocks from rich ore deposits

Hydrothermal metamorphism

Type of metamorphism that forms greenstone (metamorphosed basalt)

Regional metamorphism

• Takes place at considerable depths over an extensive area

• Rocks are formed under high pressure associated with mountain building

Regional metamorphism

Type of metamorphism that forms slate, phyllite, schist, gneiss, and migmatite

Subduction zone metamorphism

• Takes place at shallow and greater depths

• Forms rocks under high pressure and low temperature

Subduction zone metamorphism

Type of metamorphism that forms blueschist and eclogite

Burial metamorphism

• Takes place at shallow depths due to increase in overlying pressure

• Forms rocks in relatively low temperatures

Burial metamorphism

Type of metamorphism that forms metaconglomerate

Dynamic metamorphism

Takes place along fault zone where there is very high shear stress

Cataclastic rocks

Rocks formed in dynamic metamorphism through low pressures as the force breaks and grinds the rock

Mylonites

Rocks formed in dynamic metamorphism through high pressures as ductile deformation happens

Shock metamorphism

• Metamorphism that occurs at impact of a celestial body

• Pressures can reach up to 40 Gpa and raise temperatures up to 500 C in a fraction of a second

Shock metamorphism

Type of metamorphism that forms shocked quartz and shatter cones

Texture

Classification of metamorphic rocks based on size, shape, and relationships of constituent minerals

Protolith

Classification of metamorphic rocks based on original rock that was transformed by metamorphism

Mineralogy

Classification of metamorphic rocks based on the mineral assemblage present

Foliated texture

Texture that exhibits a pervasive planar structure due to the nearly parallel alignment of minerals in the rock

Non-foliated texture

Texture having no preferred mineral orientations

Slate

Foliated metamorphic rocks with very fine grains which has a well-developed slaty cleavage

Phyllite

Foliated metamorphic rocks with fine to medium grains with a silky, metallic luster

Schist

Foliated metamorphic rocks with coarse-grained crystals made up mostly of micaceous minerals

Gneiss

Foliated metamorphic rocks with coarse-grained crystals with well-developed color banding of mostly non-micaceous minerals

Slate

Identify the metamorphic rock

Slate

Identify the metamorphic rock

Phyllite

Identify the metamorphic rock

Phyllite

Identify the metamorphic rock

Phyllite

Identify the metamorphic rock

Schist

Identify the metamorphic rock

Schist

Identify the metamorphic rock

Schist

Identify the metamorphic rock

Gneiss

Identify the metamorphic rock

Gneiss

Identify the metamorphic rock

Gneiss

Identify the metamorphic rock

Quartzite

Non-foliated metamorphic rock composed of interlocking crystal grains, hard enough to scratch glass

Quartzite

Identify the metamorphic rock

Quartzite

Identify the metamorphic rock

Amphibolite

Non-foliated metamorphic rock composed of amphibole and plagioclase and is coarse-grained

Amphibolite

Identify the metamorphic rock

Amphibolite

Identify the metamorphic rock

Hornfels

Non-foliated metamorphic rock composed of pyroxene/quartz and plagioclase and is fine-grained

Hornfels

Identify the metamorphic rock

Hornfels

Identify the metamorphic rock

Marble

• Non-foliated metamorphic rock composed of interlocking calcite grains, variably colored, and is too soft to scratch glass

• Effervesces at the contact of acid

• Composed of coarsely-grained crystals

Marble

Identify the metamorphic rock

Marble

Identify the metamorphic rock

Serpentinite

• Non-foliated metamorphic rocks composed chiefly of serpentine

• Appearance is mesh-like mottled with green specks and have violet veins

Serpentinite

Identify the metamorphic rock

Serpentinite

Identify the metamorphic rock

Greenstone

Identify the metamorphic rock

Blueschist

Identify the metamorphic rock

Eclogite

Identify the metamorphic rock

Metaconglomerate

Identify the metamorphic rock

Cataclastic rocks

Identify the metamorphic rock

Mylonite

Identify the metamorphic rock

Shocked quartz

Identify the metamorphic rock

Shatter cones

Identify the metamorphic rock