Download METAMORPHIC ROCKS

METAMORPHIC ROCKS
METAMORPHISM
• Alteration of any previously
existing rocks by high pressures,
high temperatures, and/or
chemically active fluids.
Agents of Metamorphism
– high temperatures -
• most important agent because it provides energy to drive
chemical reactions
• geothermal gradient (25o C /km) & compressional heat
• proximity to magmatic intrusions
– high pressures •
•
•
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compressive stresses during mountain building
increases with depth
can cause rock to flow or bend (deform plastically)
sliding (shearing) can pulverize rock
– chemically active fluids • water is in pore spaces of most rocks
• many minerals are hydrated
• hot water contains ions in solution that form new minerals
Textural, Structural & Mineralogical Changes
• Degree of metamorphism is reflected by
– mineral composition
– texture of rocks
• Rocks become more compact (denser)
• Minerals recrystallize and grow larger in solid state
• Some new minerals form
• Preferred (structural) mineral orientations develop
– perpendicular to stress
• platy minerals
• elongated minerals (needle-like)
– causing foliation, a layered structure
• chemical mineral segregation into light and dark bands
Foliation
• Rock cleavage (slaty cleavage)– low-grade metamorphism (low pressure, low temp.)
– clay minerals recrystallize to minute mica flakes
– develops parallel fracturing
– forms slate
• Schistosity
– high-grade metamorphism
– mica crystals become visible (scaly appearance)
– forms schist
• "banding"
– highest-grade metamorphism
– silicate minerals segregate into light & dark bands
– forms gneiss
Development of Rock Cleavage (Slaty Cleavage)
• High pressure, associated with folding, develops parallel
alignment of growing microscopic mica crystals (in a former
clayey shale) that are perpendicular to the stress direction
Rock Cleavage in Outcrop
• Rock cleavage enhances
erosion
Development of Banding
• Light and dark minerals segregate out into parallel layers, such as
in a gneiss, that are perpendicular to the stress direction
Low-Grade Foliated Metamorphic Rocks
• Shale metamorphoses to slate
• Clay minerals alter to
microscopic mica crystals
• Rock cleavage develops as the
mica crystals align parallel to
each other and perpendicular
to the main stress direction
• Breaks into parallel sheets
• Once used for school
blackboards
• Slate metamorphoses to phyllite
• Microscopic mica crystals
enlarge, giving the rock a glossy
sheen
High-Grade Foliated Metamorphic Rocks
• Phyllite metamorphoses to
schist
– Mica crystals become
easily visible to the naked
eye
Highest-Grade Foliated Metamorphic Rocks
• Schists metamorphose to
gneiss
– Minerals alter to form
abundant quartz &
feldspar
– Light and dark minerals
separate out into bands
Further Metamorphism Produces Migmatites
• When the temperatures
during metamorphism are
high enough, low-meltingpoint minerals melt and
the hot fluid separates
from the still-solid, highmelting-point minerals
• Forms bands of
alternating igneous and
metamorphic rock
– light bands = igneous
– dark bands = meta.
Non-Foliated Metamorphic Rocks
• Foliation will not develop in rocks that are
– Equant grained (equidimensional in all directions)
– Monomineralic (e. g., all quartz, all calcite, etc.)
• Example #1: Quartz sandstone metamorphoses to a
quartzite which may look the same whether it forms
under low-grade to high-grade metamorphic conditions
Non-Foliated Metamorphic Rocks
• Example #2: Limestones metamorphose to marble
– Equant grained
– All calcite
Metamorphism Along Fault Zones
• Movement along a fault zone may break up and grind
the adjacent rocks, due to shear, to form a breccia that is
called a "fault breccia" near the surface where rocks are
brittle.
• The fragments in a breccia are very angular
• A high pressures deeper in Earth, rocks flow & don't
form breccias
Contact Metamorphism
• Magma comes into contact with cooler country rock
– base of thick lava flows
– near-surface dikes and sills
– around deep stocks & batholiths
• Heat and fluids from magma alter country rock
• Forms concentric zones of varying degrees of mineral
alteration
• Hydrothermal solutions from the magma can form
metallic ore deposits in the country rocks
Regional Metamorphism
• Intense metamorphism and deformation during mountain
building associated with lithospheric plate collisions
• increases towards the core or major mountain ranges
• most common type of metamorphism
Mineral Changes with Increasing Metamorphism