Download Metamorphic Rocks!

• Metamorphic rocks are formed under heat, pressure or
both deep in the Earth.
• Look very different (most of the time) from parent rock
Ig
Sed
Met
Met
FORMATION OF METAMORPHIC ROCKS
• Most metamorphic changes= elevated temperatures and
pressures
• Occur few kilometers below Earth’s surface and into
upper mantle
 Two locations for metamorphism:
① Contact: magma intrudes rock
① Regional: large areas of rock are subjected to extreme
pressures and temperatures
FORMATION: CONTACT METAMORPHISM
• When magma intrudes solid rock
• Produces low-grade metamorphism; minor changes
• High temperatures & moderate-to-low pressure
• Temperature decreases with distance from intrusion
• Metamorphic effects also decrease with distance
• Limited to thin zones because lava cools quickly
• Example: marble  forms when magma intrudes a
limestone body
FORMATION: REGIONAL METAMORPHISM
• When high temperature and pressure affect large areas of
Earth’s crust  belts of regional metamorphism
• Range in grade: low  high
• Ex: mountain building = high-grade metamorphism
• Folding and deforming of layers in the area
REGIONAL METAMORPHISM
AGENTS OF METAMORPHISM
• During metamorphism, rocks are usually subjected to 3 agents
(at the same time):
①Heat: provides energy needed to drive chemical reactions
②Pressure (stress): causes spaces between mineral grains
to close
①Hydrothermal solutions: surrounds mineral grains and help
recrystallization process
AGENTS: HEAT
• Most important agent  provides energy needed to drive chemical
reactions  cause existing minerals to recrystallize or new minerals to
form
• Heat originates from magma & change in temperature with depth
• Upper crust  increase averages between 20°C and 30°C per
kilometer!
• Different minerals recrystallize at different temperatures
• Example: 8 km 150°C-200°C
• Clay minerals: become unstable and recrystallize to form new stable
minerals (chlorite)
• Silicate minerals: stable @ these temps.  requires higher temps. to
change silicates
AGENTS: PRESSURE (STRESS)
• Pressure increases with depth
• Applied in all directions
• Causes spaces between mineral
grain to close
• Results in more compact rock 
greater density
• May cause minerals to recrystallize
 new minerals
• Cause rocks to flow versus fracture 
grains flatten & elongate
AGENTS: HYDROTHERMAL SOLUTIONS
•
•
When very hot water reacts with rock and alters chemical and mineral
composition
Hot fluids migrate in and out of the rock  original mineral composition and
texture changes
• Example: gold lodes = gold deposit fills fissure in rock usually in vein
formation
A view of an undeveloped lode gold
deposit. The gold is in the white
veins in the photo. Andes Mtns
CLASSIFICATION OF METAMORPHIC ROCKS
• Classified by texture and composition
Gneiss
• Texture can be foliated or nonfoliated:
①Foliated: layered or banded appearance
 Contact metamorphism more compact more dense
 Pressure causes microscopic minerals to compact and align
in similar direction
 Certain minerals can recrystallize in extreme conditions  flat
needle like crystals form perpendicular to direction of force
 Example: gneiss
CLASSIFICATION OF METAMORPHIC ROCKS (CONT’D)
② Nonfoliated: does not have banded texture
• most contain only one mineral & blocky crystal shapes
• Example: marble – nonfoliated contact metamorphic
rock made of calcite; parent rock
(limestone) metamorphosed, calcite
crystals combine larger
Marble
interlocking crystals seen in marble
MIGHTY MORPHING ROCKS!
Aureole = site of contact meta.
• A metamorphic aureole
in the Henry Mountains,
Utah.
• Top = greyish rock 
igneous intrusion,
consisting of porphyritic
granodiorite
• Bottom = pinkish rock
on the bottom is the
sedimentary country
rock, a siltstone.
• Middle =
metamorphosed
siltstone is visible as
both the dark layer
(~5cm thick) and the
pale layer below it.