Lecture 7: Metamorphic Rocks Flashcards
What is metamorphism?
Formed by the transformation of pre-existing rocks under the influence of high temperatures and pressures
What is contact metamorphism?
Heat + uniform pressure EXAMPLE: shale (sedimentary) baked for a million years… becomes hornfels (metamorphic) with new mineral: andalusite (large crystals)
What is regional metamorphism?
Heat + differential stress (uneven pressure)
When does contact metamorphism commonly occur?
When a pre existing rock iis heated by igneous intrusion, forming a metamorphic halo or “aureole” in the adjacent rock- generally ____ in extent
In contact metamorphism does the rocks composition basically remain the same?
Yes, (ions are just redistributed to make new minerals)
What is foliation?
Regional metamorphism generally occurs in areas where 2 lithoshperic plates are pressing against one another, rocks are subjected to differential stress. In response to this stress, platy/elongate minerals line up to produce a foliated texture (folium =leaf)
What is a foliated metamorphic rock?
Schist
What is bedding?
(stratification) is a sedimentary characteristic, produced by the deposition of sediment in horizontal layers
How do you see bedding direction?
is seen in the alternating light (quarts-rich) and dark (quartz poor) layers
What is it called when rocks break in the direction of foliation?
(slaty cleavage) following the alignment of platy mica crystals
T or F: In very few cases does a parent rock’s original texture or structure survive metamorphism
True
What kind of rocks are typically foliated?
Rocks that are subject to regional metamorphism
What does ______ result in?
an increased size of mineral grains, and the development of distinct types of foliation
How does shale transform with increasing metamorphism?
Shale Slate Phyllite Schist Gneiss
What is shale?
Parent rock—original rock before metamorphism. In this case is ____ (a sedimentary rock)… Dull, breaks into layers, may contain fossils
What is slate?
Clay minerals from shale begin to transform into mica crystals that form slate. Development of very slight shininess due to slightly larger size of mica crystals relative to original clay particles.
Foliation Type: slaty cleavage
What is phyllite?
Development of more distinct, silky sheen due to further growth of platy mica crystals
Foliation Type: phyllitic foliation
What is schist?
Obvious sparkle due to much larger mirror like mica grains. In some cases mica crystals are observed to wrap around larger round crystals of minerals such as garnet (round crystals = porphyroblasts)
Foliation Type: schistose foliation (schistosity)
What is gneiss?
Mica grains transform into more elongate crystals of amphibole or pyroxene, separated from light coloured minerals such as quartz (causing banding). Round porphyroblasts may be common
Foliation Type: gneissic foliation (gneissosity)
What are non-foliated metamorphic rocks? Give an example.
Non-foliated texture caused by lack of platy/elongate minerals required to define foliation.
Quartzite (metamorphism of quartz sandstone)
Marble (metamorphism of limestone)
What is migmatite? How does it complete the rock cycle?
Extreme temps and pressures cause non-ferromagnesian minerals to melt and produce felsic magma which produces migmitate which is now part metamorphic and part igneous
Felsic magma can then collect into magma chambers to produce igneous rocks.
