Metamorphic Rocks: • Meta = change Morph = form • Metamorphic rocks form from changes to pre-existing rock caused by heat and pressure • Metamorphism can occur locally over a small area, or over a large area covering many kilometers
Two Types of Metamorphism... • Regional Metamorphism - heat and pressure are significant over a wide area (many kilometers) • Contact Metamorphism - occurs over a small area when pre-existing rocks come into contact with hot liquid magma from an igneous intrusion.
Metamorphic Rocks... • Classified on the basis of type of metamorphism (Regional or Contact) and texture: • Foliated (occurs w/regional metamorphism) • Non-foliated (usually occurs w/ contact metamorphism)
Regional Metamorphism • Changes to rock over a large area • Layers of rock deep in the Earth’s crust experience the following: • 1. High temperatures (depth and friction) • 2. High pressures (weight and crustal movement) • 3. Exposure to hot liquids, water, and gases • Results: • Mineral grains are compressed • minerals “migrate” through rock and begin to separate • minerals are aligned within rock in response topressures to produce foliation
Regional Metamorphism... Foliation - the alignment of minerals within a rock caused by heat and pressure This sample is clearly foliated. see the mineral alignment???
Metamorphic Grade(degree of metamorphism) Medium-Grade Metamorphic Rock HIGH-Grade Metamorphic Rock Low-Grade Metamorphic Rock Medium-Grade Metamorphic Rock Sedimentary Rock SHALE SLATE PHYLLITE SCHIST GNEISS Clearly “banded” appearance as minerals separate into alternating bands Clay minerals, stratified, soft, and fragile, dull in appearance Harder, microscopic mica minerals begin to form, less air space; dull shine Micas still microscopic, but begin to reflect more light; “silky” sheen in appearance Visible mica grains; foliation obvious; other minerals may be seen.
This nice piece of schist has large, visible garnet crystals in it. This sample of schist shows mica minerals that have grown large enough to reflect light.
Here is another gneiss which shows distinct banding due to mineral alignment. The arrows indicate how this sample was squeezed by pressure at great depth below ground.
Contact Metamorphism • Occurs over as small area around magma (igneous bodies) • country rock (pre-existing, in-place) is baked by the hot temperatures associated with magma • hot liquids and gases mix with country rock • Results: • minerals migrate and fuse together • no foliation or alignment since pressure is not significant
Draw layers of rock getting metamorphosed by coming into contact with magma
Metamorphism – Parent Rock Definition • Parent Rock - the pre-existing rock from which a metamorphic rock forms
Metamorphic Rock Quiz... • Metamorphic Rocks form from changes to other rocks in response to pressure and heat • TRUE
Metamorphic Rock Quiz... • Metamorphic rocks are classified by their mineral composition primarily • FALSE. Metamorphic rocks are classified on the basis of texture and type of metamorphism
Metamorphic Rock Quiz... • Foliation in metamorphic rocks is the alignment of minerals in response to high temperatures along with great pressure. • TRUE!
Metamorphic Rock Quiz... • Only contact metamorphic rocks experience foliation. • FALSE. Regional metamorphic rocks become foliated due to high pressure in addition to high temperatures
Metamorphic Rock Quiz... • Shale is a low-grade metamorphic rock • FALSE. Shale is a sedimentary rock.
Metamorphic Rock Quiz... • Middle to high grade metamorphic rocks appear shiny because as mica minerals crystallize they begin to reflect light. • TRUE!
Metamorphic Rock Quiz... • Extremely high grade metamorphic rocks experience more alignment of minerals into distinct bands. • TRUE.
Metamorphic Rock Quiz... • Rocks that come into contact with an igneous intrusion will only change into igneous rocks. • FALSE. The high heat near an igneous intrusion will “burn” the surrounding rock and change it into metamorphic rock.
Metamorphic Rock Quiz... • The pre-existing rock that was changed into a metamorphic rock is called the “Grandma” rock. • FALSE. The original rock is called the parent rock.
Metamorphic Rock Quiz... • The sedimentary rock limestone can become changed by metamorphism into a metamorphic rock called marble. • TRUE.