Metamorphic Environments

Contact (or thermal)

Hydrothermal

Burial

Regional

Shock (impact)

Fault Zone

Contact Metamorphism

Contact or thermal metamorphism occurs when an intrusive magma heats the surrounding country (or host) rock and changes the mineralogy and texture

Contact Metamorphism

The zone where the rocks are subject to metamorphism is called the metamorphic aureole

Contact Metamorphism The sedimentary rocks are turned into metamorphic rock by contact metamorphism

Contact Metamorphism Even small dykes can form aureole of metamorphic rock a few centimeters thick

Contact Metamorphism The metamorphic aureole is the darker rock that once roofed over the igneous pluton

Hydrothermal Metamorphism Hydrothermal fluids can carry dissolved calcium dioxide, sodium, silica, copper and zinc

Ascending hydrothermal fluids can react with overlying rock, creating new minerals (which may have great economic value)

Hydrothermal Metamorphism The most widespread occurrence of hydrothermal metamorphism is along the mid-oceanic ridges

As seawater percolates through the newly created crust, it is heated and chemically reacts with the mafic (Fe and Mg rich) basalt

Hydrothermal Metamorphism The ferromagnesian igneous minerals, such as olivine and pyroxene, are changed into metamorphic minerals such as serpentine, chlorite and talc

Calcium-rich plagioclase feldspars become more sodium-rich as the sea salt (NaCl) exchanges calcium for sodium

Black Smokers Large amounts of metals, such as iron, cobalt, nickel, silver, gold and copper, are dissolved from the newly formed crust

These hot (~350oC), metal-rich fluids rise along fractures, generating particle-filled clouds called black smokers

Black smokers were first discovered in 1977 around the Galápagos Islands by the small submersible vehicle called Alvin

Smokers have now been found in all oceans

Black Smokers

Although life is very sparse at these depths, black smokers are the center of entire ecosystems

Sunlight is nonexistent, so many organisms must convert the heat, methane, and sulfur compounds provided by black smokers into energy through a process called chemosynthesis

Black Smokers

Hydrothermal vents support a large population of chemosynthetic bacteria

The bacteria then grow into a thick mat which attracts other organisms such as amphipods and copepods which graze upon the bacteria directly

Larger organisms such as snails, shrimp, crabs, tube worms, fish, and octopuses form a food chain of predator and prey

Black Smokers

Burial Metamorphism Burial metamorphism occurs when thick accumulations of sedimentary strata on the ocean floor are subducted beneath another plate

Burial Metamorphism This is a low grade metamorphism that typically begins when the subducted sediments reach a depth of 6-10 kilometers (3-6 miles) or when the temperature reaches about 200oC

Regional Metamorphism Most metamorphic rocks are created during the process of regional metamorphism associated with mountain building

During these dynamic events, large segments of the Earth’s crust are intensely deformed along convergent plate boundaries

Regional Metamorphism The mountain building applies differential stress literally over a wide regional area

Sediments and crustal rock lifted up from the ocean floor are folded and faulted

Metamorphism of all grades, from low to high occurs

Regional Metamorphism The Andes Mountains and the Himalaya Mountains (below) are prime examples where regional metamorphism has occurred along thousands of miles of mountain range

Regional Metamorphism The Swiss and Austrian Alps in Europe are other famous examples where extensive regional metamorphism has occurred

Impact metamorphism occurs when an asteroid or comet impacts the Earth’s surface

These objects can be moving as fast as 100,000 miles per hour (~28 miles per second)

Impact Metamorphism

In a fraction of a second, the energy of the rapidly moving object is transferred into heat energy and shock waves as it smashes into the Earth

Impact Metamorphism

Impact Metamorphism The impacting asteroid or comet is vaporized

The impacted rock is shattered, pulverized and sometimes even melted

Minerals in the rock are instantly subjected to both high temperature and high pressure

Rare and unusual metamorphic minerals such as coesite, which are normally never found on the Earth’s surface, are nearly instantly formed

Staggering quantities of matter are blown into the atmosphere

Impact Metamorphism

Fortunately for life on Earth, this is a rare event, but these impacts have repeatedly caused mass extinctions

A crater one mile in diameter and 500 feet deep is formed in only 30 seconds

A crater ten miles in diameter and a mile deep is formed in 90 seconds

The largest known crater on the Earth in located in South Africa and is 180 miles in diameter

Impact Metamorphism

(Above) Tenoumer Crater in Mauritania

Near the surface, rock behaves like a brittle solid

So near the surface, movement along a fault zone fractures and pulverizes the rock, creating what is called fault breccia

Fault Zone Metamorphism

In contrast, at depth under higher heat and pressure, rock is ductile and flows like plastic

At depth along a fault zone, the mineral structures are deformed by the ductile flow, giving the metamorphic rock a foliated or lineated appearance

Fault Zone Metamorphism