Dinosaur footprints in Jurassic mudstone (Golden, Colorado)

In the Jurassic, there were wetlands and many lakes in this area, providing a great place for dinosaurs to thrive.

Much later, the rocky mountains formed, lifting the Jurassic rocks to the surface.

Metamorphic Rocks

Read sections: 8.7-8.9

Review

Metamorphic Rocks

Metamorphic rocks are rocks that have undergone changes in:

a) mineralsb) texturec) chemical composition

Due to increased:

a) pressureb) temperaturec) chemical reactions in fluids

Review

Pre-existing rocks (either sedimentary, igneous, or metamorphic) may be placed into new environments with different temperature, pressure, and/or chemical fluids.

Once these rocks are placed into these new temperature, pressure, and/or chemical environments, they “metamorphose” (change) in order to equilibrate with their new surroundings.

Review

Changes in mineralogy:Some old minerals may be destroyed, andnew minerals may be formed.

Changes in texture:Minerals may “recrystallize”, meaning that they

may change their shape and size.

Changes in chemical composition: As old minerals are destroyed, they may release water with dissolved compounds and ions. New minerals may be formed by dissolved

compound and ions in water.

Temperature

There are 2 ways in which a rock experiences higher temperature:

a)Nearby molten rock (magma or lava) can increase the temperature.

b) Rocks can be buried or tectonically pushed to deeper regions where temperature is higher.

Geothermal Gradient: The increase of temperature with depth

On average, temperature increases 30o C for every kilometer depth! (roughly 90oF per mile).

That’s hot enough for water to boil at 2 miles deep.

Pressure (also called stress)

2 Kinds of pressure:

1. Confining Pressure: This is pressure due to the weight of rock above This pressure is the same in all directions. Confining pressure increases with depth Similar to pressure that divers feel

Confining Pressure

Pressure increases with depth

The unit of pressure is called a bar

One bar of pressure is equivalent to what we feel on the surface of the Earth because of the weight of the atmosphere on us.

A diver experiences about 2 bars of pressure at 10 meters depth below water.

Beneath the Earth’s surface, pressure is measured in kilobars (a kilobar = 1000 bars).

Pressure (also called stress)

2. Directed Pressure:

Occurs when the pressure is higher in one direction than other directions.

All rocks below the surface experience confining pressure due to being buried beneath lots of rock. Plate tectonics may cause additional directed pressure to the rocks.

Chemical Reactions in Fluids

Often, very hot water that contains dissolved compounds and ions flows through the rock.

This hot water is called hydrothermal fluid.

Metasomatism: When the chemicals (dissolved compounds and ions) in hydrothermal fluid react with the rock, changing its chemical composition.

Types of Metamorphism

1. Regional Metamorphism

2. Contact Metamorphism

3. Seafloor Metamorphism

4. Burial Metamorphism

5. High Pressure Metamorphism

6. Shock Metamorphism

Regional Metamorphism

Convergent plate boundaries have a large amount of directed pressure due to tectonic forces. At continental plate convergent boundaries, mountains are formed. The deeper parts of these mountain belts undergo a lot of metamorphism due to the large amount of directed pressure. The core of a mountain belt is usually made of metamorphic rocks.

(don’t worry about the details here)

Example: Northern Appalachians

Contact Metamorphism

Contact metamorphism occurs when rocks are heated (i.e., burned) by nearby magma or lava. The intense heat of the nearby magma/lava causes chemical changes in the minerals of the country rock.

Contact metamorphism usually occurs over a thin region, a few inches thick.

Seafloor Metamorphism

Close up of a spreading ridge

Seafloor metamorphism occurs at divergent plate boundaries that are below water. Divergent boundaries have magma chambers below them. Sea water seeps into the rocks. It comes into contact with the magma and gets superheated. This superheated water dissolves minerals in the rock, turning them into dissolved ions and compounds. This hot water, containing the dissolved ions and compounds, rises to the surface. As it does, the dissolved ions and compounds chemically react with the rock (a process called metasomatism), changing its composition.

Hydrothermal circulation – the flow of hot water near magma chambers

Black Smokers: where hydrothermal fluid enters the ocean. The fluid carries lots of ions and dissolved compounds, giving them a dark color.

Hydrothermal vents support a deep sea, high-temperature ecosystem

Burial Metamorphism

If rocks are buried deeper in the Earth due to (either tectonic forces or because they are buried by sediments), their minerals may not be stable under the higher pressure and temperatures. Some minerals will change into other minerals that are more stable at the new pressure and temperatures.

High Pressure Metamorphism

This type of metamorphism is similar to “burial metamorphism”. At subduction zones, one plate sinks beneath another. The plate that sinks is not that hot because it had a while to cool off at the Earth’s surface. The subducted plate experiences high pressure at moderately cool temperature. This causes a special type of minerals to form in the basalt crust that are stable at high pressures and moderately cool temperatures. These special minerals are bluish in color, and the resulting metamorphic rock is called a blueschist.

Rocks carried down to high pressures in subduction zones are metamorphosed, turning into “blueschists”

Blueschist is a metamorphic rock that is formed in subduction zones

Shock Metamorphism

A meteorite impact creates extremely high pressures for the split second during the impact. This extremely high pressure changes some of the minerals, turning them into high-pressure polymorphs. Remember what a polymorph is?

Shocked Quartz – a high pressure polymorph of quartz, formed by a meteorite impact.

We classify metamorphic rocks as either:

Foliated

or

Non-foliated

A foliated metamorphic rock has a direction to it due an alignment of minerals.

Foliated Metamorphic Rocks

Directed pressure causes some mineral grains to align themselves in a direction roughly perpendicular to the directed stress. This alignment is called foliation.

Often, new mica minerals are formed during metamorphism.

As the grade (intensity) of metamorphism increases (higher pressures and temperatures), the size of the new mica minerals becomes larger.

The mica minerals grow in directions perpendicular to the directed stress direction.

Foliated Metamorphic Rocks

Foliated Metamorphic Rocks