MetamorphismMetamorphism literally means to “change form.” These changes take thousands to millions of years to complete. During this period of change minerals within the rocks flow in solid state and can change on the atomic scale.

Reference: Tarbuck and Lutgens Pages

Metamorphism most often occurs during one of two geologic events; 1) mountain building (regional metamorphism) 2) intrusion of molten rock (contact metamorphism)

Regional Metamorphism

Occurs where tectonic plates collide, mainly at ocean – continent collision boundaries.

This type of environment produces extensive mountain building.

The greatest volume of metamorphic rocks are formed in this way.

This type of metamorphism is caused by two main conditions; 1) extreme pressure, and 2) extreme heat.

Regional Metamorphism

Pressure

This pressure causes rocks to deform and flow as if they were plastic. The minerals within the rock recrystallize and reorganize to form metamorphic textures. Folding is a common feature found in rocks that have undergone regional metamorphism.

Pressure on rocks within mountain environments is mainly caused by; 1) the weight of the material above, and 2) stress caused compressional forces as tectonic plates push against one another.

Regional Metamorphism

Heat responsible for regional metamorphism within mountain environments is produced as a result of;

Heat

2) Intrusion of hot molten rock called magma which causes the surrounding rock to heat up as heat from magma radiates outward.

1) Geothermal gradient which states that temperature increases 30 degrees Celsius for each kilometer below the surface, and

Regional MetamorphismWHERE DOES IT OCCUR?

(1) Areas of mountain building; and

(2) Subduction zones.

(1)

• A compressional environment resulting in increasing pressure. As the mountains are forming, increasing burial depth results in increasing temperature.

• Ocean-continent collision (convergent plate boundary) is an example of where subduction and mountain building occurs.

(2) This is a subduction zone. The metamorphism is not occurring within the subduction zone, but within the mountain system. As you can see, as burial depth increases so does the GRADE OF METAMORPHISM.

Shale to Slate to Phyllite to Schist to Gneiss

Regional Metamorphism

Contact Metamorphism

This type of metamorphism occurs when rock is in contact with, or near, a mass of magma (Heat).

The changes are caused primarily by the high temperatures of the molten rock, which in effect “bake” the surrounding rock.

Examples include Marble and Hornfels.

Contact Metamorphism is indicated by the “xxxx” in the diagram.

Contact Metamorphism

• WHERE DOES IT OCCUR?

• (1) Beneath lava flows (Hint: buried lava flows)

• (2) Adjacent to magma intrusions

• (3) Dykes and sills

B is a magma intrusion and R is a buried lava flow.

Dyke = Vertical

Sill = Horizontal

Contact Metamorphism

Remember: Remember: [Refer to diagram on previous slide (left side).]

• Buried Lava Flow Contact Metamorphism is located

Below the Lava Flow (Only).

• Magma IntrusionContact Metamorphism is located

Above and Below the Magma Intrusion.

Chill Out – “Chilled Margins” – Contact Metamorphism

• When lava “pours” over rock or magma “intrudes” rock, it starts to crystallize (igneous rock activity). The contact metamorphism occurs where the hot lava or hot magma “touches” the old, previously existing rock.

Look at the magma intruding through a rock unit! This magma starts to cool slowly so crystals grow to be large. HOWEVER, the magma cools faster on the outsides since the cold, previously existing rock unit causes it to cool faster. THEREFORE, YOU GET LARGER CRYSTALS TOWARDS THE MIDDLE AND SMALLER CRYSTALS BY THE OUTSIDES. The chilled effect!

Sample Problem

(A) contact metamorphism

(B) faulting

(C) regional metamorphism

(D) unconformity

What feature is represented by “xxxx” in the diagram below?

Once molten rock

Sample ProblemHow can contact metamorphism distinguish an igneous intrusion from a buried lava flow?

Answer:

Contact metamorphism would reveal if an igneous rock unit was an intrusion or a lava flow by looking at where the metamorphism is located. An intrusion would have metamorphism above and below the igneous rock unit, whereas, a lava flow would only show metamorphism below the igneous rock unit.

Sample Problem

Contrast the conditions necessary for rock to change by regional metamorphism compared to that of contact metamorphism.

Answer:

Regional metamorphism occurs in mountainous environments, where rocks are exposed to high heat from igneous intrusions and high pressure caused by compressional forces.

Contact metamorphism can occur in any environment where a heat source (molten rock) causes the surrounding rock to change.

Thus, regional metamorphism is caused by high heat and high pressure, whereas, contact metamorphism is caused by heat only.