5.3 Plate Tectonics and Metamorphism

GEOL1340

Geothermal Gradient Controls

Plate tectonic motions directly alter the geothermal gradient (the rate temperature increases with depth) across different crustal settings.

Subduction Zone Metamorphism

A low-temperature, high-pressure metamorphic environment created where cool oceanic lithosphere subducts rapidly into the mantle before warming up.

Blueschist

A blue-colored metamorphic rock formed in subduction zones under low temperatures and very high pressures; contains the blue amphibole mineral glaucophane.

Glaucophane

A blue sodium-rich amphibole mineral (Na2(Mg3Al2)Si8O22(OH)2) characteristic of high-pressure, low-temperature subduction zone metamorphism (blueschist).

Franciscan Complex

A famous geological formation near San Francisco where partially subducted blueschist was brought back to the surface instead of sinking into the mantle.

Eclogite

An ultra-high-pressure metamorphic rock formed when blueschist subducts deeper than ~35 km into the mantle.

Convergent Collisional Boundaries

Crustal compression and thickening during continent-continent collisions create widespread regional (dynamothermal) metamorphism with high differential stress.

Regional Metamorphism (Mountain Roots)

Large-scale metamorphism occurring under high pressure and temperature in mountain roots (e.g., Himalayas), producing foliated rocks from slate to gneiss.

Hydrothermal Metamorphism (Mid-Ocean Ridges)

Heated seawater circulates through hot, fractured basalt at ocean ridges (200-300°C), converting pyroxene into hydrated minerals like chlorite and serpentine.

Retrograde Metamorphism

Metamorphic reactions occurring at lower temperatures than those at which the parent rock originally formed (e.g., seawater altering 1200°C basalt to greenstone at 200-300°C).

Greenstone vs. Greenschist

Hydrated basaltic rocks altered by ridge hydrothermal metamorphism; non-foliated is greenstone, while foliated is greenschist.

Role of Hydrated Minerals in Volcanic Arcs

Subducted serpentine and chlorite release bound water (OH) at depth, lowering the melting point of the overlying mantle to trigger flux melting.

Contact (Thermal) Metamorphism

Localized metamorphism caused by hot magma bodies (~1000°C) baking cooler shallow crust under high heat and low pressure, producing non-foliated rocks.

Aureole Size in Contact Metamorphism

The contact metamorphic zone around an intrusion is small (meters to tens of meters), compared to vast regional metamorphic zones (thousands of sq km).

Volcanic-Arc Geothermal Gradient

Steeper geothermal gradient (40-50°C/km) caused by volcanic heat, allowing higher metamorphic grades to occur at shallower depths.

Typical Geothermal Gradient

Rate of temperature increase with depth in stable continental crust, averaging ~30°C per kilometer (e.g., 300°C at 10 km depth).

Subduction Zone Geothermal Gradient

Extremely low geothermal gradient (less than 10°C per kilometer) due to the refrigeration effect of subducting cold oceanic crust.

Burial Metamorphism

Metamorphism occurring deep under accumulating sedimentary piles under low temperature and pressure conditions (zeolite facies).

Zeolites

Silicate minerals formed during low-grade burial metamorphism of volcanic rocks at relatively shallow depths (5-10 km).

Shock (Impact) Metamorphism

Metamorphism caused by meteorite impacts generating extreme frictional heat (thousands of °C) and ultra-high pressure (gigapascals) instantaneously.

Exhumation

The process by which deep metamorphic rocks reach Earth's surface through a combination of tectonic uplift and erosion of overlying rock.

Continental Shields

Ancient, stable core regions of continents (e.g., Canadian Shield) where extensive exhumed metamorphic rocks are exposed at the surface.