Metamorphism, Rock Cycle, and Geological Time: Key Concepts for Earth Science

What is metamorphism?

The process by which existing rocks are changed in the solid state by heat, pressure, and/or chemically-active fluids.

What are the main causes of metamorphism?

Increase in temperature, increase in pressure, differential stress, and chemically active fluids.

What is foliation in metamorphic rocks?

A planar fabric (layering, alignment of platy minerals) produced by differential stress.

Define metamorphic grade.

A measure of the intensity of metamorphism (temperature/pressure conditions) experienced by a rock.

Name two foliated metamorphic rock types and two non-foliated types.

Foliated: slate, schist, gneiss; Non-foliated: marble, quartzite.

Where does metamorphism commonly occur

Regional metamorphism at convergent plate boundaries, contact metamorphism around igneous intrusions, hydrothermal/metasomatic environments, burial metamorphism.

What is metamorphic facies?

A set of mineral assemblages indicative of particular P-T (pressure‐temperature) conditions in metamorphism.

What does the rock-cycle diagram show?

The interrelationships among the three major rock types (igneous, sedimentary, metamorphic) and the processes that convert from one type to another (melting, crystallization, weathering, lithification, metamorphism, deformation).

Give an example path in the rock cycle: sediment → sedimentary rock → metamorphic rock.

Sediment is deposited → becomes sedimentary rock by lithification → then subjected to heat & pressure → becomes metamorphic rock.

Why is the rock cycle important for understanding Earth systems?

It shows how Earth's materials are recycled over geologic time, linking tectonics, surface processes, and Earth materials.

What is deformation in a geologic context?

The change in shape, position, or orientation of rocks in response to stress.

What are the principal types of stress?

Tension (pulling apart), compression (pushing together), and shear (sliding past).

Define brittle vs ductile deformation.

Brittle deformation: rock fractures (faults) when stress exceeds strength; Ductile deformation: rock bends or flows without fracturing (folds, foliation).

What is a fault ?

A fracture in rock along which appreciable displacement has occurred.

Name and define the main types of dip-slip faults.

Normal fault: hanging wall moves downward relative to footwall (extension). Reverse fault: hanging wall moves upward relative to footwall (compression). Thrust fault: a low-angle reverse fault.

What is a strike-slip fault?

A fault in which the predominant motion is horizontal, parallel to the strike of the fault.

What is a fold

A bend in rock layers resulting from ductile deformation.

What are anticlines and synclines?

Anticline: an arch-shaped fold where rock layers dip away from the hinge; Syncline: a trough-shaped fold where layers dip toward the hinge.

What causes earthquakes?

Sudden release of strain accumulated in rocks (usually along faults) when stress exceeds fault strength

What are seismic waves?

Waves of energy that travel through Earth when earthquakes occur , P-waves, S-waves, surface waves.

How is the size of an earthquake measured (8.4-8.5)?

Two common measures: magnitude

What is an earthquake population?

The distribution of earthquakes in time, space, and magnitude

What are some earthquake hazards?

Ground shaking, surface rupture, liquefaction, landslides, tsunamis, structural collapse.

What is earthquake engineering and zoning?

The design and planning of buildings, infrastructure, and land-use to reduce earthquake risk.

What is geologic time?

The vast span of time over which Earth has existed and geological processes have operated, typically millions to billions of years.

What is relative age?

Determining the order of events (which came first, next, last) without knowing exact ages.

What is an unconformity?

A surface that represents a gap in the geologic record, typically when deposition stopped and erosion removed previously formed rocks. Examples: angular unconformity, nonconformity, disconformity.

What is the geologic column?

A composite representation of Earth's rock layers and the distribution of fossils and ages through time.

What is numerical (absolute) age dating?

Using radioactive decay, isotopic systems, or other methods to assign an actual age in years to rocks and events.

How do we use both relative and numerical dating together?

Relative dating provides the sequence of events; numerical dating assigns ages; together they build a complete geologic timescale.

How did the universe and solar system begin?

The universe formed in the Big Bang; the solar system formed from a nebular cloud; the Earth and Moon formed by accretion and collision processes.

What are the Hadean and Archean eons?

Hadean (~4.6-4.0 Ga): Earth's initial formation, heavy bombardment. Archean (~4.0-2.5 Ga): early crust, first stable continents, first life.

What is the Proterozoic?

The eon between ~2.5 Ga and ~541 Ma, when continents grew, atmospheric oxygen increased, and more complex life appeared.