Sedimentary and Metamorphic Rocks: Formation and Time

Sedimentary Rocks

Sedimentary rocks are produced from the weathering of preexisting rocks or biological matter.

• Detrital: Rocks produced from rock fragments.
• Chemical: Rocks produced by the precipitation of dissolved ions in water.
• Organic: Rocks produced by the accumulation of biological debris.

Sediments: Loose, solid particles originating from the weathering and erosion of preexisting rocks.

Deposition: The settling and coming to rest of transported materials and the accumulation of chemical or organic sediments, typically in water.

Lithification: The general term for processes converting loose sediments into sedimentary rock.

• Breccia: Coarse-grained clastic sedimentary rocks.
• Sandstone: Medium-grained clastic sedimentary rock.
• Shale: Fine-grained clastic sedimentary rock.
• Carbonates: Contain CO3 as part of their chemical composition.
• Limestone: Composed mainly of calcite.
• Coal: Sedimentary rock forming from the compaction of partially decayed plant material.

Sedimentary Structures

Sedimentary structures are produced during or after sediment deposition.

• Bedding: A series of visible layers within a rock.
• Cross-bedding: A series of thin, inclined layers within a horizontal bed of rock.
• Ripple marks: Small ridges formed on the surface of a sediment layer by wind or water.
• Mud cracks: Polygonal cracks formed in drying mud.

Interpretation and Tectonic Setting

Interpretation: Locality that eroded and provided sediment.

Depositional environment: The location where sediment came to rest.

Tectonic setting: Plays a key role in the distribution of sedimentary rocks.

Metamorphism

Metamorphism: Solid-state changes to rocks in the Earth's interior. Produced by increased heat, pressure, or the action of hot, reactive fluids.

Factors Influencing Metamorphism

• Temperature: All minerals are stable over a finite temperature range. If the range is exceeded, new minerals result.
• Pressure: Confining pressure is applied equally in all directions. High-pressure minerals are more compact and more dense.
• Tectonic forces: Often lead to forces that are not equal in all directions. Shearing causes flattening perpendicular to stress.
• Fluids: Rising temperatures cause water to be released from unstable minerals.
• Time: Longer times allow newly stable minerals to grow larger and increase foliation.

Types of Metamorphism

Rock texture: Foliated (named by type of foliation) or non-foliated (named by composition).

• Contact metamorphism: High temperature is the dominant factor and produces non-foliated rocks.
• Regional metamorphism: High pressure is the dominant factor and results in rocks with foliated textures.
• Partial melting: During metamorphism, produces migmatites.
• Shock metamorphism: Produced by the rapid application of extreme pressure.
• Hydrothermal process: Rocks precipitated from or altered by hot water.

Geologic Time

Relative age: The order of events or objects, from oldest to youngest.

James Hutton: Realized that geologic processes require vast amounts of time.

Numerical age: The age of events or objects, expressed as a number or numbers.

Geologic Principles

Contacts: Surfaces separating successive rock layers.

Formations: Bodies of rock of considerable thickness.

Original horizontality: Beds of sediments deposited in water.

Lateral continuity: An original horizontal layer extends laterally until it tapers or thins at its edge.

Cross-cutting relationships: A disrupted pattern is older than the cause of the disruption.

Baked contacts: Contacts between igneous intrusions and surrounding rocks.

Unconformities

Unconformity: A surface that represents a gap in the geologic record.

• Disconformity: An unconformity in which the contact represents missing rock layers.
• Angular unconformity: An unconformity in which the contact separates overlying younger layers from eroded, tilted layers.
• Nonconformity: An unconformity in which an erosional surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.

Geologic Time Scale

Correlation: Determining the time equivalency of rock units.

• Precambrian: Represents 87% of geologic time.
• Paleozoic: The appearance of complex life.
• Mesozoic: Dinosaurs; the era ended by mass extinction.
• Cenozoic: Mammals and birds were abundant.