Edit · Historical Geology Lecture 8 - 16

Historical Geology

Study of Earth's history, including changes in rocks, environments, life, climate, and geography through geologic time.

Why study Earth history?

Earth's rocks and fossils preserve evidence that lets geologists reconstruct past environments, events, and the history of life.

Paleoecology

Study of ancient ecosystems and how organisms interacted with their environments.

Geologic time scale

A framework that divides Earth's ~4.56-billion-year history into eons, eras, periods, epochs, and ages.

Geologic time scale ordering

Oldest time is at the bottom and the most recent time is at the top.

Geologic history

A sequence of physical, chemical, biological, and geographic changes preserved in the geologic record.

Geologic 'ruins'

Features left behind by past environments or events that can be interpreted to reconstruct Earth's history.

Mineral

A naturally occurring, inorganic, solid substance with a crystalline structure and a consistent chemical composition.

Why minerals matter in historical geology

Minerals are basic components of rocks and can form under particular temperature, pressure, and chemical conditions.

Crystalline structure

An organized, repeating arrangement of atoms in a solid.

Silicate mineral

A mineral built around silicon-oxygen structures; silicates are the most common mineral group.

Quartz

A common silicate mineral with the formula SiO2.

Feldspar

A major silicate mineral group; common feldspars include plagioclase and orthoclase.

Plagioclase feldspar

A feldspar series ranging compositionally from sodium-rich to calcium-rich compositions.

Orthoclase feldspar

Potassium feldspar with the formula KAlSi3O8.

Pyroxene

A common silicate mineral group, often containing Mg and Fe.

Amphibole

A silicate mineral group commonly containing Ca, Mg, Fe, Al, Si, O, and OH.

Muscovite

A potassium-rich mica mineral.

Biotite

A dark mica mineral commonly containing Mg and Fe.

Mineralization

The process by which minerals form or become concentrated into solid mineral material.

Solidification

Formation of minerals as a melt cools and becomes solid.

Precipitation

Formation of solid minerals when dissolved material comes out of solution.

Solid-state diffusion

Rearrangement or movement of atoms while a material remains solid; important during metamorphism.

Biomineralization

Production of mineralized structures by living organisms, such as shells or bones.

Desublimation

Direct change from a gas to a solid.

Halite

NaCl; the mineral form of common salt and an example of a halide.

Gypsum

CaSO4·2H2O; a sulfate mineral.

Calcite

A carbonate mineral composed primarily of CaCO3.

Dolomite

A carbonate mineral containing calcium, magnesium, carbon, and oxygen.

Hematite

An iron oxide mineral, Fe2O3.

Magnetite

An iron oxide mineral, Fe3O4; iron ores in this group can provide information about atmospheric oxygen.

Rock

An aggregate of minerals or mineral-like material.

Three rock types

Igneous, metamorphic, and sedimentary.

Igneous rock

Rock that solidified from molten material such as magma or lava.

Metamorphic rock

Rock changed by heat and/or pressure through solid-state recrystallization without melting.

Sedimentary rock

Rock formed from sediments through processes such as compaction, cementation, or precipitation from solution.

Rock cycle

The continuous set of processes by which rocks can change from one rock type into another.

Intrusive igneous rock

Igneous rock formed when magma solidifies within Earth's crust.

Extrusive igneous rock

Igneous rock formed when lava reaches the surface and cools there.

Slow cooling

Generally produces larger mineral crystals.

Rapid cooling

Generally produces small crystals or no visible crystals.

Metamorphism

Solid-state alteration of an existing rock due mainly to heat, pressure, and deformation.

Contact metamorphism

Metamorphism caused by heat from nearby magma.

Deep metamorphism

Metamorphism occurring under substantial pressure and temperature at depth.

Bowen's Reaction Series

A sequence showing the relative crystallization of common silicate minerals from cooling magma and their relative stability at Earth's surface.

Outcrop

A portion of rock exposed and visible at Earth's surface.

Stratigraphic section

An outcrop or series of adjacent outcrops exposing a vertical sequence of strata.

Bed

A distinct sediment layer more than 1 cm thick.

Lamina

A distinct sediment layer less than 1 cm thick.

Bioturbation

Mixing or disruption of sediment by burrowing organisms.

Weathering

Breakdown of rock at or near Earth's surface.

Mechanical weathering

Physical breakdown of rock without necessarily changing its mineral composition.

Chemical weathering

Chemical alteration of minerals at or near Earth's surface.

Erosion

Removal and movement of weathered material.

Sediment transport

Movement of sediment by water, wind, ice, or gravity.

Deposition

The settling or accumulation of sediment when transporting energy decreases.

Lithification

Conversion of sediment into sedimentary rock, especially through compaction and cementation.

Compaction

Squeezing of sediment grains closer together by the weight of overlying material.

Cementation

Binding of sediment grains together by minerals precipitated from groundwater.

Diagenesis

Post-depositional physical and chemical changes that affect sediment and sedimentary rock.

High-energy environment

An environment capable of moving relatively large grains, such as gravel or cobbles.

Low-energy environment

An environment where fine particles such as mud can settle.

Grain rounding

Process in which transported grains become smoother and more rounded through collisions and abrasion.

Grain sorting

Degree to which sediment grains are similar in size.

Alluvial fan

Fan-shaped deposit of sediment that commonly forms where streams or debris flows leave steep mountain fronts.

Braided river

A river system with multiple channels separated by sediment bars; commonly associated with high sediment supply and energy.

Meandering river

A winding river with a relatively low gradient; commonly deposits sand in channels and finer sediment on floodplains.

Delta

Sediment accumulation where a river enters a larger body of water and loses transporting energy.

Beach sediment

Sediment commonly reworked by waves, often becoming relatively rounded and well sorted.

Turbidite

A deposit produced by a dense, sediment-laden current moving downslope underwater.

Graded bedding

A sedimentary layer in which grain size changes systematically, commonly becoming finer upward.

Mudstone

Fine-grained sedimentary rock commonly deposited in relatively calm water.

Shale

Fine-grained sedimentary rock commonly associated with low-energy depositional settings.

Siliciclastic sediment

Sediment made mainly from fragments of pre-existing silicate-rich rocks and minerals.

Biogenic sediment

Sediment produced from biological material.

Evaporite

Sedimentary mineral deposit formed when water evaporates and dissolved minerals precipitate.

Coal

Organic sedimentary rock formed from accumulated plant material, commonly progressing from peat to coal.

Limestone

Carbonate sedimentary rock composed largely of calcium carbonate.

Chalk

Fine-grained carbonate sediment commonly formed from accumulated coccoliths.

Ooid

Small rounded carbonate grain with concentric layers, commonly formed in warm, shallow, wave-agitated water.

Reef

A rigid carbonate structure that forms in place or from bound components and creates relief on the seafloor.

Playa

A shallow, temporary lake in an arid environment; may contain fine sediment, evaporites, and mudcracks.

Fossil

Preserved remains, impressions, or traces of a once-living organism.

Body fossil

Preserved part or remains of an organism.

Trace fossil

Evidence of an organism's activity or behavior, such as footprints or burrows.

Chemical fossil

A chemical signature associated with past biological activity.

Mudcracks

Cracks formed when wet sediment dries; can indicate subaerial exposure.

Symmetrical ripples

Ripples produced by oscillating flow, such as waves.

Asymmetrical ripples

Ripples produced by predominantly one-directional current flow.

Cross-beds

Inclined layers within sedimentary deposits that record migration of ripples or dunes.

Evolution

Change through time in a population or species.

Natural selection

A process in which heritable variation leads to differences in survival and reproductive success, changing trait frequencies over generations.

Variation

Differences among individuals in a population; the raw material of natural selection.

Heritability

The extent to which traits can be passed from one generation to the next.

Differential reproductive success

Some individuals leave more surviving offspring than others, contributing to evolutionary change.

Biostratigraphy

Use of fossils and their ranges to help correlate and determine the relative ages of rocks.

Ecology

Study of interactions among organisms and between organisms and their environments.

Climate and ecology

Environmental conditions such as temperature and precipitation help determine where organisms can live.

Tropical energy input

The tropics receive more solar energy per unit area because sunlight strikes them more directly.

Global air circulation

Large-scale atmospheric circulation redistributes heat and helps create global precipitation patterns.