geo 111 exam 2

Detrital Sedimentary rocks

• Made of sediment that is transported as solid particles

• Grain size is the primary basis for distinguishing various detrital

• sedimentary rocks

• Grain size indicates environment of deposition

• CLASTIC TEXTURE

Shale

• Detrital Sedimentary rocks

• (mudstone, siltstone)

• (>50% of all sedimentary rocks)—silt and clay

• Small grain size = settling from quiet, non-turbulent water

• Forms in lakes, lagoons, swamps, deep ocean

• Quiet water setting

Sandstone

• Detrital Sedimentary rocks - 2.

• sand sized particles (1/16 – 2 mm, 25% of all sedimentary rocks

• Rivers, coastal environments (beaches), wind blown dunes

• Grain shape and sorting important for determining environment

• Sorting:

  • well sorted = wind & wave

  • Poorly sorted = streams

• Grain shape: 

  • well rounded = water or wind transported long distances

  • Angular = glacier transport, debris flows, transported short distances

Conglomerate and Breccia

Detrital Sedimentary rocks 3.

Gravels (pea to large boulders; >2 mm in diameter)

Conglomerate: rounded pieces—very energetic mountain streams or coasts

Breccia: angular pieces—did not travel far—glaciers, landslides

Chemical Sedimentary Rocks

• Interlocking crystals forming from precipitation.

• Composition is important for rock name and interpretation

• Usually have a CRYSTALLINE TEXTURE

• Either inorganic or organic from organisms secrete CaCO3 minerals

Limestone

• (by far the most abundant)

• (10% of all sedimentary rocks)

• composted of calcite, CaCO3

• Marine organisms extract the ions from the water to form their shells

• When they die, the shells accumulate on the bottom of the ocean

• Compaction, recrystallization, & cementation

  • Microscopic algae

  • Foraminifera (forams)

  • Microscopic animals

Chert

• Chemical Sedimentary Rocks

• Interlocking crystals form from precipitation

• (jasper, flint, agate)—SiO2

• Inorganic: can precipitate from silica-rich water

• Organic: some marine organisms make their shells of silica

  • Radiolaria: single celled animals 

  • Diatoms

  • Single-celled plants

  • Marine sponges & larger animals

Evaportites

• Chemical Sedimentary Rocks

• form when ion-rich water evaporates and leaves minerals behind.

  • Salt - NaCl

  • Gypsum - CaSO4 + 2 H2O

  • Sylvite KCl

Coal

• Chemical Sedimentary Rocks

• made of terrestrial organic matter, leaves, bark, wood, plant matter

• Dead organic matter accumulates in oxygen poor environments (swamps)

• Buried and compacted

• Pressure  expels water, CO2 and other gases

Sedimentary Structures

Usually in detrital sedimentary rocks

Bedding (stratification)

• Sedimentary Structures.

• Usually in detrital sedimentary rocks

• Arrangement into distinct layers

• Produced by transitions between different beds indicate a change in the energy of the environment and/or a change in the material being deposited

Graded Beds

• within a layer, the sediments continuously change size

• Produced by rapid deposition by water

• Heaviest grains fall out first

• Turbidity currents: loose, unstable material shed from continents is dislodged by an earthquake and forms an underwater landslide out to the ocean.

Cross-bedding

• Sedimentary Structures in detrital sedimentary rocks

• sedimentary layers deposited at an angle

• Forms when material dropped from a moving current

• Sand dunes or ocean dunes or river dunes

• Change in deposition direction

• Changes the direction of the beds

• Represents lee side of dunes: records direction of flow

Ripple Marks

Ripples at top of deposit - records direction of flow

Mudcracks

• Wet fine-grained sediment exposed to the air, it dries out and shrinks.

• Indicates a wet environment that dried up.

Sole Marks

sedimentary structures found on the base of a rock bed that were formed by scouring, tool marks, or deformation of the underlying soft sediment before the overlying bed was deposited.

Salt casts

sedimentary structures found on the base of a rock bed that were formed by scouring, tool marks, or deformation of the underlying soft sediment before the overlying bed was deposited.

Lignite

This is the lowest rank of coal, also known as "brown coal." It has the highest moisture content, lowest carbon content (65-70%), and lowest energy content.

Subbituminous

This is the next grade up, with a higher carbon content (70-76%) and lower moisture content than lignite. It is often used for electricity generation.

Bituminous

This is a higher-rank coal with higher energy content and a carbon content ranging from 76-86%. It is the most abundant type of coal used in the United States and is used for both electricity generation and steel production.

Anthracite

This is the highest rank of coal, with the highest carbon content (86-92%), lowest moisture content, and highest energy content. It is also called "hard coal" and burns with a short, smokeless flame

Differences between detrital and chemical sedimentary rocks

Detrital rocks form from the accumulation of weathered rock fragments (detritus). Chemical rocks form from minerals that precipitate out of a solution, such as water, through chemical processes

Metamorphic Rock

A rock that has been changed (in mineral composition and/or texture) by heat, pressure, and/or chemically active fluids.

Protolith

The original, unmetamorphosed rock from which a given metamorphic rock is formed.

Slate

A fine-grained, foliated metamorphic rock formed from the low-grade metamorphism of shale (the protolith). It exhibits slaty cleavage (the ability to split into thin, flat layers).

Phyllite

A fine-grained, foliated metamorphic rock intermediate in grade between slate and schist. Formed from shale (the protolith), it has a glossy sheen due to the development of fine mica crystals.

Schist

A medium-to-coarse-grained, strongly foliated metamorphic rock formed from shale or basalt (protoliths). Characterized by schistosity (the alignment of visible platy mineral grains, like mica).

Gneiss

A coarse-grained, high-grade foliated metamorphic rock formed from shale, granite, or volcanic rocks (protoliths). It exhibits a distinct banding of light and dark minerals.

Marble

A nonfoliated metamorphic rock formed from the metamorphism of limestone or dolostone (the protoliths). It's primarily composed of large, interlocking calcite crystals.

Quartzite

A nonfoliated metamorphic rock formed from the metamorphism of quartz-rich sandstone (the protolith). It's extremely hard and composed of interlocking quartz grains.

Fault

A fracture or zone of fractures between two blocks of rock where there has been relative movement (displacement).

Hangingwall

The block of rock that lies above the fault plane. (Imagine where you would hang a lantern if you were in a mine on the fault).

Footwall

The block of rock that lies below the fault plane. (Imagine where you would stand).

Normal Fault

A fault where the hangingwall moves down relative to the footwall. It results from tensional stress (pulling apart) and causes an extension of the crust.

Reverse Fault

A fault where the hangingwall moves up relative to the footwall. It results from compressional stress (pushing together) and causes a shortening of the crust.

Thrust Fault

A specific type of reverse fault that has a very shallow dip (typically less than 45 degree). It also results from compressional stress.

Fold

A bend or warp in a layer or series of layers of rock, typically resulting from compressional tectonic forces.

Syncline

A downward-arching fold (U-shaped) where the rock layers are youngest near the center of the fold.

Anticline

An upward-arching fold (A-shaped) where the rock layers are oldest near the center of the fold.

Dome

A large, circular or elliptical structure where rock layers dip outward in all directions from a central point. (Oldest rock in the center).

Basin

A large, circular or elliptical structure where rock layers dip inward in all directions toward a central point. (Youngest rock in the center).

Weathering

The process by which rocks and minerals break down at or near the Earth’s surface.

Erosion

The process by which moving water, wind, ice, or gravity carries pieces of weathered rock away from the bedrock and deposits them somewhere else.

Sedimentary Rocks

Rocks made from sediment at the Earth’s surface. They tell a story about surface processes.

Detrital Sedimentary Rocks

Rocks made of sediment that is transported as solid particles.

Mass Movement (Slope Failure)

The process that transports Earth material (bedrock, sediment, soil) down slopes by the pull of gravity.

Mechanical Weathering

Breaks a mineral or rock into smaller pieces without changing their chemical makeup. This process increases surface area.

Frost Wedging

Water freezes in rock fractures, expanding by 9% and pushing the rock apart.

Crystal Growth (Salt Wedging)

Water evaporates from a crack, leaving growing salt crystals that put pressure on the rock and push the fracture apart.

Exfoliation

Occurs when overlying rock is eroded, reducing pressure and causing the underlying rock to expand and fracture into sheets (like at the surface of batholiths).

Biological Weathering

Plant roots grow and expand into rock fractures, putting force on the rock and widening the cracks.

Chemical Weathering

Alters the composition of rocks and minerals, usually through chemical reactions involving water.

Dissolution

The dissolving of ions by water (e.g., salt or limestone), which are then carried away.

Carbonic Acid

An acid (H2CO3) formed when H2O reacts with CO2, which is highly effective at dissolving rock like limestone.

Oxidation

Positive ions (like iron) combine with oxygen to form an oxide (rust), such as hematite Fe2O3

Hydrolysis

Ions from water (OH- or H+) replace other ions in minerals, converting minerals like Feldspar into clay (e.g., kaolinite).

Earthflows

A flow involving relatively dry soil and small particles; they have high viscosity and are slow (meters/hour to meters/min).

Mudflows

A rapid mass movement flow that is saturated with water, fast, and generally composed of particles smaller than sand.

Debris Flows

A rapid mass movement flow that is water-saturated and fast, but is composed of debris larger than sand-size particles up to boulders.

Soil Profile

The vertical cross-section of the soil, typically consisting of distinct layers called horizons.

Topsoil (A Horizon)

The uppermost mineral layer of soil, which is rich in organic matter.

Leached Zone (E Horizon)

A layer below the A horizon where soluble minerals are leached out, or removed by water.

Accumulation Zone (B Horizon)

A layer often characterized by the accumulation of iron and clays that were transported downward from above.

Weathered Bedrock/Sediment (C Horizon)

The deepest layer of the soil profile, consisting of partially weathered bedrock or sediment.

Precipitation (Chemical)

When water leaves, the dissolved ions are left behind to form solid compounds.

Principle of Superposition

In an undeformed sequence of sedimentary rocks, the oldest layers are on the bottom and the youngest are on the top.

Principle of Original Horizontality

Layers of sediment are generally deposited in a horizontal position; any tilting or folding occurred after deposition.

Principle of Cross-Cutting Relationships

A geologic feature (fault, dike, intrusion) that cuts across another feature is younger than the feature it cuts.

Principle of Inclusions

The rock fragments (inclusions) found inside another rock layer are older than the rock layer containing them.

Unconformity

A surface that represents a break or gap in the geologic rock record, typically caused by a period of erosion or non-deposition.

Angular Unconformity

An unconformity where titled or folded sedimentary rocks are overlain by flat-lying sedimentary rocks.

Nonconformity

An unconformity where younger sedimentary layers overlie older intrusive igneous or metamorphic rocks.

Disconformity

An unconformity where the rock layers above and below the erosional surface are parallel.

Absolute Age Dating

The process of assigning a specific numerical age (in years) to a rock or geologic event.

Radiometric Decay of Isotopes

The spontaneous, predictable, and constant rate at which an unstable parent isotope transforms into a stable daughter isotope.

Half-Life

The time required for half of the unstable parent isotope atoms in a sample to decay into stable daughter isotope atoms.

Dendrochronology (Tree Rings)

A dating method that uses the annual growth rings of trees to determine the age of wood and the climate conditions during the year the ring was formed.

Basis of Geologic Time Scale

Eras and periods are primarily based on major changes in the fossil record and evidence of mass extinction events (e.g., the end of the Paleozoic Era).

Absolute Age Assignment of GTS

Achieved by radiometric dating of igneous rock layers (like volcanic ash beds) that lie within or bracket the sedimentary rock layers containing the fossils.

Age of the Earth

Approximately 4.54 billion years (Ga).

Age of the Oldest Continental Rocks

Approximately 4.03 billion years (Ga) (e.g., Acasta Gneiss).

Age of the Oldest Oceanic Rocks

Approximately 180 million years (Ma) (due to constant destruction/renewal via subduction and seafloor spreading).