Rock Identification and Weathering

Rock Identification

• Texture and composition apply to all rock type identification.
• Grain size, composition, texture, and layering aids in identifying rocks
• Igneous, sedimentary, and sandstone are 3 rock types

Weathering

• Weathering is the breakdown of rock
• Sediment consists of loose rock or mineral fragments broken off of bedrock
• It also consists of mineral crystals that precipitate directly out of water, shells, or shell fragments.
• Loose debris or soil is known as regolith
• Sediments are produced by weathering pre-existing rock.
• Physical and chemical weathering are sediment production types

Physical Weathering

• Mechanical weathering includes the breakage of intact rocks into unconnected grains/chunks, which are clasts.
• An accumulation of clasts is detritus
• Examples of physical weathering activities: thermal expansion from repeated heating and cooling, animal attacks, frost, salt root wedging, wind, and jointing of cracks in a rock.

Chemical Weathering

• Dissolution occurs when minerals completely dissolve in water, which primarily affects salts and carbonates. Quartz may also partially dissolve.
• Hydrolysis occurs when water chemically reacts with minerals, and breaks them down, creating new minerals through the reaction.
• Oxidation is the reaction of oxygen with iron-bearing minerals in rock where the iron atom loses electrons and precipitates as another mineral which creates rust.
• Hydration occurs when minerals absorb water into their structure.

Acid Formation

• Natural acids form in water at Earth's surface, causing chemical weathering, which breaks down minerals that are not ionically bonded.
• Gravestones made of limestone slowly dissolve because of carbonic acid in water.
• Physical weathering speeds up chemical weathering, with both types occurring simultaneously.
• Chemical and physical reactions take place at the material's surface.
• Chemical weathering rate depends on the ratio of surface area to volume where greater surface area results in faster weathering.
• Differential weathering refers to weathering that occurs at different rates.
• Bowen's reaction series dictates the typical order of crystallization of silicate minerals such that during crystal formation, some minerals will be stronger or more resistant, while others will be weaker/less resistant.

Rock Cycle

• Sedimentary rocks come from weathered pieces of other preexisting rocks.
• Metamorphic rocks are preexisting that changed when temperature and pressure increased.
• Igneous rocks are made from any that melted and recrystallized.
• All rock types are connected in the rock cycle, and recycled.
• The rock cycle shows that genetic rock classification connects the three classes by the processes that are responsible for each different rock type
• Most rocks on Earth are recycled, which makes very old rocks scarce

Sedimentary Rocks

• Sedimentary rocks are products of weathering.
• Fossils are commonly found in sandstone.
• They form at or near the Earth's surface.
• Sedimentary rock formation includes the cementing together of loose clasts, precipitation of minerals from water, growth of shell masses/cementing together of shells, as well as organic matter from dead plankton/plants.
• Coal is a common resource found in sedimentary rocks

Sedimentary Rock Classification

• The three types of classes are detrital, chemical, and biochemical.
• Detrital or clastic rocks are composed of mineral grains weathered from pre-existing rock and cemented together by precipitated minerals.
• Chemical rocks are composed of minerals precipitated directly from water.
• Biochemical rocks consist of fossils and/or carbon-rich relicts.
• Classification relies on origin, texture, and composition.
• Texture dominates detrital terminology.
• Composition dominates chemical and biochemical terminology.
• Clastic rocks can be composed of mineral grains weathered from pre-existing rock and cemented together by precipitated minerals.
• Clastic rock formation: weathering, erosion, transportation, deposition, then lithification.
• Compaction occurs from decreased volume due to loss of pore space as well as the movement of minerals.
• Cementation occurs when dissolved ions in pore space fluids precipitate out, forming a cement that sticks minerals together, decreasing/eliminating pore space.
• Compaction and cementation is lithification
• Clastic characteristics: clast size, composition, angularity and sphericity, sorting, and cement composition.
• Clast size measures fragment/grain size from very coarse to very fine where grain size decreases as transport distance increases.
• Grain size control: grain size is controlled by the force of the transporting agent where coarse material is transported less distance than finer material.
• Coarse conglomerates/breccia are classified from gravel, medium quartz sandstone/arkose/greywacke, and fine siltstone.
• Fine clay creates shale and mudstone.

Rock Types

• Breccia has angular clasts that are close to the source.
• Conglomerates have rounded clasts.
• Quartz sandstone mostly contains quartz grains and may exhibit fine laminations.
• Sand arkose is poorly sorted and lacks fine laminations.
• Sand greywacke contains rock fragments and angular quartz, also including clay.
• Silt sandstone is not visible, but feels gritty.
• Clay shale displays fissility, breaking in one direction parallel to bedding
• Limestone contains calcite and fizzes with acid.

Clast

• Clast composition refers to the mineral makeup of sediment grains and yields clues about the original source rock.
• Different clast compositions can hint at source area and transport processes.
• Clast angularity is the degree of edge or corner smoothness.
• Clast sphericity measures how close a clast shape comes to a sphere.
• Angularity and sphericity indicate the amount of grain abrasion during transport.
• Clast sorting measures the uniformity of grain sizes in sediment and sorting increases with transport distance.
• Cement composition is an important consideration in a sedimentary rock's history where cements are derived from dissolved ions in groundwater that move through pores in the original sediment.

Chemical Sedimentary Rocks

• Chemical sedimentary rocks are composed of minerals precipitated from water solutions and have a crystalline or interlocking texture.
• Several classes of sedimentary rocks exist including: evaporites, travertine, replacement chert, oolitic limestone, and diatomite.
• Evaporites are rocks from evaporated seas or lakes where a thick deposit requires large volumes of water with minerals to include halite/rock salt and rock gypsum.
• Travertine is calcium carbonate precipitated from groundwater that reaches the surface and expels CO2.
• Hot springs and caves/speleothems contribute to travertine.
• Replacement chert is nonbiogenic, cryptocrystalline silica that gradually replaces calcite long after limestone was deposited.
• Petrified wood preserves wood grain with silica, while agate precipitates in concentric rings.

Biochemical Sedimentary Rocks

• Sediments are created from the shells of living organisms such that hard mineral skeletons accumulate after death.
• The classes of sediments include limestone, dolostone, chert, and organic material (peat, lignite, and bituminous coal).
• Limestone forms from biologically precipitated calcium carbonate/CaCO3. It also contains a fine-grained mass, cream to gray color, and recrystallization of tiny fossils
• Fossiliferous rock contains visible fossil shells in a fine-grained matrix.
• Coquina is mainly composed of shell fragments.
• Chalk is soft, and made up of single-celled algae.
• Oolitic rock is composed of small spheres compacted together.
• Dolostone is limestone (CaCO3) replaced with Dolomite.
• Exposure to high Mg groundwater means Mg replaces 50% of Ca, forming Dolostone. It only reacts to HCl when in powdered form.
• Chert can be Chemical or Biochemical as silica-secreting plankton accumulates on the seafloor, becoming buried, dissolves, forms a silica-rich gel, and solidifies.
• Replacement with silica solidifies to chert.
• Coal is an organic carbon-rich relict of plants that accumulates in lush tropical wetland settings. Deposition requires an absence of oxygen.

Sedimentary Rock Classification

• Clastic sedimentary rocks classification relies on grain size, then rounding, sorting, and composition.
• Size, rounding, and sorting reveals travel time and distance.
• Chemical/biochemical sedimentary rock classification relies on composition, which reveals the rock from which the sediment was derived, as well as transport and weathering.

Sedimentary Rock Structures

• Sedimentary structures are the layering of sedimentary rocks, surface features on layers formed during deposition, and arrangement of grains within layers.
• Structures provide a record of ancient environments.
• Sedimentary rocks are usually layered (stratification) arranged in planar or close-to-horizontal beds with the boundary between two beds being a bedding plane.
• Bedding reflects changing conditions during deposition
• Changes in the transporting medium and sediment source are responsible for bedding formation
• Conglomerate is rounded material

Rock Formation

• Depositional changes vary the rock feature stacking and a distinct rock package can display geologic formation distribution maps
• Ripples are elongated ridges formed on a bed at right angles to the current in shallow flowing water or deserts.
• Cross bedding in sandstones produced from desert dunes are large; those produced from ripple marks are small.
• They yield information on the flow direction of wind or water that produced sediment.
• Mud cracks tell us an area alternates between wet and dry, like a floodplain or tidal flat that contains fine sediment which means a low energy environment.
• Turbidity currents and graded beds occur when sediment moves down a slope as a pulse of turbid water.
• As velocity decreases, grains settle, forming graded beds in turbidite deposits where the bottom has heavier pieces.

Depositional Environments

• Alternating layers of rock mark sedimentary rocks that deposit one layer at a time such that transition from one rock to another tells us of environmental change.
• Transgressive seas rise, while regressive seas fall.
• Limestone is ocean rock.
• Characteristics used to name sedimentary rocks can interpret the geologic history where rocks are found.
• Terrestrial, intermediate/coastal, and marine environments can name sedimentary rocks.
• Fossils can also interpret past environments, climate, and vegetation.

Terrestrial Depositional Environments

• As ice moves down, it grinds rocks to deposit in the ocean.
• Rivers are always changing such that the flatter the river, the slower it is.
• Glacial environments remove rock from the underside and sides of a glacier.
• Piles of debris are called moraines, which are composed of glacial till that have poor sorting, multiple grain sizes, and angular shapes
• Fast flowing water carries large clasts in mountain stream environments
• Conglomerates are characteristic of the setting where during low flow, cobbles and boulders are immobile.

Alluvial Fan Environments

• Sediments pile up at a mountain front, with the rapid stream velocity drop creating a cone-shaped wedge.
• Breccia, conglomerates, arkose, and shale are resulting rocks
• Sand dune environments occur in dry climates
• Medium-sand-sized and smaller sediment can be easily moved by wind.
• The resulting sand is well-sorted sandstone such that the Sahara quartz sand can be deposited into quartz sandstone.
• Rivers can carry anything from clay to boulders, depending on energy and type
• Sand and gravel fill concave-up channels with fine sand, silt, and clay depositing on flood plains. The resulting rocks are sandstones, mudrocks, and conglomerates.
• Lake environments accumulate fine-grained sediment and algae and the resulting rocks are mudrocks and shale and where rivers enter, deltas can form in a low energy environment.
• Delta environments (intermediate) accumulate sediment where a river enters a sea and grow over time, building out into a basin, with the sediment dropping as river velocity is dumped.
• Sediments are transported along the coasts due to ocean currents and surfaces creating a well-sorted, well-rounded, medium-grained sandstone in beach environments. Rounded minerals show up such as quartz.
• Shallow-marine clastic environments have deeper offshore that limited to no wave energy with fine-grained, well-sorted, silt and mud
• Siltstone and mudstone contain marine fossils
• Shallow-marine carbonate environments contain sediments composed of shell organisms with warm clear marine water relatively free of clastic sediments where limestone is the dominant rock type
• Deep-marine carbonate environments have fines that settle out far from land with the skeletons of planktonic organisms that make chalk or chert.

Sedimentary Basins

• Sedimentary basins form where tectonic activity creates space
• Foreland basins crash into mountains to deposit sediment to the basin next to it.
• Rift basins are produced from downward slip on faults.
• Intracontinental basins form in the interior of a continent over an old rift.
• Passive-margin basins are along the coast in areas where tectonic activity is nonexistent, while glacier rocks are angular.

Metamorphism and Metamorphic Rocks

• Meta means change; morph form or shape; and ism implies the changing of form or shape.
• Metamorphism is defined as the alteration of preexisting rock to form another where the preexisting rock is known as a protolith or parent rock.
• Two types of changes occur, one being in texture and the other in mineralogy.
• Original rock type is important because its chemistry controls reactions and it is a solid-state change in response to specific agents that is underground.
• Commonly found exposed in actively forming mountains and the center of eroded ancient mountain belts.
• New conditions cause a rock that is no longer at equilibrium to equilibrate at very slow rates
• Processes include recrystallization, neocrystallization, pressure solution, or plastic deformation
• Recrystallization of protolith turns it into a metamorphic rock such that tiny separate clasts form large grains together
• Neocrystallization of protolith turns it into metamorphic rock of clay and quartz becomes quartz, garnet, and mica.
• New growth after previous minerals are “digested” makes the element different than previous minerals through chemical reactions.
• Grains dissolve on sides with more pressure from pressure solutions that need water. Ions from dissolution then migrate and precipitate new growth.
• Plastic deformation occurs when mineral grains soften and deform.
• Agents of metamorphism include heat, stress, and chemical activity often co-occur.

Agents of Change

• Heat causes atoms to vibrate rapidly, stretching and bending chemical bonds that lock atoms to their neighbors.
• If bonds stretch too far and break, atoms detach from original neighbors, move slightly, and form new bonds with other atoms.
• Repetition of this process leads to rearrangement of atoms within grains, or to migration of atoms into and out of grains, solid-state diffusion
• Heat sources are temperatures from below the Earth's surface, sediment burial, mountain building, and rising magma or igneous intrusions.
• Stress is force applied to a given area that affects mineral stability and influences crystal size and orientation.
• Pressure sources include Sediment burial, mountain building, and compression/shear stress along faults.
• Tectonic burial begins as sediment near the surface then undergoes major changes in heat and pressure
• Metamorphism is adjacent to magma intrusion.
• Fluids reacting with protolith minerals contributes to chemical activity.
• Water is the main fluid and carries the ions in a solution, may be acidic, and is more reactive than cold. It typically occurs at low grade metamorphism.
• Water can occur in pore spaces between grains, seafloor spreading zones, released from magma, and hydrothermal vents
• Altering texture can occur from normal stress parallel to a surface.
• Shear stress texture is altered by changes in size, shape, and orientation of minerals and acts parallel to its surface.
• Directed pressure (differential stress) leads to mineral grain alignment causing layering, which is a consequence of metamorphism.
• Recrystallization is instigated by temperature and pressure that causes minerals to change in shape and sizes.
• Stable minerals have ranges in temperature and pressure

Metamorphic Effects and Classification

• Minerals may transform as temperature and pressure changes
• Some minerals form through a limited range, acting as index minerals
• Regional metamorphism buries pressure, large regions, which elevates temperature
• Contact metamorphism “bakes” the surrounding rocks from contact with magma
• Fault metamorphism happens through shearing and has tiny grains with parallel lines.
• Texture and composition are the primary criteria to classify metamorphic rocks and is grouped into foliated/non-foliated types.
• Foliated rocks are characterized by aligned or layered minerals.
• Non-foliated rocks are identified by their component minerals.
• The degree of foliation is important for foliated rocks, starting with sedimentary shale.

Foliated Rocks

• Slate exhibits low heat and pressure exhibits fine clays, mica, chlorite and quarts that split into sheets and is parent shale or mudstone.
• Phyllite exhibits low heat with mica, some chlorite, and feldspars that may form and has a “sheen” with slate or shale parent rocks.
• Schist exhibits medium heat where chlorite is gone, garnet may occur, and parallel minerals can grow larger.
• Gneiss exhibits high heat and much stress that causes banding with alternating bands of felsic and mafic minerals and parent rocks of schist, granite, or rhyolite.
• Quartzite comes from the metamorphism of quartz sandstone where the fused grains recrystallize to form a hard rock.
• Marble is formed from carbonate rocks used by sculptor as that contains impurities impact its colors and patterns.
• Stretched pebble conglomerate occurs from shear stress of conglomerate rocks that stretches and flattens the grains.
• Greenstone rocks are tough, dark altered mafic rocks that contain a fine-grained texture and have a higher density than glass.
• Serpentinite rocks have a greasy or silky feel, is a soft rock, and contains thin veins.
• Anthracite coal is metamorphosed bituminous coal that displays a shiny but low density.

Geologic Time & Fossils

• Human understanding stemmed from fieldwork observations made by James Hutton.
• Relative dating is a typical example and realization of geologic time depth.
• To reconstruct geologic history, we determine the order of geologic events, construct the timescale, recognize gaps in the rock record, know how we found absolute ages, and how we determined Earth's age.
• Earth's history involves order sequence and time required for steps, while the 2 ways to determine geologic time are relative and absolute dating.
• Relative dating observes rocks in the field and determines event order without knowing how long ago it occurred.
• Absolute dating quantifies when an event occurred or rock was formed.

Dating

• Dating of numerical geologic events has analytical uncertanties, it is also the realm of mass spectrometry.
• It necessitates lab analysis of naturally occurring radioactive elements.
• The 6 dating “rules” or principles are superposition, original horizontality, cross-cutting, inclusions, lateral continuity, and faunal succession.
• Sedimentary/volcanic rocks are created in successions, with oldest rocks at the bottom from the principle of superposition.
• When flat rocks are not longer horizontal, it indicates tectonic activity after deposition from the principality of original horizontality.
• Geologic features cutting across rock must be younger than the rock that has been cut, as per the principality of cross-cutting.
• Objects enclosed in rock are older than the rock itself in the principality of inclusions.
• Rock layers are continuous until encountering an obstruction in the principality of lateral continuity.
• Fossils of different organisms first appear at different times with related organisms exhibit regular progressively younger changes.
• When extinct, fossil organisms disappear from everywhere simultaneously in younger rocks as used by William Smith's map.
• Index fossil are units of study that exists only for a brief time which means rocks containing an assemblage of rock must be the same age. Correlation matches up rock ages in different place to one another.

Geologic Time Scale

• The timescale was created originally from fossils but became quantified using isotopic dating to find absolute time divisions such as eons.
• Eras subdivide eons.
• Periods are subdivisions of eras, while epochs subdivide periods.
• Unconformities occur are gaps in the rock because erosion has occurred rather than deposition of geological significance.
• Angular unconformity is where 2 rock layers meet when inclined at different angles to each other.
• Nonconformity is where sedimentary rocks overlie much older intrusive igneous/ metamorphic rock.
• Disconformity is a gap between parallel of sedimentary rock with erosion but no tilition.

Dating in Geology

• Age dating examples are basalt dikes, cross-cutting faults, granite plutons, folds (original horizontality), intrusions and superposition.
• Relative age dating calculates the event sequence and absolute is numerical.
• Material entirely consisting of one type of atom is an element.
• The smallest amount of an element that can exist is an atom.
• Atoms in an element have the atomic number but can vary in neutrons.
• Elements with different number of nuetrons and different atomic weights are known as an isotope.
• Stable isotopes are non radioactive, while some are radioactive and decay.
• Decay produces an element of a different isotope where the original is the parent and new is the daughter.
• Isotopes that decay are repeatedly measured for a long period of decays and are well understood in their decay and time to alter.
• Based on how a parent and daughter isotopes can change w/ time, rock crystallizes when magma is present.
• Crystals have to be able to cool for both daughter and parent isotopes.

Dating Process

• Radioactive parent isotopes has to decay into a stable state for measuring isotopes.
• The decay rate has to be known of the rock or minerals for determining the age.
• Decay is the amount of time it requires for ½ of isotopes to decaying into daughters.
• This works best for igneous/metamorphic and crystalized magma, while Earth's formation requires accretionary processes.
• Ages must agree agree in relative aging princips, historical observations, and ages confirmed by dating principles.
• Relative and absolute dating combines for the most complete form of dating.

Fossils

• Paleontology is the study of prehistoric life that includes studying all living orginisms that used to pre exist.
• Stratigraphic columns have been stacked to create a geologic column after global correlation
• Recognizing what a fossil is requires that fossils contain all fossils that are older than 10,000 years
• Preservation includes rapid burial, hard parts, and anoxic environments.
• Types are preserved teeth, body, and shells.
• Molds tell us how life gets turned into sediment.
• Mass extinction occurs in relatively short time periods and contains large percentages of species.

Metamorphic Overview

• Metamorphism can have its minerals increase such as heat, pressure and mineralogy.
• Regional refers to large areas and contacts.
• A high/low metamorphic grade will dictate which direction the rock types take and will form slate,phylite,and schist.
• Slate can have its grain increases until they becomes gneisses and have alternating bands of colors.
• Heat is the most significant to metamorphism.
• Slate begins as a shale parent rock.