Geol 102 Midterm

Geology

The study of the earth, the physical and chemical processes that act on the earth, and the history of the earth and its residents

Physical Geology

The study of the origin, classification, and composition of earth materials and the study of the processes that act deep in the interior

Historical Geology

The study of the origin, evolution, and changes in the earth and solar system

Nicolas Steno - Danish, late 1600's

Principle of Superposition, Principle of Original Horizontality, Principle of Lateral Continuity, Stratigraphy

Principle of Superposition

In undisturbed strata, oldest rocks are at the bottom, and youngest rocks are at the top

Principle of Original Horizontality

Most sedimentary particles settle due to gravity; Sediments must have been deposited nearly horizontal and parallel to the underlying surface

Principle of Lateral Continuity

Strata extend until they terminate by thinning, grading into another sediment, running into a barrier

Stratigraphy

The study of layered rocks, including their: texture, composition, arrangement, correlation

Abrahm Werner - England, 18th Century

Lead the Neptunists

James Hutton - Scottish Farmer, late 18th Century

Plutonist: Believed crystalline rocks formed from melted matter deep below the surface

Principles: Uniformitarianism & Actualism

Uniformitarianism

Laws of nature haven't changed so that the "present is key to the past"

Actualism

Natural laws governing both past and present processes on Earth are the same

Charles Lyell - Early 1800's

Principle of Crosscutting Relationships & Principle of Inclusions

Principle of Crosscutting Relationships

A geologic feature that cuts across another feature must be younger

Principle of Inclusions

Rock fragments included in another rock must be older than the surrounding rock

William "Strata" Smith - late 18th Century

Principle of Biological Succession

Principle of Biological Succession

Rock units often contain unique fossils

These fossils represent a unique time period

Unique fossils can be used to identify time-equivalent rocks

Cuvier and Brongniart - 18th century France

Founded vertebrate paleontology & Catastrophism

Paleontology

The study of all ancient life forms, their interaction, and their evolution

Catastrophism

Believed history of earth was marked by violent floods, crustal upheavals, and extinctions

Charles Darwin - 18th century biologist/geologist

Developed the theory of natural selection

Provided a mechanism for evolution

Geologic Time Scale

System is used for rocks ; Period is used for time

Phanerozoic- "evident life"

Precambrian- "hidden life"

Relative Age Dating

order of events tell younger vs older

Uses the principles of geology

Geologic history of an area: must be aware of many contributors (layering, fossils, cross-cutting, etc)

Uses correlation to establish the relative age of two rocks in different locations. Correlations can be made using:

Lithology - rock type and character

Index fossils - fossils with wide spatial distribution but narrow time window of existence (e.g., lived for only a short time, geologically speaking)

Absolute Age Dating

method that enables scientists to determine the actual age of certain rocks and other objects- use radioisotope/radioactive/radiometric dating

incorrect methods of absolute age dating

Bishop Usher- used ages of people in the Bible- problem is that they don't match geologic evidence

Sedimentation Rates- use modern rates of sediment accumulation; measure thickness of ancient deposits to calculate age- problem is that sedimentation rate is not uniform, compaction, or erosion

Seawater salinity- doesn't account for salts leaving water

Cooling rate- Kelvin- problem is that heat from radioactive decay slowed this process

Radioisotope Dating

THE PROCESS BY WHICH THE ABSOLUTE AGE OF A ROCK OR EVENT IS DETERMINED

Isotopes: have same atomic number but different mass numbers

Unstable isotopes want to decay at a stable rate

They do this by emitting radiation

Once an unstable nucleus decays it forms a daughter element (will decay again if also unstable)

half life concept

Half-Life

THE AMOUNT OF TIME NECESSARY FOR HALF OF THE UNSTABLE PARENT ATOMS TO DECAY TO STABLE DAUGHTER ATOMS

Each radioactive isotope has its own specific half-life

Why are rocks and minerals important

because they form under unique conditions which tell us about the environment in which they were formed

Minerals

naturally occurring, crystalline solid, definite chemical composition, inorganic

Most rocks are made up of only about 30 minerals

ways minerals form

Solidification (cooling of molten material)

Precipitation (forming a solid from ions dissolved in water

Rearrangement (atoms in solid are rearranged- requires water, heat, and/or pressure

Polymorphs

same chemical composition, but different crystal structure (diamond and graphite)

Major types of minerals

Silicates: over 90% of earth's crust

Quartz (hard)

Feldspars (most abundant)

Micas

Amphiboles

Pyroxene

Oliyene

Clay minerals

Rocks

assemblage of minerals

Igneous Rock

solidified of magma (melted rock)

Sedimentary Rock

accumulation and cementation of mineral grains ; chemical precipitation

Metamorphic Rock

existing rocks altered by heat or pressure

Igneous Rock Names

based on mineral comp and texture- basalt, quartz, micas, amphibole, granite, gabbro, etc

Intrusive Igneous Rock

Solidified underground

Slow cooling→ large crystals

Intrusive Igneous Rock Forms

Pluton: massive igneous bodies formed at depth

Sill: horizontal intrusion between existing layers of rocks

Dike: intrusion that cuts across existing layers

Vein: deposit of foreign minerals within a rock fracture

Volcanic glass: either lava or pyroclastic, no crystals

Obsidian: volcanic glass with no bubbles, solid rock

Porphyry: a mix of fine crystals with larger crystals (phenocrysts)

Slow-cooling magma begins forming crystals. If then cooled slowly, small crystals form and encase the larger phenocrysts

Extrusive Igneous Rock

Solidified at the surface- usually forms from Volcanoes

Fast cooling→ small crystals

Extrusive Igneous Rock Forms

Lava: magma (molten rock) that has flowed to the surface (ex: basalt)

Pyroclastic Rock (Tuff): hot ash and magma thrown into the air, settled, and cooled

Sedimentary Rock categories

clastic & chemical

Clastic Sedimentary Rocks

formed from accumulation of particles (10x more common)

Types are based on size of particles... coarse-fine:

Conglomerates

Breccia

Sandstone

Siltstone

Mudstone / shal

Chemical Sedimentary Rocks

formed by precipitation of dissolved particles

Limestone- - Calcareous sand, mud, coral and/or shells (CaCO3)

Dolomite - Similar to limestone but some Ca replaced with Mg

Evaporite - Salts (gypsum, halite, borax)

Chert- Nodular - SiO2 replacement in limestone bedded - siliceous shells (microscopic)

Organics - organic debris (peat and coal)

Iron Oxides - common in deep soils

Metamorphic Rock Grades

high- formed at high temp or pressure ; low- formed at low temp or pressure

Types of metamorphic alteration:

Compositional and Textural

Compositional alteration

rearrangement of atoms in minerals resulting in new minerals

The types of Minerals are controlled by temperature and pressure (stability relationships). Results in a progression in mineral types

Textural alteration

Alteration of the texture of the rock

Recrystallization - results in a new texture (ex: larger or intergrown crystals), caused by pressure or heat

Deformation - altering the shape of minerals (pressure)

Foliated Metamorphic Textures

Minerals show preferred orientation

Flat minerals align perpendicular to primary force (Slate, Phyllite, Schist, Gneiss)

Non-Foliated Metamorphic Textures

no preferential orientation

No minerals capable of aligning in original rock

(Quartzite, Marble)

Each continent has a craton consisting of a:

Shield (ancient crystalline rocks)

Platforms (ancient flat lying sedimentary rocks)

Craton is bounded by:

orogenic belts (regions of deformed younger rocks)

Environments of Deposition:

Continental

Marine

Transitional

Continental Environments:

Fluvial (River) Systems

Most important process for transporting sediment from mountains to lowlands and the oceans

Erodes mountains and flattens topography

River type depends on the gradient

High Gradient River

Braided stream (mountains)

Steep gradient and relatively straight river

Consist of gravels, silt, and sands

Low Gradient River

meandering stream (MS Delta)

Contorted pathway

Sands accumulate in channels while silt and clays accumulate in the floodplain

Sediments in flowing water move by:

Traction

Saltation

Suspension

Water moves faster on steeper slopes (w high gradients)

Narrow channel width: fast- more erosion (bigger particles)

Wide channel width: slow (deposition of particles occurs)

Continental Environments:

Alluvial Fans

Form at the foot of mountains

Coarse sediments near the apex, fine sediments in the distal fan

Continental Environments:

Eolian Deposits (Sand Dunes)

Form in arid environments from sediment (sand) transported by wind

Wind moves the sand up the slope and down the slip face

Continental Environments:

Glacial Deposits

Form in cold regions (in mountains or as continental ice sheets)

Ice moves fastest at the top, but is warmer at the base

Glaciers deposit boulders to clay-sized material

Glaciers erode mountain valleys into a U-shape (ex: valley near Jackson Hole)

Fiords are also U-shaped valleys that have been flooded by sea water- ocean levels rose after the last ice age

Continental Environments:

Lacustrine Deposits (Lakes)

Lake cycle:

Summer: stratified waters (cold on bottom, warm on top)- coarse sediment

Fall: turnover (waters mix)

Winter: stratified waters (cold on top from ice, less cold on bottom)- fine sediments (clay) settle out

Spring: turnover (waters mix)

In glacial lakes, turnover controls sedimentation: Coarse sediment in the summer ; Fine sediments (e.g., clay) settle out in the winter → Forms alternating layers of coarse and fine called varves

Continental Environments:

Playa Deposits (Dry Lakes)

Form in arid environments- may be mudflats

May contain salt deposits

Transitional Environments:

Delta Deposits

Where rivers deposit sediments into the sea

Delta deposits: broad expanses of mud deposits, crossed in places by meandering "ribbons" of sand deposited in the river channels

The tremendous weight of sediment in a delta can cause the area to sink

Transitional Environments:

Tidal Flats

Form in low-lying coastal areas

Sediments consist of muds, organic materials, and some sands

Transitional Environments:

Lagoon

Restricted form the ocean- often contains mud, silt, organic material, and may contain evaporites

Transitional Environments:

Beach

Beach sediments reflect nearby source rocks can vary from sands to gravel

Transitional Environments:

Barrier Island

Separated from the shore by a lagoon

Has a combo of environments- mud/saltmarsh | mud/mud-flat | sand/dune | sand/beach | open water

Marine Environments:

Shallow Marine: coast to continental shelf

Clastic: sands near the beach, fine grained mud away from the beach, shells throughout, local organic rich sediments in deeper water

Reefs: coral reefs that grow in shallow water

Reefs grow on themselves, shed talus into deeper water, create shallow lagoons between the reef and the land

Marine Environments:

Continental slope/rise: sloping region from continental shelf to deep sea

Sands, silts, and muds carried by turbidity currents in submarine canyons are deposited in the deep marine (abyssal plain) environment

Marine Environments:

Deep Marine (Abyssal Plane): Deep ocean beyond slope

Mud, fine-grained carbonates, and chert deposits accumulate here

Clastic rocks consist of:

- Clastic grains: individual particles of rock or mineral

- Cements or matrix: bonds the grains together

Cements are often calcite or quartz

Matrix is often clay

Clastic sediments are classified based on grain size

Conglomerate: a mix of large grain sizes ex: lithified gravel

Sandstone: consists mainly of sand

Siltstone: mainly silt

Shale: mainly clay

Clastic Textures

Sorting (well or poorly)

Shape (angular v rounded) & Sphericity (spherical v oblong)

Orientation of grains

Maturity

a measure of distance traveled

Mature sands

high quartz content, well-rounded grains, well-sorted

Immature sands

high feldspar content, angular grains, poorly sorted

Common sandstones:

Quartz sandstone: mature

Arkose: immature

Graywacke: clay and silt matrix

Lithic sandstone: immature

Quartz sandstone comes from

sedimentary rocks with sandstone being brought into the ocean, producing mature quartz sandstone

Arkose comes from

granitic igneous rocks (containing feldspar) in an uplift of a mountain eroding, providing sand-sized material with quartz and feldspar deposited in alluvial fans or streams, creating arkose sands along the mountains

Grawacke comes from

narrow continental shelf (containing different rocks) that is drained, carrying fragments of the rocks into the ocean moving along the shore, hitting canyons in the abyssal plane, where turbidity currents are that form a submarine fan- forms graywacke sediments

Lithic sandstone comes from

place with lots of source rocks surrounding the ocean, the sediments are carried into the ocean as lithic sandstone

Sedimentary Structures:

Larger features formed during or shortly after deposition (before lithification when sediment is turned into stone)

Key for interpreting depositional environment

Mudcracks

tell us that the rock was once exposed to the surface of the air- it has been wet and later dried out; the sediment could not have formed entirely underwater- we must be in some kind of land environment rather than marine

Ripple Marks

Symmetric ripples form when a current moves back and forth over an area (commonly in the intertidal zone of a beach)

Asymmetric ripples form when a current (water or air) moves in one direction (sand dune or stream)

Cross Bedding

large features that tell us that either water or wind moving in a current created dunes; tend to develop in the direction of the current

Planar: laminae/beds are parallel within each group

Trough: circular sections

Large-scale: sandbars in a river

Small-scale: cross-lamina (ripples) in a river

Graded Bedding

Consists of repeated beds that get finer upwards

Common in turbidity currents formed by underwater earthquakes or landslides

Front part of current contains coarse material, back part- fine

Geopetal Indicators

tells us up vs down

Sole marks: formed when scours are eroded in the bottom of a channel; point downwards under normal circumstances

Fossils like corals have a specific shape when growing upwards

Burrows- normally U-shaped- upside down U means overturned strata

Limestone

Shallow marine environments

Primarily from organic activity

Calcareous shells, corals, and excretions

Dolomite

Forms when calcium in a limestone is replaced by magnesium

Evaporites

Forms in arid regions- playa lakes

Forms in restricted marine environments

Requires high evaporation rates (dry)

Formations

a single group of rocks given a stratigraphic name

Can be grouped into "groups"

Can be subdivided into "members"

GROUP→ Formations→ members

Facies

lithologic or biogenic characteristics from which the depositional environment can be inferred

Lithofacies can cross timelines

A single type of rock may have different ages in different locations

Facies onlap

when the sea-level rises (transgression)

Facies off-lap

the sea-level drops (regression)

Lateral succession of Facies

Often the lateral succession reflects the vertical succession (Walther's Principle)

The sea-level has changed with time. This is due to:

Ice ages

Displacement along the seafloor

Subsidence

Angular unconformity

an area has undergone an uplift and the uplift was accomplished by either folding or tilting, with erosion of the strata prior to later subsidence and continued deposition

Non-conformity

uncomform-able erosional surface that involves and separates older igneous or metamorphic rocks from younger overlying sedimentary strata

Disconformity

uncomform-able surface that separates essentially parallel sedimentary strata

Invertebrate Paleontology

study of animals with no backbone