Geology 102 1st assignment
time-stratigraphic unit 100
A body of strata that was deposited during a specific interval of geologic time; for example, the Devonian System, a time-stratigraphic unit, was deposited during that part of geologic time designated the Devonian Period.
sand dune 117
A ridge or mound of wind-deposited sand.
angular unconformity
An unconformity below which strata dip at a steeper angle than those above. (See disconformity, nonconformity, and unconformity.)
unconformity
An unconformity indicates that the geologic record is ________ at a particular location, and may indicate a period of ________ . incomplete; non-deposition
biogenic sedimentary structures
Any feature such as tracks, trails, and burrows in sedimentary rocks produced by the activities of organisms. (See trace fossil.)
Marine Environments
Continental shelf Inner shelf Mostly cross-bedded sandstone with wave-formed ripples, marine fossils, and bioturbation Outer shelf Mostly mudrocks with subordinate sandstone; marine fossils and bioturbation Continental slope and rise Turbidite sequences in submarine fans with graded bedding in sandstone and mudrocks Carbonate shelf Limestone (dolostone). Limestone varies from coquina (made of shell fragments) to ooliticlimestone to micrite (carbonate mud). Cross-beds, mud cracks, ripple marks common; marine fossils Deep-ocean basin Pelagic clay and calcareous and siliceous oozes Evaporite environments Rock salt and rock gypsum the most common, but others, including potassium and magnesiumsalts, may be present
delta seaward; progradation
Deltas build ________ by a geologic process called ________ .
evaporites deposited? What are the two most common types of evaporite rocks?
Evaporites are mostly rock gypsum and rock salt, formed in saline lakes, playa lakes, and some marginal marine environments. Evaporites form in arid environments with high evaporation rates.
Evaporites 81
Evaporites, such as rock salt, result when evaporation exceeds precipitation, such as in desert regions or along hot, dry shorelines
bioturbation
Extensive burrowing by organisms may produce ________ , in which sediments may be so disturbed that other sedimentary structures are destroyed
Cross-bedding
Layers deposited at an angle to the surface on which they accumulated Deposition on a sloping surface as the downwind side of a sand dune
Laminations (or laminae)
Layers less than 1 cm thick Form mostly as particles settle from suspension
Beds
Layers more than 1 cm thick. Form as particles settle from suspension and from moving sediment as sand in a stream channel
micrite
Many limestones are made up of sand- and gravel-sized particles and calcium carbonate mud called
Lower Sularian Strata 124
New Jersey, Penn. deposited in braided streams that flowed from east to west
deposition
Sedimentary structures are formed during ________ or shortly thereafter.
cross dating 79
The procedure of matching ring patterns from numerous trees and wood fragments in a given area is called cross-dating.
Atrypa 102
and the trilobite Paradoxides are guide fossils because they are well suited for time-stratigraphic correlation
Current ripple marks 110
are asymmetrical in profile and are invariably generated by currents, either water or wind, that flow in one direction
metamorphic rocks 77
difficult to accurately date bc of heat and pressure affect parent daughter ratio
atoms 73
extremely small particles that make up elements
disconformity 87
is an erosion surface in sedimentary rocks that separates younger rocks from older rocks, both of which are parallel to each other
daughter element 73
new more stable element
discontinuities or unconformities
represent times of nondeposition, erosion, or both
depositional environment 117
Any area where sediment is deposited; a depositional site where physical, chemical, and biological processes operate to yield a distinctive kind of deposit.
Numerical dating 66
Assigning an age in years before the present to geologic events before the present; numerical dates are determined by radioactive-decay dating techniques
marine transgression 90
, when sea level rises with respect to the land, a marine transgression takes place that accounts for the vertical sequence of facies
alpha decay 73
2 protons and 2 neutrons are emitted from the nucleus
playa lake 117
A temporary lake in an arid region.
outwash 117
All sediment deposited by streams that issue from glaciers. (See drift.)
stratigraphy
Although the branch of geology known as stratigraphy applies to all the major rock groups, it focuses primarily on ________ rocks. sedimentary
till
Any sediment deposited directly by glacial ice is called
pelagic clay and ooze 121
Beyond the continental shelf, the seafloor is nearly covered by fine-grained deposits known as pelagic clay and ooze
marine transgression vertical sequence of strata can be attributed to a marine transgression
During a transgression, sea level becomes higher, and the shoreline migrates towards land. As the shoreline migrates landward, offshore facies are deposited over nearshore facies, so that in the rock record the nearshore facies sandstone is overlain by shale and then limestone.
Walther's Law
During marine transgressions and regressions, several facies are deposited at the same time, giving rise to vertical and lateral relationships. Which law summarizes these relationships?
John Wesley Powell 66
Grand Canyon,
half-life 1/8
If a radioactive element has a half-life of 32 million years, what fraction of the original amount of parent material will remain after 96 million years?
daughters atoms migrate out 73
If a rock is heated during metamorphism and the daughter atoms migrate out of a mineral that is subsequently radiometrically dated, an inaccurate date will be obtained. Therefore, this date will be younger than the actual date;
members and beds, 98
Many formations are subdivided into members and beds,
geologic history The photograph on the right was taken on Fish Creek Trail in the Sierra Nevada, California. It shows two black metavolcanic erratics on granodiorite bedrock that has been intruded by a white dike. Using your knowledge of igneous rocks and processes, glacial features, and relative dating principles, provide a geologic history of this area based on what is shown in this image
The first event was the emplacement of the intrusive granodiorite batholith. The white dike then intruded this plutonic material. Erosion occurred. Later glacial processes transported the glacial erratics and deposited them on the plutonic material and dike.
rounding 109 texture
The process involving abrasion of sedimentary particles during transport so that their sharp edges and corners are smoothed off.
Tree-ring dating
The process of determining the age of a tree or wood in a structure by counting the number of annual growth rings.
Relative dating
The process of placing geologic events in their proper chronological order with no regard to when the events took place in terms of the number of years ago. (See numerical dating.)
marine transgressions and regressions
Uplift and subsidence of continents, the amount of water frozen in glaciers, and the rate of seafloor spreading are responsible for marine transgressions and regressions
ripple marks
Wavelike structure on a bedding plane, especially in sand, formed by (1) unidirectional flow of air or water currents or (2) by oscillating currents as in waves.
ripple marks 110
Wavelike structure on a bedding plane, especially in sand, formed by (1) unidirectional flow of air or water currents or (2) by oscillating currents as in waves.
fossil footprint
What is the best example of a biogenic sedimentary structure
discordant 77
ages do not agree
Carbon-14 dating 78
can be used only on organic matter such as wood, bones, and shells, and is effective back to approximately 70,000 years ago
conformable 86
conformable Refers to a sequence of sedimentary rocks deposited one after the other with no discontinuities, or only minor ones.
James Hutton 70
considered the founder of modern geology
uranium thorium dating 78
cross check d=carbon 14 dating
uranium-lead and thorium-lead 77
date igneous rocks lunar sample and meteorites
samarium - neodymium dating 78
date old igneous and metamorphic rocks and meteorites
Niobrara Formation 124
deposition in a marine environment
continental rise 121
detrital marine env The gently sloping part of the seafloor lying between the base of the continental slope and the deep seafloor.
Mud cracks are one type of biogenic sedimentary structure.
false
igneous rocks 75
gives most accurate radiometric dates
sedimentary rocks are important because they
may contain fossils
rock ages 103
rocks of Ordovician age are younger than those of the Cambrian and older than Silurian rocks,
Eras Eras are subdivided into periods.
true
unconformities All unconformities are created during periods of erosion and/or non-deposition.
true
George Lewis De Buffon 67
Earth Gradually cooled, 75,000 years old
original horizontality
Imagine a stack of tilted sedimentary beds. We know that these beds were deposited horizontally, and then later tilted, because of the principle of ________ .
superposition 72
Imagine that you are standing in front of a stack of sedimentary beds. You know the beds at your feet are older than those above your head because of the principle of ________
lithostratigraphic unit 97
Lith and litho mean "stone" or "stonelike."
Till 117
Sediment deposited directly by glacial ice, as in an end moraine.
lagoons
Silt and clay are fine-grained sediment and tend to be deposited in low-flow environments such as
sedimentary facies
Simultaneous deposition in adjacent but different environments yields sedimentary facies, which are bodies of sediment (or sedimentary rock) with distinctive lithologic and biologic attributes.
Ripple marks
Small (< 3 cm high) ridges and troughs on bedding planes
Navajo sandstone 124
Studies show that the Navajo sandstone is made up of well-rounded, well-sorted quartz grains with tracks of land dwelling animals, including dinosaurs, and it has cross-beds up to 30 meters high. These are good indicators that this was formed in a ________ environment. coastal dune
Fission-track dating
The dating process in which small linear tracks (fission tracks) resulting from alpha decay are counted in mineral crystals.
fossil succession - allows geologists to determine the relative ages of sedimentary rocks in widely separated areas.
The fossil record reveals that animal and plant species (or assemblages of species) have occurred in a specific, non-repeating sequence that is consistent from area to area. Therefore, by correlating fossils or fossil assemblages from one area to another area, geologists can conclude the rocks in both locales were deposited at similar times.
period 101
The fundamental unit in the hierarchy of time units; part of geologic time during which the rocks of a system were deposited.he most commonly used time unit
superposition and faunal succession;
The geologic column and relative geologic time scale were established by the 1840s based on
Radioactive decay 73
The spontaneous change in an atom by emission of a particle from its nucleus (alpha and beta decay) or by electron capture, thus changing the atom to a different element.
Paleogeography
The study of Earth's ancient geography on a global as well as a regional scale.
Paleogeography 125
The study of Earth's ancient geography on a global as well as a regional scale.
till poorly; glaciers
Till is ________ sorted sediment deposited by ________
Tillites 81
Tillites (glacial sediments) result from glacial activity and indicate cold, wet environments.
elements 73
all matter is made up of elements
Stratigraphy is primarily the study of sedimentary rocks.
false
Unconformities can only develop between layers of sedimentary rock.
false
Uniformitarianism is a limited concept, because it fails to account for sudden events such as earthquakes, volcanoes, and tsunamis.
false
correlation The strongest evidence that rocks are time correlated is derived from lithological similarities.
false
Continental Environments 117
*Fluvial * Braided stream Mostly horizontally bedded conglomerate and cross-bedded sandstone; mudrocks not common Meandering stream Mostly mudrocks deposited on floodplains; subordinate but distinctive lenticular sandstones deposited in point bars *Desert * Alluvial fan Poorly sorted conglomerate from debris flows and sandstone- and conglomerate-filled channels Sand dune Well-sorted, rounded sandstone with large-scale crossbeds Playa lake Laminated mudstone/siltstone; evaporites such as rock salt, rock gypsum, and others *Glacial* Outwash Much like braided stream deposits Moraines Unsorted, nonstratified deposits of sand and gravel
sorting 109 texture
, which refers to the size variation in a sedimentary deposit or rock. If most of the particles are of about the same size, the sediment or rock is well sorted, but if a wide range of sizes is present, the material is poorly sorted
sedimentary rocks are important because 108
1.they preserve evidence of the surface processes responsible for deposition (for example, running water, wind, waves, and glaciers) and 2.many contain fossils that are rare or absent in most other rocks. So here we focus on areas where sediment is deposited—that is, depositional environments—as well as the criteria used to recognize deposits of specific environments.
Inclusions /B
3.Which one of the following statements is correct? a. A formation is made up of two or more biozones; b. Inclusions are older than the rock layer containing them; c. Time units are defined by rock type and thickness; d. Guide fossils have very long geologic ranges; e. A disconformity is the fundamental type of lithostratigraphic unit.
unconformities 86
A break or gap in the geologic record resulting from erosion or nondeposition or both; the surface separating younger from older rocks where a break in the geologic record is present. (See angular unconformity, nonconformity, and disconformity.)
Tidal flats 119
A broad, extensive area along a coastline that is alternately water-covered at high tide and exposed at low tide
drift 117
A collective term for all sediment deposited by glacial activity; includes till deposited directly by ice, and outwash deposited by streams discharging from glaciers. (See outwash.)
Walther's law 90
A concept holding that the facies in a conformable vertical sequence will be found laterally to one another.
alluvial fan 117
A cone-shaped accumulation of mostly sand and gravel where a stream flows from a mountain valley onto an adjacent lowland.
mud cracks
A crack in clay-rich sediment that forms in response to drying and shrinkage.
mud cracks 110
A crack in clay-rich sediment that forms in response to drying and shrinkage.
varve 117
A dark-light couplet of sedimentary laminations representing an annual deposit in a glacial lake.
prograding delta explain why prograding deltas yield a deposit that becomes coarser-grained upward, whereas in prograding tidal flats, sediments become finer-grained upward.
A delta commonly forms by the deposition of sand and clay as a river or stream enters the quiet water of an ocean or lake. Hence, near-shore deposits, which commonly include sands and silts deposited at the river mouth, prograde over deep water deposits, which commonly include clay minerals that remain in suspension as the river water moves away from the coast toward deeper water. In contrast, tidal flats commonly prograde over beach deposits. The fine-grained sediments of the tidal flats overlie coarser-grained sands of the beach.
delta
A deposit of sediment where a stream or river enters a lake or the ocean.
delta 118
A deposit of sediment where a stream or river enters a lake or the ocean.
geologic column 96
A diagram showing a composite column of rocks arranged with the oldest at the bottom, followed upward by progressively younger rocks. (See geologic time scale.)
biozone 98
A general term referring to all biostratigraphic units such as range zones and concurrent range zones.
rocks
A geologist is least likely to collect good data on recent climate change (within the past 1,000 years) from what source?
barrier islands 119
A long sand body more or less parallel with a shoreline but separated from it by a lagoon.
carbon-14 dating 78
A numerical dating technique relying on the ratio of to in organic substances; useful back to about 70,000 years ago.
principle of cross-cutting relationships 72
A principle holding that an igneous intrusion or fault must be younger than the rocks it intrudes or cuts across.
principle of inclusions 86
A principle holding that inclusions or fragments in a rock unit are older than the rock itself—for example, granite inclusions in sandstone are older than the sandstone.
principle of lateral continuity 72
A principle holding that rock layers extend outward in all directions until they terminate.
principle of superposition 72
A principle holding that sedimentary rocks in a vertical sequence formed one on top of the other so that the oldest layer is at the bottom of the sequence whereas the youngest is at the top.
principle of original horizontality 72
A principle holding that sediments are deposited in horizontal or nearly horizontal layers.
graded bedding
A sediment layer in which grain size decreases from the bottom up.
graded bedding 109
A sediment layer in which grain size decreases from the bottom up.
graded bedding
A sediment layer in which grain size decreases from the bottom up. Most graded beds are deposited by turbidity currents, which are underwater flows of sediment and water that have a greater density than sediment-free water.
meandering streams 116
A stream with a single, sinuous channel with broadly looping curves.
braided streams 116
A stream with an intricate network of dividing and rejoining channels.
cross-bedding
A type of bedding in which individual layers are deposited at an angle to the surface on which they accumulate, as in sand dunes.
cross-bedding 110
A type of bedding in which individual layers are deposited at an angle to the surface on which they accumulate, as in sand dunes.
Biostratigraphic units 98
A unit of sedimentary rock defined solely by its fossil content.
nonconformity
A(n) ________ is an erosional surface cut into plutonic or metamorphic rocks that is overlain by sedimentary rocks.
superposition; younger 72
According to the principle of ________, the sedimentary rocks on the top layer, are ________ than those on the bottom layer. .superposition; younger
sedimentary structures
All features in sedimentary rocks, such as ripple marks, cross-beds, and burrows, that formed as a result of physical or biologic processes that operated in a depositional environment.
sedimentary structures 109
All features in sedimentary rocks, such as ripple marks, cross-beds, and burrows, that formed as a result of physical or biologic processes that operated in a depositional environment. formed during deposition or shortly thereafter, but before lithification.
guide fossils 102
Any easily identified fossil with a wide geographic distribution and short geologic range; useful for determining relative ages of strata in different areas.
biogenic sedimentary structures 110
Any feature such as tracks, trails, and burrows in sedimentary rocks produced by the activities of organisms. (See trace fossil.)
trace fossils 92
Any indication of prehistoric organic activity such as tracks, trails, burrows, and nests. (See biogenic sedimentary structure, body fossil, and fossil.)
Time units 101
Any of the units such as eon, era, period, epoch, and age referring to specific intervals of geologic time.
parent vs daughter isotopes decreases; increases
As a rock ages, the percentage of parent isotopes ________ , whereas the percentage of daughter isotopes ________ .
Current ripple marks
Asymmetric ripple marks Result from deposition by water or air currents flowing in one direction
Baculites reesidei biostratigraphic zone 103
Baculites reesidei biostratigraphic zone in the Bearpaw Formation in Saskatchewan, Canada, is about 72 to 73 million years old because numerical ages have been determined for associated volcanic ash layers
coals 81
Coals form in freshwater swamps where climatic conditions promote abundant plant growth.
Transitional Environments 117
Delta (marine) Mudrocks and sandstone in coarsening-upward sequences; associated rocks of marine origin;fossils of marine and land-dwelling organisms Beach Rounded sandstone with variable sorting, commonly with shells or shell fragments, wave-formed ripple marks, and small-scale cross-bedding Barrier island Beach As above Sand dunes Much like desert dunes but with sand-sized shell fragments Tidal flat Mudstone and sandstone in fining-upward sequences; distinctive herringbone cross-bedding in sandstone
Paleocene-Eocene Thermal Maximum 80
Earth has also undergone large-scale periods of soaring global temperature, such as during what is known as the Paleocene-Eocene Thermal Maximum (Figure 4.15), beginning around 56 million years ago and lasting more than 150,000 years
continental shelf, continental slope, and continental rise
From land to sea, the order of marine environments is ________
radiometric
Geologists depended solely on relative dating methods prior to the development of ________ dating techniques.
rock sample closed; long
Geologists use a rock sample that is a(n) ________ system to date events that occurred early in Earth's history as well as a radioactive isotope with a relatively ________ half-life.
graded bedding? How does it form? In which sedimentary environment would you expect it to be common
Graded bedding is a sedimentary structure characterized by rock layers that become progressively finer in an upwards direction and is usually the result of turbidity currents. These sediment-laden currents move along the bottom of the seafloor. As the velocity diminishes, they deposit large particles followed by progressively smaller ones, forming graded bedding.
uniformitarianism 70
Hutton viewed Earth history as cyclical, with periods of uplift, erosion, and deposition. He believed Earth was old, and the cycles had vast amounts of time to operate. Lyell supported Hutton and postulated a "steady-state" Earth system in which the same geologic processes operating today have prevailed at the same rate over geologic time. By accepting this, if we understand Earth's processes today, we can use them to reconstruct Earth's history. However, it is now accepted that the rates of some geological processes may vary with time. In this way, the principle of uniformitarianism has been adapted to incorporate processes that may be relatively rare, such as major bolide impacts. Uniformitarianism is significant because it allows geologists to use current observations to interpret the rock record and determine past conditions.
younger than; cross-cutting relationships
If a fault cuts three beds of rock, it must be ________ the three beds of rocks. We know this because of the principle of ________ .
Johannes Walther (1860-1937). 90
In our discussion of marine transgressions and regressions, we considered both vertical and lateral facies relationships, the significance of which was first recognized by Johannes Walther (1860-1937).
Graded-bedding
Individual layers with an upward decrease in grain size Deposition by turbidity currents or during the waning stages of floods
sedimentary deposits of the inner and outer continental shelf differ? Do they share any features
Inner shelf deposits are mostly sand, contain marine fossils, are bioturbated, and are commonly cross-bedded. Bedding planes are marked by wave-formed ripple marks. This is the high-energy area that is periodically stirred up by waves and tidal currents. Outer shelf deposits are typically mud with marine fossils. This is the low energy part of the shelf. Both inner and outer shelf environments are marine environments where fossils and bioturbation are present.
Mud cracks
Intersecting cracks in clay-rich sediments Drying and shrinkage of mud along a lakeshore, on a floodplain, or on tidal flats
Lord Kelvin 71
Kelvin assumed that Earth had begun as a molten mass, and then he calculated the temperature under those conditions. By estimating Earth's current temperature and rate of heat loss, Kelvin determined how long it would have taken for Earth to cool to its present temperature. His calculations appeared scientific and convincing, but the calculations produced a date that was far too young. Kelvin's basic flaw was his failure to account for a heat source within Earth since its formation. He was unaware that radioactive decay within the Earth generated heat. However, even if radioactivity had not been discovered, there is other scientific evidence for the ancient age of Earth. Scientists eventually would have accumulated more data, which would have led to acceptance of the ancient age of Earth.
cooling; refuted
Lord Kelvin analyzed ________ rates for Earth to determine the age of Earth. He believed that his work ________ uniformitarianism as outlined James Hutton-Charles Lyell.
Marine environments 119
Marine environments include the continental shelf, slope, rise, and the deep seafloor
Mold—93
Mold—a cavity with the shape of a bone or shell;
turbidity currents 109
Most graded beds are deposited by turbidity currents, which are underwater flows of sediment and water that have a greater density than sediment-free water
point bar 116
One of the most distinctive features of meandering streams is point bar deposits consisting of a sand body overlying an erosion surface that developed on the convex side of a meander loop
relative dating; 66
Placing geologic events in sequential or chronologic order as determined by their position in the geologic record is
Charles Lyell 70
Principles of Geology changes over long periods of time have cumulative effects
fluvial 116
Relating to streams and rivers and their deposits.
elongate, or shoestring, geometry, 115
Some sand deposits have an elongate, or shoestring, geometry, especially those deposited in stream channels or barrier islands.
Wave-formed ripple marks
Symmetric ripple marks; generally with sharp crests and broad troughs Formed by oscillating currents (waves
facies 89
Tapeats Sandstone is overlain by the Bright Angel Shale and finally by the Muav Limestone (Figure 5.7). Each of these rock units represents a facies,
formation 97
The basic lithostratigraphic unit; a mappable unit of strata with distinctive upper and lower boundaries
lithostratigraphic unit
The basic, mappable, lithostratigraphic unit is the ________ , which has ________ upper and lower boundaries. formation; distinct
determine numerical ages of sedimentary rocks
The best way to determine numerical ages of sedimentary rocks and their contained fossils is to obtain dates for associated igneous rocks and metamorphic rocks.
bioturbation
The churning of sediment by organisms that burrow through it.
bioturbation 111
The churning of sediment by organisms that burrow through it.
sorting
The degree to which all the particles in a detrital sedimentary rock are of about the same size
meandering stream deposits
The deposits of a meandering stream system consist mostly of mud with subordinate sand;
cross-bedding and ripple marks
The direction of ancient wind and water currents can be deduced from analyzing ________
average sedimentation rate Suppose that the upper ash layer in Figure 5.21 formed 1.7 million years ago and the lower one is 1.9 million years old. What was the average sedimentation rate in cm/yr for the deposits between the ash beds for the column on the right? Do you think your calculated average is accurate for the actual rate of sedimentation? Explain.
The distance between the upper and lower ash beds is approximately 21m using the scale between the columns. Therefore, in 0.2 my, 21m of sediment were deposited. Rate of sedimentation=21m×102cmm×1200,000yr=0.011 cm/yr . This is probably not the average actual rate. It is obvious from the column on the left that either much less deposition or much more erosion took place at another location.
radioactive decay 73
The element changes to a new element.
relative geologic time scale 96
The geologic time scale as deduced from the geologic column, showing only relative time; that is, Silurian rocks are younger than those of the Ordovician but older than those designated Devonian.
rock
The geologic time scale is a dual scale based on ________ sequences with radiometric dates expressed as years before the present
stratification .
The layering in sedimentary rocks; layers less than 1 cm thick are laminations, whereas beds are thicker
stratification
The layering in sedimentary rocks; layers less than 1 cm thick are laminations, whereas beds are thicker.
stratification or bedding 109
The layering in sedimentary rocks; layers less than 1 cm thick are laminations, whereas beds are thicker.
dust 121
The main sources of deep-sea sediments are dust blown from continents or oceanic islands, volcanic ash, and shells of microorganisms that dwelled in the ocean's surface waters.
radiometric dating techniques 66
The most common method used for numerical ages, enabled geologists to determine numerical ages for rock units and geologic events
numerical dating
The most common way to numerically date a sedimentary rock is to ________ .numerically date an associated igneous or metamorphic rock
system 100
The most commonly used time-stratigraphic unit is the system, which is based on a stratotype consisting of rocks in the area where the system was first described.
different ages A volcanic ash fall was radiometrically dated using the potassium 40-argon 40 and rubidium 87-strontium 87 isotope pairs. The isotope pairs yielded distinctly different ages. What possible explanation could be offered as to why these two isotope pairs yielded different ages? What would you do to rectify the discrepancy in ages?
The most likely explanation for the discrepancy between the calculated ages is that the potassium 40-argon 40 system involves a gas as a daughter product. It is more likely that argon gas escaped from the system resulting in depletion in the daughter product. In order to confirm this hypothesis, the calculated dates using the two isotope systems should be compared. If argon gas has escaped from the system since the deposition of the ash, the date for the ash using the potassium 40-argon 40 method should be younger than the date calculated using the rubidium 87-strontium 87 isotope pair.
present is the key to the past"?
The natural laws that operate today operated precisely the same in the past
rounding
The process involving abrasion of sedimentary particles during transport so that their sharp edges and corners are smoothed off.
sorting
The process whereby sedimentary particles are selected by size during transport; deposits are poorly sorted to well sorted, depending on the range of particle sizes present.
relative age While doing geologic field work, you make the following observation: a thick layer of sandstone lies adjacent to a large body of granite, and the sandstone contains small pieces of the granite. How can you determine the relative ages of the sandstone and granite? What additional information at this rock exposure would support your conclusion?
The relative dating principle of inclusions reveals that the granite is older and the sandstone is younger. In order for the sandstone to contain inclusions of the granite, the granite had to be present first. Additional evidence supporting your conclusion could come from examining the sandstone for contact metamorphism. If the granite intruded after the sandstone, you would expect the underside of the sandstone to be baked.
progradation 118
The seaward (or lakeward) migration of a shoreline as a result of nearshore sedimentation.
Study the illustration at right and determine the relationship that exists between the tilted older rocks and the overlying horizontal rock layers. What sequence of events can you infer that led to the origin of these rocks and their present relationships?
The sedimentary rocks below the angular unconformity represent an early period of deposition and lithification. Afterward, deformation tilted these strata and erosion removed the upper portion of the section. A second period of deposition deposited the sediments above the angular unconformity. The limestone at the bottom of this upper section was probably deposited in a marine environment.
body fossils 92
The shells, teeth, bones, or (rarely) the soft parts of organisms preserved in the fossil record. (See fossil and trace fossil.)
global warming Given the current debate over global warming and the many possible short-term consequences for humans, try to visualize how the world might look in 10,000 or even 1 million years. To make your prediction, use what you have learned about plate tectonics and the direction and rate of movement of plates as well as what you know about how plate movement and global warming will affect ocean currents, weather patterns, weathering rates, and other factors. Do you think that such short-term changes can be extrapolated to long-term trends in trying to predict what Earth will be like using a geologic time perspective?
There are a variety of answers for this question. Using the principles of uniformitarianism, we can attempt to extrapolate conditions 10,000 years into the future. Many researchers attempt to do this in an attempt to show human effects on the Earth system. However, the conclusions are not yet definitive, so students may produce a variety of answers based upon their reading and understanding of current science news articles. It would be much more difficult to extrapolate one million years into the future, since student models will probably not include possible catastrophic events that we cannot accurately predict. Additionally, geologic time is so immense that it would not be representative to take conditions over a short-term period and apply them to a long-term period.
Trace fossils
Tracks, trails, tubes, and burrows Indications of organic activity. Intense activity results in bioturbation involving disruption of sediment
depositional environments Geologists are able to interpret depositional environments not only by the sedimentary structures they contain, but also by comparing ancient deposits with present-day deposits.
True
Unconformity 86
Unconformity is a general term that encompasses three distinct types of surfaces: disconformity, nonconformity, and angular unconformity.
radiometric dating 73
What is being measured in radiometric dating is the time of crystallization of a mineral containing an isotope;
period
What is the correct time unit for the Quaternary, Pennsylvanian, Ordovician and Cretaceous?
marine transgression
When a marine transgression occurs, the shoreline migrates ________ , and sea level ________ . landward; rises
continental shelf
Which depositional environment is considered to be marine?
sedimentary structures
Which feature of sedimentary rocks would be most useful to determine the depositional environment?
species-identical fossil assemblages
Which piece of evidence would best illustrate that rocks in widely separated areas were deposited at the same time? species-identical fossil assemblages
deep ocean basin pelagic clay and calcareous and siliceous oozes
Which rock type is predominant in deep ocean basins?
cities and time record
Why are cities unlikely to be preserved in the geologic record? They are situated on land, which tends to erode.
marine regression 90
Withdrawal of the sea from a continent or coastal area caused by emergence of the land with a resulting seaward migration of the shoreline.
Philip Henry Gosse, 85
a British naturalist, who in 1857 claimed that such features were created to look as if Earth had a long history. He proposed that Earth is only a few thousands of years old and that all features in rocks, including fossil bones and shells, that looked as though they formed long ago were created with that appearance.
sedimentary facies 88
a body of sediment with distinctive physical, chemical, and biological attributes.
Navajo Sandstone 124
a coastal dune deposit, wind blew from northeast
biostratigraphic unit;
a concurrent range zone is a type of biostratigraphic unit;
beta decay 73
a fast moving electron is emitted from a neutron in the nucleus, increases atomic number by 1
erosional surface 70
a gap in geologic record- Hutton
outcrops 86
a number of separate rock exposures, or what geologists call outcrops.
concordant 77
ages obtained from parent daughter ratios agree closely, reflects time of crystallization of the magma
radioactive decay 3 types 74
alpha, beta, electron capture
nonconformity 87
an erosion surface cut into plutonic rocks or metamorphic rocks that is overlain by sedimentary rocks is a nonconformity
Range zones
are most beneficial for identifying ________ . the total time species existed
Body fossils
are the actual remains of organisms, most of which are bones, teeth, and shells.
most reliable dates
are those obtained by using at least two different radioactive decay series in the same rock.
Desert dunes 81
are typically well sorted and exhibit large-scale cross-bedding.
Numerical dates for rocks
are usually obtained by determining how many half-lives of a radioactive parent element have elapsed since the sample originally crystallized.
fossil 92
as the remains or traces of prehistoric organisms preserved in rocks. They are by far most common in sedimentary rocks, but they may also be found in volcanic ash deposits and volcanic mudflows—but rarely in other igneous rocks or metamorphic rocks.
lateral gradation 88
as their composition and/or texture becomes increasingly different
relative geologic time scale 71
at first just relative dating then radiometric dating helped with numerical dating
bedding planes 86
bedding planes separate individual strata from one another (Figure 5.1) or the strata grade vertically from one rock type into another.
trace fossil a. shells b. teeth c. burrows d. bones e. jellyfish
burrows, tracks,
James Ussher, archbishop 67
calculated Earth's age at approximately 6,000 years, thus providing the idea that Earth was very young .Based on Old Testament genealogies, October 23, 4004
sedimentary rocks 75
cannot be radiometrically dated bc it would measure a mineral rather than time
ooids 123
carbonate barriers from in high energy area and may be reefs, skeletal particles or accumulations of spherical carbonate grains
cast—93
cast—a mold filled by minerals or sediment
chemical sedimentary rocks 109
chemical sedimentary rocks are composed of minerals extracted from solution by inorganic chemical processes or by the activities of organisms
color 108
color is a useful feature of some sedimentary rocks: red rocks likely were deposited on land, whereas greenish ones more likely were deposited in a marine environment. However, exceptions are numerous, so color must be used with caution.
atomic mass number 73
combined # of protons and neutrons
nucleus 73
composed of protons and neutrons with electrons encircling it
fossil record ________ are least likely to be preserved in the fossil record because ________ . a. Horse teeth; horses live in areas not conducive to fossilization b. Corals; they frequently undergo mineral replacement c. Brachiopods; they frequently undergo mineral replacement d. Jellyfish; they consist mainly of soft parts e. Clams; they are very scarce
d. Jellyfish; they consist mainly of soft parts
potassium-argon 77
date fine grained volcanic rocks
argon-argon dating 78
date fined grained volcanic rocks and metamorphic rocks
fission track dating 78
date samples 40k to 1.5 million years old
Madison Group 124
deposition in a shallow marine env.
principle of superposition 86
determine the correct relative ages of underformed strata by their position in a sequence; the oldest layer is at the bottom of the sequence with successively younger layers upward in the sequence
numerical age 73
determined by measuring the decay rate of unstable isotopes
mass spectrometer 75
determines parent daughter ratio
Lithostratigraphic correlation involves
determining the original lateral extent of a formation or group;
continental shelf 119
detrital marine env The area where the seafloor slopes gently seaward between a shoreline and the continental slope.
continental slope 121
detrital marine env The relatively steep part of the seafloor between the continental shelf and continental rise or an oceanic trench.
Correlation of biostratigraphic zones,
especially concurrent range zones, demonstrates that rocks in different areas are of the same relative age even though they may differ in composition.
Alpha decay involves the emission of two protons and two electrons.
false
Evaporites form in moist, humid marine environments. 123
false
Fossil evidence must be examined for accurate determinations of depositional environments.
false
Fossils are useful to a point in that they tell scientists what types of animals and plants existed in the past, but they can't provide climatological data.
false
Geologists expect the most noticeable changes that present-day activities will leave in the geologic record will be the relics of concrete and steel from cities, factories, and utilities.
false
Grand canyon Rocks in the Grand Canyon are in undisturbed horizontal sedimentary layers. Therefore, the youngest rocks are at the bottom of the sequence, and the oldest rocks are at the top.
false
Relative dating principles like superposition are rarely applied by stratigraphers, as they rely mostly on numerical dates.
false
Sedimentary structures are unique to specific environments. For example, ripple marks are only found in stream channels.
false
William Smith's principle of fossil succession states that fossil assemblages succeed one another through time in regular and determinable order.
false
carbonate rocks Unlike detrital sedimentary rocks, carbonate rocks do not have cross-bedding, ripple marks, or other sedimentary structures.
false
earths date Early naturalists used various scientific methods to date Earth, and erred by only +/- 2-3 million years.
false
fossils Soft-bodied organisms are less well represented in the fossil record because they were less common in ancient geologic history.
false
geologic time scale Originally, the geologic time scale was developed using numerical dates.
false
numerical age Lava flows and ash falls interbedded with sedimentary rocks are most useful for determining numerical ages.
false
uranium 238 The short half-life of uranium-238 allows it to be used to date ancient events.
false
Sedimentary structures form during lithification.
false- before lithification
Rocks in a formation can belong to multiple lithostratigraphic units.
fasle
geologic record 85
geologic record—that is, the evidence for physical and biological events preserved in rocks.
Cross-bedding;
geologists cross bedding to determine ancient current directions
William Smith 95
had found a way to determine the relative ages of rocks in different areas by matching up fossil assemblages
Paleogeography deals primarily with producing maps of ________ 125
historical distribution of the continents at various times
Uniformitarianism 70
holds that the laws of nature have been constant through time, and that the same processes operating today have also operated in the past, although not necessarily at the same rates.The uplift mechanism was thermal expansion
Amanz Gressly, 88
in 1838, was the first to use the term facies when he carefully traced sedimentary rocks in the Jura Mountains of Switzerland and noticed lateral changes such as sandstone grading into shale.
Lithostratigraphic correlation
involves demonstrating the original continuity of a rock unit over an area even though it may not now be continuous over this area.
stratigraphy 86
is a branch of geology concerned mostly with the composition, origin, age relationships, and geographic extent of layered, or stratified, rocks. It deals primarily with sedimentary rocks because almost all of them are stratified
Period
is a time unit
Lingula 102
is a type of brachiopod that meets the first two criteria, but its geologic range makes it of little use
Tree-ring dating 79
is another useful method for dating geologically recent events. The age of a tree can be determined by counting the growth rings in the lower part of the trunk. Each ring represents one year's growth, and the pattern of wide and narrow rings can be compared among trees to establish the exact year in which the rings were formed
range zone 102
is defined as the geologic range (total time of existence) of a particular fossil group, such as a species or a group of related species called a genus.
angular unconformity 87
is present if the strata below an erosion surface are inclined at some angle to the strata above
fossil record
is strongly biased toward those organisms that have durable skeletons and that lived where burial was likely.
William Smith
is the basis for the principle of fossil succession, which holds that fossil assemblages succeed one another through time in a predictable order.
banded iron formation, 108
is the main source of the world's iron ore and is important in the economies of Minnesota, Michigan, and Ontario, Canada
half-life 73
is the time it takes for one-half of the original, unstable radioactive parent element to decay to a new, more stable daughter element
Tree-ring dating
is useful for ________ dating of ________ events. numerical; recent
Evaporites are ________ common than limestone in the geological record, and consist mostly of ________
less, rock salt and rock gypsum
micrite 123
limestones are made up of gravel and sand sized grains and microcrystalline carbonate mud called micrite
marine transgression
marine transgression, a vertical sequence of facies results with offshore facies superposed over nearshore facies.
groups and supergroups 98
more inclusive units such as groups and supergroups
closed system 75
needed for accurate radiometric dates neither parent or daughter has been removed or added since mineralization
numerical ages for sedimentary rocks 103
numerical ages for sedimentary rocks and their contained fossils must be determined indirectly by dating associated igneous and metamorphic rocks
Nicolas Steno 71
oldest layer at bottom youngest on top, principle of superposition
Wave-formed ripple marks 110
on the other hand, tend to have symmetrical profiles and, as their name implies, are generated by the to-and-fro motion of waves
parent element 73
original unstable elem
Relative dating 66
places geologic events in sequential order as determined from their position in the geologic record.
preservation of body fossils 92
preservation of body fossils are (1) that an organism has a durable skeleton and (2) that it lives where burial in sediment is likely.
principle of fossil succession 95
principle of fossil succession A principle holding that fossils, especially groups or assemblages of fossils, succeed one another through time in a regular and determinable order.
principle of lateral continuity 88
principle of lateral continuity, meaning that layers of sediment or sedimentary rock extend outward in all directions until they terminate
electron capture 73
proton captures one electron from an electron shell converting the proton to a neutron
Numerical dating 66
provides specific dates for geologic rock units or events that are expressed in years before the present.
radioactivity
radioactivity provided a reliable geologic clock with which we now assign numerical ages to many ancient events.
correlation 101
refers to the act of matching up the same rock unit in areas where it is no longer continuous (lithostratigraphic correlation), or determining which events took place at the same time in different areas (time-stratigraphic correlation).
Texture 109
refers to the size, size distribution, shape, and arrangement of clasts in detrital sedimentary rocks
evaporate environments 123
rock salt and rock gypsum, are found in playa and saline lakes, but mostly formed in seas, found in Mediterranean sea, Michigan Ohio, New York, Gulf coast states western Canada
detrital sedimentary 109
rocks as those made up of detritus, that is, the solid particles of preexisting rocks such as gravel, sand, silt, and clay
blanket, or sheet, geometry 115
rocks cover hundreds or thousands of square kilometers but are perhaps only a few tens to hundreds of meters thick. That is, they are not very thick compared with their dimensions of length and width and thus have a blanket, or sheet, geometry
John Joly 69
salt in streams, 90 million years
sedimentary rocks 85m
sedimentary rocks because those rocks preserve evidence of surface processes (running water, glaciers, and wind) responsible for deposition.
Guide fossils
should be geographically ________ , with a relatively ________ geologic range. widespread; short
sill—86
sill—a sheetlike intrusive body—it is younger than the layers below it and younger than the layer immediately above it
Adam Sedgwick 95
studied rocks in northern Wales and described what he called the Cambrian System, and Sir Roderick Impey Murchison named the Silurian System in southern Wales
Charles Lapworth 96
suggested that the strata in the area of overlap be assigned to a new system, the Ordovician
Transgressions and regressions
take place in response to any tectonic movement that elevates the land with respect to sea level or causes subsidence (downward movement) of the land Another cause of transgressions and regressions is the volume of water in the oceans..
Transgressions and regressions 91
take place in response to any tectonic movement that elevates the land with respect to sea level or causes subsidence (downward movement) of the land.
Walther's law,
the facies in a conformable vertical sequence replace one another laterally
James Hutton- 70
the principle of uniformitarianism
time transgressive, 90
time transgressive, meaning that as the shoreline migrated landward, the facies were not deposited simultaneously over their entire geographic extent.
Cross-bedding allows geologists to determine ancient wind directions.
true
Fluvial systems, lakes, and deserts are classified as continental environments
true
Larger particles indicate higher energy depositional environments
true
Lord Kelvin's estimates of Earth's age were incorrect because he did not account for heat supplied to Earth by radioactive decay.
true
Marine transgressions and regressions are events where sea levels change due to tectonic or climatic events
true
Numerical dating was not used until the discovery of radioactive decay.
true
Paleogeographic maps show the distribution of land and seas at various times in Earth's history
true
Range zones indicate the total time a species existed in the geologic record.
true
Relative dating principles allow geologists to correlate rock units and interpret the geologic history of an area.
true
Sand is commonly found in fluvial deposits.
true
Sedimentary rocks are important because they preserve evidence of the surface processes responsible for deposition.
true
Sediments are originally deposited in horizontal layers.
true
Systems of rock units were established on the basis of superposition and fossil content.
true
Walther's law states that facies seen in a conformable vertical sequence will also replace one another laterally.
true
deep sea sediments The main sources of deep-sea sediments are dust blown from continents or oceanic islands, volcanic ash, and shells of microorganisms that lived in the ocean's surface waters.
true
fossils Dinosaur bones would be body fossils, and dinosaur tracks would be trace fossils.
true
fossils Guide fossils are confined to a narrow range of time.
true
metamorphosed rock After a rock is metamorphosed, it is impossible to determine an accurate date for when the original rock formed.
true
sedimentary rates One reason sedimentation rates cannot be used to calculate the age of Earth is variations in erosion rates.
true
varve consists of a pair of dark and light colored laminations that represent deposition in lakes produced by glacial processes over a one-year period.
true
long-lived radioactive isotope pairs
uranium 238-lead 206, uranium 235-lead 207, thorium 232-lead 208, rubidium 87-strontium 87, and potassium 40-argon 40
James Ussher
used an unscientific method to determine the age of Earth. In the 1600s, ________ analyzed the Old Testament and determined that Earth was formed on October 23, 4004 B.C.
isotopes 73
variable forms of the same element
Superposition and fossil succession
were used to piece together a composite geologic column, which is the basis for the relative geologic time scale.
La Brea Tar Pits in Los Angeles, California, 93
where hundreds of fossil mammals, birds, insects, and other organisms have been recovered