Geologic Time

What are the 3 eras of the Phanerozoic eon?

Paleozoic era Mesozoic era Cenozoic era

Define the principle of fossil succession and the theory of evolution

Principle of fossil succession - specific groups of fossils occur in particular rock layers. Each layer differs, and changes in life forms can be observed from layer to layer Theory of evolution - life forms have evolved, or changed over time through natural selection

Compare the techniques of relative dating to those of radioactive decay dating

Relative dating correlates rock layers based on their positions. There is no computation involved; it is all based on observation in the field. Numerical dating uses radiometric methods that involve calculations. This is all done in a laboratory

What is Precambrian time?

The long span of time from Earth's formation about 4.56 billion years ago to the beginning of the Cambrian period about 540 millions years ago

What happens to atoms that are radioactive?

The nuclei decay or react by emitting alpha or beta particles

What is uniformitarianism?

The same laws that operated in the past still operate today

How did the acceptance of uniformitarianism change the way scientists viewed Earth?

They learned that Earth was very old, that Earth's landscape is always changing, and that the processes they observed had also been at work in the past

Angular unconformity

This type of UNCONFORMITY has rock layers that are tilted, and younger sediment layers are deposited horizontally on top of the eroded and tilted layers.

carbon film fossils

a carbon-rich remnant of a plant or animal that appears like a flat two-dimensional imprint

Which unit of geologic time is the greatest span of time? a. era b. eon c. period d. epoch

b. eon

The periods of the Cenozoic era are divided into smaller units known as?

epochs

relative dating

identifies rock units that formed first, second, third, etc. In relative dating, geologists follow several principles: the law of superposition, the principle of original horizontality, and the principle of cross-cutting relationships

what is a trace fossil?

indirect evidence of prehisotric life such as an animal track or burrow

Why do all organisms contain a small amount of carbon-14?

it cirulates in the atmosphere and is absorbed by living matter

radiometric dating

process wherein scientists measure the ratio between the radioactive parent isotope and the daughter products in a sample to be dated. The older the sample, the more daughter product it contains

Describe the conditions that favor the formation of fossils

quick burial, possession of hard parts

Eons

represent the longest intervals of geologic time. They are divided into eras, which are subdivided into periods, which are further subdivided into epochs

What is a half-life?

the amount of time necessary for one-half of the nuclei in a sample to decay to its stable isotope

What is compared when dating with carbon-14?

the ratio of carbon-14 to carbon-12

How would heat, pressure, and melting affect fossils?

they would destroy fossils

What is an adaptation?

trait that enables an organism to survive and reproduce

To what type of rock can the law of superposition and the principle of original horizontality be best applied?

undisturbed sedimentary rocks

What occurs during radioactive decay?

unstable atomic nuclei spontaneously break apart, or decay, releasing energy also known as radioactivity

radiocarbon dating

Dates organic material using the stable and known rate of decay of Carbon 14, and is only useful for materials less than 45,000 years old.

List the geologic organization of time:

Eon Era Period Epoch

Geologists divide Earth's history in four ____?

Eons

What subdivisions make up the geologic time scale?

Eons, eras, periods and epochs

T/F - Sediments are deposited through the process of weathering

False

List the four eons - oldest to youngest

Hadean Archean Preterozoic Phanerozoic

A geologist finds layers of sedimentary rock immediately above an eroded anticline. What type of unconformity is this?

It is an angular unconformity because sedimentary rock layers overlie the eroded surface of tilted sedimentary layers in the anticline

List three main ideas of the science of geology

1. the rock record provides evidence of Earth's history 2. Processes at work today were also at work in the past; 3. Earth is very old and has changed over time

petrified fossil

A fossil formed when minerals replace all or part of an organism

Who was the scientist that formulated the theory of uniformitarianism? a. Charles Darwin b. James Hutton c. William Smith d. Louis Agassiz

B. James Hutton

After Precambrian time (consisting of what 3 eons?) came the Phanerozoic eon, which is made up of what eras?

(Hadean, Archean, Proterozoic) Three eras make up the Phanerozoic eon: Paleozoic, Mesozoic and Cenozoic

Traditionally, geologists divide the Cenozoic era into these 2 periods?

Tertiary and Quarternary

Inclusions

A piece of one rock unit contained within another. Inclusions are used in relative dating. The rock mass adjacent to the one containing the inclusion must have been there first in order to provide the fragment.

principle of cross-cutting relationships

A principle stating that geologic features that cut across a rock must be younger than the rock they cut through

What is the role of isotopes in radiometric dating?

A radioactive isotope decays into its stable daughter product at a constant rate. The time it takes for half of the isotope in a sample to decay is known as the half-life of the isotope. A radioactive isotope can thus serve as a "clock" that can be used to determine the age of a substance.

Why are there more subdivisions of the time scale for the Phanerozoic eon than for earlier eons?

There is more information in the rock and fossil record about the geologic events of this eon than there is about previous eons

Why does the law of superposition apply primarily to sedimentary rock?

These are the only rocks that are laid down horizontally, except for igneous rock extrusive lava flows, ash flows, and falls

How old is Earth?

4.6 billion years

What is a half-life?

A common way of expressing the rate of radioactive decay. It is the amount of time necessary for one-half of the nuclei in a sample to decay to its stable isotope

How is a dike formed?

A dike is formed when magma is injected into fractures in rock

Using relative dating methods, which of the following are scientists able to do? a. Identify the order in which rock units formed. b. Assign a numerical date to each rock layer studied c. Determine the age of the fossils within each layer d. Identify what rock types are present

A. Identify the order in which rock units formed

What is the geologic column?

All the rocks in the rock record arranged in order of relative age

Why is the rock record for any given location on Earth incomplete?

Because erosion leads to the formation of unconformities representing breaks in the rock record

How are petrified fossils and carbon fossils similar? Different?

Both preserve the shape of an organism, but in different ways. In a petrified fossil, minerals replace the matter that made up the organism, forming a 3-dimensional copy. In a carbon film fossil, traces of carbon from the organism may remain. Or, the carbon may be lost, leaving only a delicate impression

Why is the ratio of carbon-14 to carbon-12 constant during an organism's lifetime?

Carbon-14 is continually replaced

What occurred during the first 3 eons of earth's history?

Earth formed, the atmosphere and oceans deveoped, and early life evolved

What did Darwin propose as the mechanism of evolution?

Evolution occurred through the process of natural selection. In natural selection, organisms that are well adated to their environment survive and pass on their traits to the generations that follow. Those that are not well adapted become extinct

principle of fossil succession

Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.

About 88% of geologic time is made up of the first three eons known as ___, ___, and ____

Hadean, Archaean, and Proterozoic

Describe the formation of igneous, metamorphic, and sedimentary rocks

Igneous rocks form by melting and solidification Metamorphic rocks form by heat and pressure Sedimentary rocks form by compaction and cementation

What is the potassium-argon method of radiometric dating

In potassium-argon dating, scientists measure the ratio of radioactive potassium-40 to argon (its decay product) in a sample.

Definitions of inclusion, unconformity and correlation

Inclusion - piece of one kind of rock contained in another rock; Unconformity - surface that represents a break in the rock record; Correlation - way of determining that two separate rock units are the same

What major life forms evolved during each of the eras of the Phanerozoic eon?

Paleozoic ("ancient life") - most major groups of organisms evolved (vertebrates and plants) Mesozoic ("middle life") - forest developed on land, many reptiles, first mammals Cenozoic ("recent life") - mammals, birds, flowering plants

To which era does each of the following periods belong: Ordovician, Tertiary, Permian, Triassic?

Paleozoic era - Ordovician period Cenozoic era - Tertiary period Paleozoic era - Permian period Mesozoic era - Triassic period

Each era is divided into ________?

Periods; Different geologic events, environmental conditions, and life forms characterize each period

List the different types of fossils

Petrified fossils, molds and cases, carbon films, preserved remains, and trace fossils

The Hadean, Archaean and Proerozoic eons are also known as "_______ time"

Precambrian time

Why can't radiometric dating be used with accuracy on metamorphic rocks?

Radioactive decay begins as soon as a radioactive mineral is formed. When such a mineral is metamorphosed, it can lose its daughter products. Dating this mineral would result in a younger date than is actually correct.

Explain how radioactivity and radiometric dating are related

Radiometric dating makes use of the known decay rates of some radioactive isotopes

Describe radiocarbon dating

Scientists measure the ratio between the amount of radioactive isotope atoms and daughter product atoms in a substance. The ratio reflects how much time has passed since the substance formed

How do geologists use radiometric dating to date sedimentary rock layers indirectly?

Sedimentary rock layers that lie between two igneous rock units must have an age that lies somewhere between the absolute ages of the two igenous rock units. The latter can be dated using radiometric methods

What type of rock likely contains most fossils?

Sedimentary rock likely contains most fossils because it has not been affected by melting, heat, or pressure

molds and casts

Shells often leave empty cavities called molds within hardened sediment. When sand or mud fills a mold and hardens, a natural cast forms.

Why can certain fossils, such as corals, be used to indicate former water temperature?

Some corals live in warm water today. Using the theory of uniformitarianism, scientists can infer that some fossil corals must have lived in warm waters too

Why can't carbon-14 be used to date material that is older than 75,000 years?

The decay rate of carbon-14 is very short

Explain why the time periods of the Paleozoic and Mesozoic are subdivided into early, middle, and late instead of named epochs, as they are in the Cenozoic

The fossil record in the Paleozoic and Mesozoic, although extensive, is not sufficient to subdivide periods based on the percentage of organisms presented

law of superposition

The geologic principle that states that in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it.

What is the geologic time scale?

The geologic time scale divides Earth's history into units that each represent specific amounts of time; represents both geologic events and major developments in the evolution of life

How have the processes that affect Earth's surface changed through time?

The rate at which they occur and the size or scope of their occurrence have changed

List and briefly describe Steno's principles

The law of superposition states that in a sequence of undisturbed rock layers, the oldest layer is on the bottom; the upper layers are progressively younger. The principal of original horizontality states that sedimentary rocks are generally deposited horizontally. The principal of cross-cutting relationships states that features such as faults and intrusions are younger than the features they cut across

How are the law of superposition and the principle of fossil succession related?

The law of superposition states that the oldest rocks are at the bottom of a sequence of rocks. The principle of fossil succession states that layers of rock contain specific fossils that change from layer to layer. Thus, the oldest fossils would be in the oldest layer of rock, which in turn would be at the bottom

What two conditions increase an organism's chance of becoming a fossil?

The presence of hard parts and rapid burial after death

principle of horizontality

The principle that states that layers of sediment are generally deposited in a horizontal or nearly horizontal position.

A geologist wants to use potassium-argon dating to date a granite rock found on the surface. What is a possible source of inaccuracy in dating the rock?

The rock may have been contaminated by other sources of potassium-40 on the surface. For radiometric dating to be accurate, a sample must remain in a closed system

How are cross-cutting relationships used in relative dating?

The rock that is cut or broken by an intrusion or fault is older

What can you infer about the age of sedimentary rock layers relative to the age of a sill intruded into those layers?

The sedimentary rock layers are older than the sill

Describe how a clam might become a fossil

The shell falls to the bottom and is buried under mud and sediment. Mineral-rich water soaks into the pore spaces, leaving minerals behind. Over time, the shell becomes incorporated into the mud. As the mud turns to rock, the shell becomes a fossil

Describe two ways that geologists can use fossils to interpret Earth's history

To correlate rock layers and reconstruct past environments

List and briefy describe 3 different types of fossils

Unaltered remains are mummified or frozen animals Altered remains are shells that have been filled in with minerals such as quartz Carbon films form when liquid and gaseous matter is driven off by pressure

How do unconformities form?

When deposition stops and/or surfaces are eroded, thus leaving a gap in the rock record

Sill C was intruded after deposition of layer D, but before layers A through D were uplifted. You can tell by noting that sill C and layers A, B, and D are eroded at the top, indicating that they were uplifted at the same time

When was sill C intruded? Explain

C

When were rock layers A through D uplifted and tilted? a. after deposition of layer G b. after deposition of layer F and before deposition of layer G c. after deposition of layer D and before deposition of layer E d. after deposition of layer A and before deposition of layer G

D

Which choice correctly lists the order in which rock layers A through D formed? a. A, B, C, D b. D, C, B, A c. B, C, D, A d. A, B, D, C

Dike B is older because additional sediments have deposited above it.

Which dike is older, A or B

Fault A is older because it does not reach all the way to the top layer. Fault B does reach the top so it formed after the final layers of rock were deposited.

Which fault is older, A or B?

Unconformities

a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

index fossil

a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found

Which of Steno's principles states that most layers of sediments are deposited in flat-lying layers? a. original horizontality b. cross-cutting relationships c. fossil succession d. superposition

a. original horizontality

Which atoic particles have a positive electrical charge a. protons b. neutrons c. electrons d. all of the above

a. protons

What principle states that the physical, chemical and biological laws that operate today have also operated in the geologic past? a. uniformitarianism b. theory of evolution c. principle of original horizontality d. law of superposition

a. uniformitarianism

What percentage of geologic time is made up of the Cenozoic era?

about 1.4%

What name is given to layers of tilted rocks that are overlain by younger, more flat-lying rock layers? a. disconformity b. angular unconformity c. nonconformity d. fault

b. angular unconformity

What is the name of the process that matches up rocks of similar ages in different regions? a. indexing b. correlation c. succession d. superposition

b. correlation

What are atoms with the same atomic number but different mass numbers called? a. protons b. isotopes c. ions d. nucleotides

b. isotopes

Which atomic particles orbit the nucleus? a. protons b. neutrons c. electrons d. elements

c. electrons

What are remains or traces of prehistoric life called? a. indicators b. replicas c. fossils d. fissures

c. fossils

About how old is Earth? a. 4,000 years b. 4.0 million years c. 5.8 million years d. 4.56 billion years

d. 4.56 billion years

What name is given to pieces of rock that are contained within another younger rock? a. intrusions b. interbeds c. hosts d. inclusions

d. inclusions

What name is given to fossils that are widespread geographically, are abundant in number, and are limited to a short span of time? a. key b. succeeding c. relative d. index

d. index

What is the name of the process during which atomic nuclei decay? a. fusion b. correlation c. nucleation d. radioactivity

d. radioactivity

At what point does the amount of carbon-14 in an organism begin to decrease?

when the organism dies and starts to decay