geo chapter 10

two different methods of correlating formations.

Lithologic correlation is the use of physical and chemical characteristics of rocks to determine that spatially isolated strata were once continuous (through original lateral continuity). Fossil correlation uses fossils with known stratigraphic ranges to determine the approximate temporal equivalence of two bodies of rock (which may be of disparate lithologies). Because a single locality does not provide strata that span in age throughout Earth's history, fossil correlation was required to provide time equivalence for units that are geographically isolated. With successful correlation, rock sequences from all parts of the world could be brought together to form an accurate sequence of relative time: the geologic column. A stratigraphic formation is a recognizable layer of a specific (usually sedimentary) rock type or set of types that were deposited within a certain time interval and can be traced over a broad region.

Describe the Principles that allow us to determine the relative ages of geological events.

The Principle of Uniformitarianism- Physical processes we observe operating today also operated in the past (at roughly comparable rates). (Physical processes do not occur at "exactly" the same time.) Superposition- Sequence of sedimentary rock layers. (Layer below = older than one above.) Original horizontality- When layers of sediment are originally deposited, they are fairly horizontal. (When we see folds & tilted beds, we are seeing the consequences of deformation that postdates deposition) Original Continuity - Sediments generally accumulate in continuous sheets.Cross-Cutting Relations -If one geologic feature cuts across another, the feature that has been cut is older. Inclusions- If an igneous intrusion contains fragments of another rock, the fragments must be older than the intrusion. (The fragments in sedimentary layers are inclusions. The rock containing the inclusion must be younger than the inclusion.) Baked or Chilled Contacts- An igneous intrusion "bakes" (metamorphoses) surrounding rocks. The rock that has been baked must be older than the intrusion.

What does the age of an igneous rock mean?

To determine relative ages of rocks geologists also study Extrusions and intrusions of igneous rock, faults and gaps in the geological record.

stratigraphic column

A cross-section diagram of a sequence of strata summarizing information about the sequence.

geologic map

A map showing the distribution of rock units and structures across a region.

uniformitarianism

Charles Lyell's idea that geologic processes have not changed throughout Earth's history.

isotope

Atoms of the same element that have different numbers of neutrons

stratigraphic group

Several adjacent stratigraphic formations in a succession.

fossil assemblage

a group of fossil species found in a specific sequence of sedimentary rock

daughter atom

the product atom from the radioactive decay of a parent atom

How does an unconformity develop?

-Strata are not deposited continuously at a location. Unconformity is an interval of non-deposition and/ or erosion .-An unconformity forms whenever the land surface does not receive & accumulate sediment.Angular :-forms where rocks were either folded or tilted by faulting before being uplifted and eroded.

how to calculate half life

1. First calculate the number of half-lives that will have passed in the allotted time. 2. Divide the mass of the sample in half for each half-live that has passed.

What is the age of the oldest rocks on the Earth?

4.03 Ga, 4.57 Ga.

unconformity

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

Cambrian explosion

A burst of evolutionary origins when most of the major body plans of animals appeared in a relatively brief time in geologic history; recorded in the fossil record about 545 to 525 million years ago.

stratigraphic formation

A recognizable layer of a specific sedimentary rock type or set of rock types, deposited during a certain time interval, that can be traced over a broad region.

geologic time scale

A record of the geologic events and life forms in Earth's history.

Nonconformity

A type of unconformity in which layered sedimentary rocks lie on an erosion surface cut into igneous or metamorphic rocks

nonconformity

A type of unconformity in which layered sedimentary rocks lie on an erosion surface cut into igneous or metamorphic rocks

geologic time

Earth's history as revealed by layers of rock

Discomformity

Exists where part of a sequence of parallel rock layers are missing. Can happen when sedimentation stops and layers get eroded away

Precambrian

Name for the time in earths early history that accounts for ninety percent of earth's time, but only cellular organisms lived.

Contrast numerical age with relative age.

Numerical Age: the age of a feature given in years Relative Age: the age of one feature with respect to another.

How was correlation used to develop the geologic column?

Over broad areas fossil correlation is used, because sources of sediments and depositional environments may change from one location to another. Sediment may look very different, but can still be the same age.

What happens during radioactive decay,

Parent isotopes turn into daughter isotopes.

stratigraphic

Sequences of rock layers. Correlating the sequences of rock layers in different areas enables scientists to trace a particular geologic event to a particular period.

relative age

The age of a rock compared to the ages of rock layers

closure temperature

The temperature at which parent and daughter isotopes can no longer escape from a mineral, so the ratio of parents to daughters can be used for isotopic dating.

How are growth rings and ice layering useful in determining the ages of geologic events?

They can help determine how long an organism survived. Ice cores can keep track of what the climate was like, and the temperatures.

What is the current estimate of the numerical age of the Earth? Why is there a difference?

They come off of different planets, so they can e older than Earth rocks

Distinguish among the three kinds of unconformities.

Three kinds of unconformities are recognized: nonconformities, disconformities, and angular unconformities.

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

era

a long and distinct period of history with a particular feature or characteristic.

epoch

a period of time in history or a person's life, typically one marked by notable events or particular characteristics.

disconformity

a type of unconformity in which the sedimentary layers above and below the unconformity are parallel

How does the principle of fossil succession help determine relative ages?

allows geologists to arrange fossil species in a progression from older at the bottom to younger at the top, that the range in which a particular species occurs may overlap with the range of other species, and once a species becomes extinct, it does not reappear higher in the sequence. (With the Principle, we can define the relative ages of strata by looking at fossils.)

period of time

an amount of time

geologic column

an ordered arrangement of rock layers that is based on the relative ages of the rocks and in which the oldest rocks are at the bottom

angular unconformity

an unconformity in which younger sediment or sedimentary rocks rest on the eroded surface of tilted or folded older rocks

Why can't we date sedimentary rocks directly?

dating would only give the time that minerals in a sedimentary rock first crystallized as a metamorphic/igneous rock, not the time they were deposited/ solidified.

What does the age of a metamorphic rock mean?

often is defined by the time at which these changes - called "metamorphism" - took place. Metamorphism generally takes a very long time - sometimes millions of years.

correlation

refers to the methods by which the age relationship between various strata of Earth's crust is established. ... Correlation is an important geological technique because it provides information with regard to changes that have taken place at various times in Earth history.

How do we assign numerical ages to intervals on the geologic column to produce a geologic time scale?

researchers use some type of clock to determine the date it was formed. Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events.

formation

something that is made or formed

geochronology

study of the ages of geologic events

numerical age

the age of a geologic feature given in years

parent atom

the atom that undergoes radioactive decay in a nuclear reaction

geologic contact

the boundary surface between two formations

eon

the largest division of geologic time

radioactive decay

the process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element

radiometric dating

the process of measuring the absolute age of geologic material by measuring the concentrations of radioactive isotopes and their decay products

What does half-life mean?

the time taken for the radioactivity of a specified isotope to fall to half its original value.

half-life

the time taken for the radioactivity of a specified isotope to fall to half its original value.

isotopic dating

the use of naturally occurring radioactive elements to determine the ages of rocks

How to Obtain an Isotopic Date

An isotope is an atom of a chemical element whose nucleus has the same atomic number but which has a different atomic weight. What this means is that an isotope can be thought of as a different version of a standard element on the periodic table. Due to its difference though, isotopes are atomically unstable, and prone to change into other, more stable elements. This process is referred to as radioactive decay. The rate at which radioactive decay occurs is that particular isotope's half-life.