Geologic Time: Concept and Principles
principle of fossil succession
a principle holding that fossils, and especially groups or assemblages of fossils, succeed one another through time in a regular and predictable order.
describe unconformity
a surface of erosion with younger strata above and older below
sill (define)
a tabular or sheetlike tabular pluton
excellent marker beds for dating sedimentary rocks
ashfall and lava flow are excellent marker beds or time equivalent surfaces for ..
Carbon 14 is rapidly
assimilated into the carbon cycle
Lord Kelvin
assumed that Earth is gradual losing heat and that, by measuring this heat loss, he could determine its age.
lava flow
baking zone at bottom of flow
sill
baking zones on both sides of
extrapolate
extend the application of (a method or conclusion, especially one based on statistics) to an unknown situation by assuming that existing trends will continue or similar methods will be applicable.
electron capture
is when a proton captures an a electron from an electron shell and thereby converts to a neutron, resulting in the loss of one atomic number, but not changing the atomic mass number.
that which cuts something is older than that which it cuts
principle of cross cutting relations
sediment was laid down, to- gently non-sloping surfaces due to gravity settling
principle of original horizontality
basis of relative age dating of fossils and rock strata
principle of superposition
Absolute dating
provides specific dates for rock units or events expressed in years before the present.
Charles Lyell
published a landmark book, principles of Geology, in which he championed Hutton's concept of uniformitarianism as the guiding principle of geology
we have no way of knowing how many years ago these events occurred unless we can obtain _ for the igneous rocks.
radiometric dates establish absolute ages.
principle of superposition is the basis for..
relative age determinations of strata and their contained fossils
principle of lateral continuity
states that sediment extends laterally in all directions until it thins and pinches out or terminates against the edge of the depositional basin
can be used to demonstrate the relative ages of the fossil assemblages.
superposition
When conducting accurate radiometric dating on igneous rocks, the time being measured is _ and not _
the time being measured is the time of crystallization of the mineral that contains the radioactive atoms, and tnot the formation of the radioactive atoms themselves.
Most commonly used isotope pairs
uranium-lead and thorium-lead series.
6 fundamental principles of relative dating. Memorize definitions for all
1. Principle of superposition- relative age determination, oldest youngest, 2. Principle of original horizontality- water, gravity, deposition, tiltation after deposition 3. Principle of lateral continuity- all directions until terminated 4. Principle of cross-cutting relationships- fault younger than rocks intruded or displaced 5. principle of inclusions- inclusions older than rock 6. principle of fossil succession
Nicolas steno
1638-1686. steno observed that when streams flood, they spread out across their floodplains and deposit layers of sediment that bury organisms dwelling on the floodplain.
James Hutton
1726-1797 considered to be the father of modern geology.
William Smith
1769-1839 was an English civil engineer involved surveying and building canals in southern England, independently recognized the principle of superposition by reasoning that the fossils at the bottom of a sequence of a strata are older than those at the top of the sequence.
principle of superposition
A principle holding that in a vertical sequence of undeformed sedimentary rocks, the relative ages of the rocks can be determined by their position in the sequence- oldest at the bottom followed by successively younger layers.
principle of uniformitarianism
A principle holding that we can interpret past events by understanding present day processes, based on the idea that natural processes have always operated in the same way.
disconformity
A type of unconformity. It is a surface of erosion or nondeposition separating younger from older rocks, both of which are parallel with one another.
Colorado Plateau
About 90% of the area is drained by the Colorado River and its main tributaries: This region provides a record of events occurring over approximately two billion years.
principle of original horizontality
According to this principle, sediments are deposited in horizontal or nearly horizontal layers.
Law of Cross-Cutting
Any feature that cuts across a rock or body of sediments must be younger than the rock or sediment that it cuts across
when an organisms dies
Carbon 14 decays back to nitrogen by a single beta decay step.
Carbon 14 formation
Carbon 14 is constantly formed in the upper atmosphere when cosmic rays-high energy particles- (mostly protons), strike the atoms of the upper atmospheric gases, splitting their nuclei into protons and neutrons.
Carbon 14 formation chart
Cosmic radiation- Nitrogen 14 neutron capture= Carbon 14 ; C14 is absorbed along with C 12 13 into the tissue of living organisms @ constant ratio- Organism dies C14 converted to N
correlation
Demonstration of the physical continuity of rock units or biostratigraphic units, or demonstration of time equivalence as in time-stratigraphic correlation
correlation
Demonstration of the physical continuity of rock units or biostratigraphic units, or demonstration of time equivalence as in time-stratigraphic correlation.
guide fossils or index fossils
Fossils that are easily identified, are geographically widespread and existed for a rather short interval of geologic time.
why angular conformity is the easiest type of unconformity to recognize and understand
It consists of tilted or folded sedimentary rocks beneath younger, more horizontal strata, creating an angle across the unconformity.
It is important to distinguish between a nonconformity and an intrusive contact because
It is important to distinguish between _ and _ because they represent different sequences of events.
principle of igneous intrusion and who discovered
James Hutton recognized that an igneous intrusion or fault must be younger than the rocks it intrudes or displaces
In an angular unconformity, the erosional surface between the _ and the _ meant that a significant gap existed in the geologic record.
The _ between the older tilted rocks and the younger flat lying strata meant that a _ existed in the geologic record
principle of inclusions
This principle holds that inclusions, or fragments of one rock contained within a layer of another, are older than the rock layer itself.
Nitrogen to C 14
When a neutron strikes the nucleus of a nitrogen atom, (atomic number 7, mass number 14), it may be absorbed into the nucleus and a proton emitted. atomic number decreases by 1, atomic mass stays same. atomic number changed = new element!
unconformity
a break in the geologic record represented by an erosional surface separating younger strata from older rocks
beta decay
a fast moving electron is emitted from a neutron in the nucleus, changing that neutron to a proton and consequently increasing the atomic number by one with no resultant atomic mass number change
concordant
ages obtained from each parent daughter ratio should agree closely, thus reflecting the time of crystallization of magma; disconcordant is when ages do not agree, and other samples must be taken and ratios measured to see which, if either, date is correct.
pluton
an intrusive igneous body that forms when magma cools and crystallizes within the crust, such as a batholith or sill.
this surface separating the strata and represents a major break in our record of geologic time.
an unconformity
layers below the unconformity are not parallel to it, but layers above it are
angular unconformity
To obtain accurate radiometric dates, geologist must be sure they are dealing with a _
closed system, meaning neither parent nor daughter atoms have been added or removed from the system since crystallization.
Because most fossils have fairly long geologic ranges, geologist construct _ to determine the age of the sedimentary rocks containing the fossils.
concurrent range zones to determine ..
time stratigraphic units
consist of rocks deposited during a particular interval of geologic time. System is the basic unit.
time units
designations for certain parts of geologic time. period is the basic unit.
according to the principle of cross-cutting relationships, it _ must be younger than all of the rocks it intrudes
dike
frequently resemble ordinary bedding plane
disconformity
layers below the unconformity are parallel to it, as are the layers above
disconformity
Geophysical instruements may be lowered down the drill hole to record such rock properties as
electrical resistivity and radioactivity, providing a record or well log of rocks penetrated
to correlate rock units over a large area or to correlate age-equivalent units of different composition, _ and _ must be used
fossils and principle of fossil succession are generally used to correlate _ over a large area.
biozone
fundamental biostratigraphic unit
Sedimentary radiometric dates can be obtained when _ is present; it forms when sedimentary rock forms.
glauconite useful because
in the case of an intrusion, the _ are younger
igneous rocks
the most accurate radiometric dates are obtained from _
igneous rocks
excellent indicators of relative age
inclusions, older than the rock they are contained in are excellent indicators of
stratigraphy
is a branch of geology which studies rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
angular unconformity
is an erosional surface on tilted or folded strata over which younger strata were deposited.
nonconformity
is an unconformity between younger sedimentary rocks and subjacent metamorphic or igneous rocks.
Carbon-14 dating technique
is based on the ratio of carbon 14 to 12 and is generally used to date formerly living material.
relative dating
is placing geologic events in a sequential order as determined from their position in the geologic record.
half-life
is the time it takes f or half of the atoms of the original unstable parent element to decay to atoms of a new, more stable daughter element.
That which is at the top, is relatively younger than materials below (based upon gravity settling principles)
law of superposition
parent-daughter ratio is determined by,
mass spectrometer is an instrument that measures the proportions of atoms of different masses.
larger and smaller subdivisions of formations
members and beds; groups and supergroups
mineral crystallizing in _ will contain radioactive parent atoms but no _
mineral crystallizing in cooling magma will contain _ but no stable daughter atoms.
Major John Wesley Powell
no one has contributed to the understanding of the grand Canyon as _
sedimentary layers are in contact with an igneous or metamorphic rock body
nonconformity
radioactive decay occurs at a _ rate rather than a _ rate
occurs at a geometric rate rather than a linear rate.
buried lava is _ than the rocks above it, whereas a sill, resulting from later igneous intrusion is _ than all of the beds below it and _ than the immediately overlying bed as well.
older, younger, younger
basic time units
period; two or more periods =era, two or more eras = eon.. periods also consist of shorter designations such as epoch and age.
biostratigraphic unit
An association of sedimentary rocks defined by its fossil content.
unconformity significance: Unconformities represent
represent missing time, periods of ersion and/or non-deposition of sediment
energy pulses, such as those from explosions, travel through rocks at a velocity determined by _
rock density determines
well cuttings under microscope reveal ...
rock type, porosity, permeability, presence of oil stains and variety of micro-fossils.
in the case of erosion the _ are younger
sedimentary rocks
Criteria used to correlate units are
similarity of rock type, position in a sequence, and key beds
a sequence of sedimentary rock layers that is _ from the horizontal must have been tilted after deposition and lithification
steeply inclined
difficulty of recognizing unconformity in a disconformity
strata exhibit parallel bedding or stratification.
fission track dating
the absolute dating process in which small linear tracks (fission tracks) resulting from aplha decay are counted in mineral crystals.
formation
the basic lithostratigraphic unit is the formation, which is a mappable body of rock with distinctive upper and lower boundaries.
hiatus
the interval of geologic time not represented by strata
batholith
the largest of all plutons, must have at least 100km2 of surface area and most are far larger.
stratigraphy
the study of the composition, origin, areal distribution and age relationships of layered rocks.
alpha decay
two protons and two neutrons are emitted from the nucleus, resulting in the loss of two atomic numbers and four atomic mass numbers.
lithostratigraphic
units defined by the physical attributes of the rocks, such as rock type, (for example sandstone or limestone), with no consideration of time of origin.
key beds are
units, such as coal beds or volcanic ash layers, that are sufficiently distinctive to allow identification of the same unit in different areas
principle of original horizontality
Sedimentary layers (strata) and lava flows were originally deposited as relatively horizontal sheets, like a layer cake. If they are no longer horizontal or flat, it is because they have been displaced by subsequent movements of Earth's crust.
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.
