geologic time Flashcards
James Hutton
recognized the immensity of Earth history and the importance of time as a component in all geologic processes in the late 18th century
Sir Charles Lyell
demonstrated that Earth had experienced many episodes of mountain building and erosion, which must have required great spans of geologic time in the 19th century
Cambrian time was how many million years ago
540
numerical date
the number of years that have passed since an event occurred
relative date
the chronological order of events, determined by placing rocks and structures in their proper sequence or order
Nicolas Steno (1638-1686)
a Danish anatomist, geologist, and priest, was the first to recognize a sequence of historical events in an outcrop of sedimentary rock layers`
Principle of Superposition
A principle which states that in any undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below
Principle of Original Horizontality
A principle by which layers of sediment are generally deposited in a horizontal or nearly horizontal position
Principle of Lateral Continuity
A principle which states that sedimentary beds originate as continuous layers that extend in all directions until they grade into a different type of sediment or thin out at the edge of a sedimentary basin
Principle of Cross-Cutting Relationships
the geologic principle which states that geologic features that cut across rocks must form after the rocks they cut through
Principle of Inclusion
The principle which states that a rock mass adjacent to one containing inclusions must have been there first in order to provide the rock fragments and is therefore the older rock mass
conformable
referring to rock layers that were deposited without interruption
unconformity
a surface that represents a break in the rock record, caused by erosion and nondeposition
angular unconformity
an unconformity in which the older strata dip at an angle different from that of the younger beds
disconformity
a type of unconformity in which the beds above and below are parallel
nonconformity
an unconformity in which older metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata
fossils
the remains or traces of organisms preserved from the geologic past
paleontology
the systematic study of fossils and the history of life on earth
permineralization
when mineral-rich groundwater permeates porous tissue such as bone or wood, minerals precipitate out of solution and fill pores and empty spaces
petrified wood
permineralization with silica, often from a volcanic source such as a surrounding layer of volcanic ash
mold
When a shell or other structure is buried in sediment and then dissolved by underground water, a mold is created.
cast
If these hollow spaces are subsequently filled with mineral matter, a mineral or rock replica of the organism, called a cast, is created
carbonization
the process that is particularly effective at preserving leaves and delicate animal forms
impression
film of carbon is lost from a fossil preserved in fine-grained sediment, a replica of the surface, called an impression, may still show considerable detail
amber
the hardened resin of ancient trees
tracks
animal footprints or trails made in soft sediment that later turned into sedimentary rock
burrows
tubes in sediment, wood, or rock made by an animal. These holes may later become filled with mineral matter and preserved. Some of the oldest-known fossils are believed to be worm burrows.
coprolites
fossil dung and stomach contents that can provide useful information about the size and food habits of organisms
gastroliths
highly polished stomach stones that were used in the grinding of food by some extinct reptiles
two specific conditions increase the probability that an organism will be fossilized
a rapid burial in sediment after death, and the possession of hard parts
correlation
the process of establishing the equivalence of rocks of similar age in different areas
principle of fossil succession
a principle by which fossil organisms succeed one another in a definite and determinable order, and any time period can be recognized by its fossil content
index fossils
a fossil that is associated with a particular span of geological time
fossil assemblage
the overlapping ranges of a group of fossils (assemblage) collected from a layer. by examining such an assemblage, the age of the sedimentary layer can be established
earths age
4.6 billion years
deep time
immense space of geologic time
where is practically all of an atom’s mass?
the nucleus
mass number
protons+neutrons
radioactive decay
the spontaneous decay of certain unstable atomic nuclei (nuclear decay)
alpha decay
An alpha particle is composed of 2 protons and 2 neutrons. Thus, emission of an alpha particle means that the mass number of the isotope is reduced by 4, and the atomic number is lowered by 2.
beta decay
electron is emitted to produce the electron plus a proton. Because the nucleus now contains one more proton than before, the atomic number increases by 1—and it’s no longer the same element
electron capture
when an electron is captured by the nucleus. The electron combines with a proton and forms an additional neutron. As with beta decay, the mass number remains unchanged. However, because the nucleus now contains one fewer proton, the atomic number decreases by 1.
parent
unstable radioactive isotope
daughter products
isotopes resulting from the decay of the parent
radiometric dating
the procedure of calculating the absolute ages of rocks and minerals that contain certain radioactive isotopes
half-life
the time required for one-half of the atoms of a radioactive substance to decay
uranium-238
lead-206
4.5 billion years half life
uranium-235
lead-207
704 million years
thorium-232
lead-208
14.1 billion years
rubidium-87
strontium-87
47.0 billion years
potassium-40
argon-40
1.3 billion years
earths oldest rocks so far
may be as old as 4.28 billion years
radiocarbon dating
dating of events from the very recent geological past (the past few tens of thousands of years) based on the fact that the radioactive isotope of carbon is produced continuously in the atmosphere
geologic time scale
the division of earth history into blocks of time (eons, eras, periods, and epochs) the time scale was created using relative dating principles
eon
the larges time unit on the geologic time scale, next in order of magnitude above era
Phanerozoic eon
the part of geologic time that is represented by rocks containing abundant fossil evidence. The eon extending from the end of the Proterozoic eon (540 million years ago) to the present. means “visible life”
deformation
general term for the processes of folding, faulting, shearing, compression, or extension of rocks as the result of various natural forces
where does deformation of rock most commonly occur?
along plate boundaries
outcrop
sites where bedrock is exposed at the surface
geologic/rock structure
all features created by the processes of deformation from minor fractures in bedrock to a major mountain chain
three types of tectonic structures
folds, faults, and joints
stress
the force per unit area acting on any surface within a solid
confining pressure
stress that is applied uniformly in all directions
what is the result of confining pressure?
this type of force compacts mineral grains and reduces the volume of a rock body
differential stress
forces that are unequal in different directions
what is the result of differential stress?
deformation
what are the three types of differential stress?
compressional, tensional, and shear
compressional stress
differential stress that shortens a rock body
what are compressional stresses most often associated with?
convergent plate boundaries
what happens to the crust due to compressional stress?
crust becomes laterally shortened and vertically thickened
tensional stress
the type of stress that tends to pull a body apart
what are tensional stresses most often associated with?
divergent plate boundaries
what happens to the crust due to tensional stress?
crust is stretched, thinned, and fractured (deep rift valleys)
shear
stress that causes two adjacent parts of a body to slide past one another
what is shear most often associated with?
transform plate boundaries
what happens to the crust due to shear?
earth’s crust slips horizontally past one another
strain
an irreversible change in the shape and size of a rock body caused by stress
how can you infer the type of stress that deformed a rock?
by observing and measuring the strain imprinted on a rock body
three types of deformation changes (strain)
elastic, brittle, and ductile
elastic deformation
rock deformation in which the rock returns to nearly its original size and shape when the stress is removed
brittle deformation
deformation that involves the fracturing of rock. associated with rocks near the surface
ductile deformation
a type of solid-state flow that produces a change in the size and shape of a rock body without fracturing. occurs at depths where temperatures and confining pressures are high
four factors that influence how a rock deforms
temperature, confining pressure, type of rock, and time
how does temperature affect deformation?
high temperatures-ductile
low temperatures-brittle
how does confining pressure affect deformation?
higher temperatures enhance ductile behavior, and greater pressures tend to keep the rock intact—and thus more likely to bend rather than fracture
strong, brittle rocks
granite, basalt, quartz sandstones
weak, ductile rocks
rock salt, shale, limestone, and schist, and glacial ice
how does time affect deformation?
compressional strain (shortening) can be accommodated by ductile deformation->slow process if stress is applied to a rock unit too quickly, the rock will deform elastically until its strength is exceeded, and then it will fracture
folds
a bent layer or series of layers that were originally horizontal and subsequently deformed
hinge line
imaginary axis in which each layer is bent around
axial plane
surface that connects all the hinge lines of the folded strata
anticline
a fold in sedimentary strata that resembles an arch
synclines
a linear downfold in sedimentary strata; the opposite of anticline
dome
a roughly circular unfolded structure
what happens to an eroded dome?
erosion has stripped away the highest portions of the overlying sedimentary beds, exposing older igneous and metamorphic rocks in the center
basin
a circular downfolded structure
what happens to an eroded basin?
the youngest rocks are at the center
monoclines
a one-limbed flexure in strata. the strata are usually flat-lying or very gently dipping on both sides of the monocline
fault
a break in a rock mass along which movement has occurred
strike
the compass direction of the line of intersection created by a dipping bed or fault and a horizontal surface
a strike is always ______ to the direction of dip
perpendicular
dip
the angle at which a rock layer or fault is inclined from the horizontal
dip-slip fault
a fault in which the movement is parallel to the dip of the fault
hanging wall block
the rock surface immediately above a fault
footwall block
the rock surface below a fault
dip-slip fault (normal fault)
a fault in which the rock above the fault plane has moved down relative to the rock below
horst
an elongate, uplifted block of crust bounded by faults
graben
a valley formed by the downward displacement of a fault-bounded block
detachment fault
a nearly horizontal fault that may extend for hundreds of kilometers below the surface. such a fault represents a boundary between rocks that exhibit ductile deformation and rocks that exhibit brittle deformation
reverse fault
a fault in which the material above the fault plane moves up in relation to the material below
thrust fault
a low-angle reverse fault
mega thrust fault
the plate boundary separating a subducting slab of oceanic lithosphere and the overlying plate
strike-slip fault
a fault along which movement occurs horizontally
transform fault
a major strike-slip fault that cuts through the lithosphere and accommodated motion between two plates
oblique-slip fault
a fault that exhibits both dip-slip and strike-slip movement
faut scarp
a cliff created by movement along a fault. it represents the exposed surface of the fault prior to modification by weathering and erosion
slickensides
polished and grooved rock surfaces etched as crustal rocks slide past one another
fault breccia
loosely coherent rock composed of broken and crushed rock fragments
joint
a fracture in rock along which there has been no movement
what do geologists infer if sedimentary strata are horizontal?
the area is undisturbed
geologic map
graphic depiction of an area of geologic study, with labels and annotations
block diagram
a three-dimensional view of a portion of earth’s crust that makes it possible to visualize rock layers at the surface as well as underground
