Geologic History Flashcards
geologic history
study and interpretation of earth’s past
relative age
the age of rocks or events compared to the age of other rocks or events
(ex. i am a teenager)
absolute age
the age of a rock, object, or event in years
(ex. i am 14 years old)
how do scientists determine the relative ages of rock layers
- uniformitarianism
- principle of original horizontality
- principle of superposition
uniformitarianism
the geologic processes that took place in the past are generally similar to those that take place now
principle of original horizontality
sediments are deposited in horizontal layers that are parallel to the surface they were deposited on
what do tilted or folded layers indicate
the crust has been deformed
principle of superposition
in undisturbed layers, the oldest layer is on the bottom, and each overlaying layer is younger
- does not apply if layers have been overturned or faults have forced over rock over younger rock
igneous intrusions and extrusions
as hot magma squeezes into cracks and zones of weakness, the cooling magma pastes its heat energy to the nearby rock
how to tell if this intrusion is younger or older
if its younger there will be burn marks
unconformity
a gap in the rock record caused by the burial of an eroded surface by younger sediments. shown by a wavy line
how do unconformities happen
- submergence
- deposition of sediments
- emergence
- erosion
- submergence
- deposition
where does the rock need to be eroded
above water
where does the rock need to be to be created from deposition
underwater
folding
layers of sedimentary rock that have been bent or warped by crustal forces
tilting
formally horizontal rock layers that have been tilted at an angle by crustal activity
faulting
a crack of weakness in earth’s crust along which movement occurs during an earthquake
law of cross cutting
a disruption that cuts through rock or another geologic feature must be younger than the rock or the other geologic feature
when is there contact metamorphism
when the intrusion is younger
how do you know when there is contact metamorphism
there would be dashed lines
how to know when rock layers are deposited before the fault
when the fault line goes through it
what do you know about the geologic cross-sections when the bottom layer is sedimentary
submergence is the first thing that happened
erosional surface
a surface where rock has been worn away by wind, water, or ice, often creating an unconformity
subsidence
the sinking or downward movement of the earth’s surface
uplift
the rising of earth’s surface due to tectonic forces
emergence
when land rises above sea level, often due to uplift or falling sea levels
submergence
when land sinks below sea level, often due to subsidence or rising sea levels
when did earth form
about 4.6 billion years ago
geologic time scales
geologists have subdivided geologic time into units based on fossil evidence
fossil
any evidence of former life
what are the 4 major divisions of time
precambrian eon
paleozoic era
mesozoic era
cenozoic era
how much of earth’s history is the precambrian eon
88%
how much of earth’s history have humans existed
0.04%
what are the 4 types of divisions in time in size order
- eon
- era
- period
- epoch
life has not existed for _______
most of earth’s history
fossils provide evidence of
evolution
outgassing
volcanoes began releasing water vapor, carbon dioxide and nitrogen or early earth
evolution
change from simple forms to more complex forms
isotopes
atoms of an element that have the same number of protons and electrons, but a different number of neutrons
they are the same they just have different masses
how do scientists give events or fossils an exact age
each radioactive isotope has a known rate of decay that can be used to give an absolute age to a material
radioactive decay
can be used to give an absolute age to a material
what does carbon-14 decay into
nitrogen-14
half-life
the amount of time it takes for half of the atoms in a radioactive sample to decay
can a half-life change
the half-life will never change (even if you apply heat, pressure, break the substance into pieces, etc)
as radioactive decay progresses, the amount of the radioactive element decreases by _____ each life
half
can a radioactive sample get to 0
no, because you can always divide the amount remaining in half (even if it gets really really small)
different radioactive substances have _____ half lives
different
half life
the time it takes for half of a radioactive isotope to break down into something else and change
radioactive isotopes
isotopes that are unstable slowly break down over time, releasing radiation (energy)
a half life tells us how fast this happens
the isotope will decay until it becomes a stable element
what makes an isotope become a radioactive isotope
when the nucleus is unstable, the balance between protons and neutrons isn’t quite right, causing the atom to break down over time and release radiation
how does decaying make isotopes more stable
it helps the nucleus fix its imbalance of protons and neutrons
when it decays it releases radiation to adjust its structure and become a more balanced, stable atom
