geological time Flashcards
what is the principle of uniformitarianism?
that physical processes we observe today also operate in the past and were responsible for the formation of geological features.
what is the principle of original horizontality?
sediments settle out of a fluid by gravity. if tilted, you know it must have been horizontal. tilted sedimentary rocks must be deformed.
what is the principle of superposition?
the unreformed sequence of altered rocks.
what is the principle of cross-cutting relations?
younger features truncate older features.
what is the principle of inclusion?
a rock fragment within each other.
what is the principle of lateral continuity?
strata often formed lateral sheets. erosion dissects once continuous layers.
what is the purpose of fossil succession?
fossils speak of past depositional environments. specific fossils are only found within a limited time range. fossil succeed one another in a known order. a time period or zone, is recognised by its fossil content
how are there missing periods of time?
horizontal marine sediment deformed orogenesis. mountains eroded completely away, renewed marine invasion. new sediment is deposited, vertical beds put on overlaid beds show millions of year missing. once sequence of rock exposed there consisted of alternating beds, of different martirals. when tilted and truncated by erosion, there shows a time interval.
what is an unconformity?
where there are time intervals missing in rock formation.
what are the 3 types of unconformity?
angular unconformity
non-conformity
disconformity
what is an angular unconformity
rocks were tilted or folded before unconformity developed. this angular unconformity cuts across the underlying layers, and they have different orientation.
what is a non-conformity
type of conformity at which sedimentary rocks overlie intrusive igneous or metamorphic rocks. they must have been cooled, uplifted and exposed to erosion, to form the substrate on which the new sedimentary rocks were deposited.
what is a disconformity
a sequence of sedimentary beds that have been deposited beneath a shallow sea, then the sea level drops, and the recently deposited beds were exposed for some time. no new sediment accumulates and some of the pre-existing sediment gets eroded away. later, sea-level rise and a new sequence of sedimentary beds were deposited. the boundary between these 2 boundaries is conformity. this is hard to find due to the no difference in deposited layers.
what are stratigraphic columns?
draw column to show the thickness of each layer in the outcrop.
what are stratigraphic formations?
recognizable intervals of a specific rock type or group of rock type deposited during a specific time interval. the boundary surface between two formations is a type of geological contract.
what is lithologic correlation?
correlate formations bweteen nearby regions based on similarities on rock types. to correllate units over broader areas, we must apply on fossisl to define the ages.
what is a geological column?
composite stratigraphic column can be constructed. assembled from incomplete sections of the globe. by crrealting rocks from locality at millions of places in the world.
when did life first appear on earth?
3.8 Ga.
what is the eon which visible life appeared?
phanerozoic, <543 Ma to present
when was the Proterozoic eon?
before life, 2.5-0.543 Ga
what happened in the Proterozoic eon?
tectonic plates build up of oxygen in atmosphere
when was the Archaen eon?
3.8-2.5 Ga
what happened in the Archaen eon?
birth of the continents
when was Hadean eon?
4.6-4.8 Ga
what are the subdivisions of geological time?
The largest subdivisions are the eons: Hadean, Archaean, Proterozoic, and Phanerozoic. the first 3 make up the Precambrian (early life). the Phanerozoic era is subdivided into eras, the oldest being Paleozoic, Mesozoic, and Cenozoic. and we can divide these into periods, and each period into epochs.
what is geochronology?
many relative ages can now be assigned to actual dates. using radiomatric dating, radioactive deca and mass spectronomy
how can you use radioactive decay to find ages?
concept of half-life- all atoms of a given element have the same number of protons in nucleus. not all the atoms have the same number of neurons (isotopes). they are stable, therefore last forever. radioactive isotopes arent stable, therefore they decay, which c converts them into different elements.
what are the different types of decay for radioactive elements?
alpha decay (ejecting 2 protons and 2 neutrons), beta decay (transforming a neutron into a proton by ejecting an electron), and electron capture (joining of a proton with an electron to form a new neutron)
what are the 4 steps of radioacitve dating tehcnique?
collecting the rocks
separating the minerals
extracting parent and daughter isotopes
analysing the parent-daughter ratio
how can you determine numerical age?
growth rings- annual layers of a tree or shells. rhythmic layering- annual layers in sediments or ice.
magnetostratigraphic: magnetic signatures in strata are compared to global reference column.
fission track analysis: measuring decay paths, scar crystals etc.
what do rings or layers show?
this shows the growth rate of trees, the organic productivity of lakes and seas, the sediment supply carried by rivers, the growth rate of chemically precipitated sedimentary rocks, and the growth rate of shell-secreting organisms.
