GEL topic 4: Deep time

How long ago was the birth of the solar system?

4.6 BYA

what is relative age-dating?

referring to events as “younger than” and “older than”

what are the 4 principles of relative age dating?

1. superposition

2. original horizontality

3. cross cutting relations

4. faunal succession

what is superposition?

layers of rock at the bottom of a stack of sedimentary rocks are the oldest, while layers at the top are the youngest

what does originally horizontality mean?

sediment eroded from highlands is eventually deposited in horizontal layers. gravity forces particles of sediment to come to rest at its lowest possible elevation, with the end result being horizontal layering

• sediments are rarely deposited on slopes preferring to end up at the very lowest elevation

what does cross-cutting relations mean?

geological feature that “cuts across” another geological feature must be younger

• folded and faulted sedimentary rocks had to have been deformed by some mountain building even after they were originally deposited horizontally

what is faunal succession?

based on progressive, non-repetitive evolutionary change - rocks with evolutionarily older fossils are older than rocks with evolutionarily younger fossils

• fossils are only preserved in sedimentary rocks

• species of fossils succeed each other in a specific order that will always be the same in many locations around the planet

• geologist can expect rocks containing fossils xyz in Nevada will be the same age as layers with same fossils in Wyoming

what occurred during precambrian era?

• first 88% of earth’s history

• oldest evidence of microbial life

• cyanobacteria developed ability to photosynthesize in shallow ocean water

• stromatolites built by colonial cyanobacteria was built

what are stromatolites?

oldest incontrovertible fossils of organisms which are rock structures and laminated internally with layers of organic matter alternating with layers of lime mud

what is cyanobacteria?

precambrian microbes which developed ability to photosynthesize

what is most important metabolic innovation in history of life?

photosynthesis invented by cyanobacteria

what was the cyanobacteria process of photosynthesis?

made first solar cells from sugar from sunlight, throwing away leftover oxygen in the process

what led to development of the ozone layer and oxygenated planet?

the evolutionary development of photosynthesis

what is the ozone layer?

part of the atmosphere that shields life on earth from harmful ultraviolet radiation from the sun

• permitted more complex life to evolve in the shallow waters of the oceans and eventually on the continents

what did an oxygenated atmosphere lead to?

eventual development of complex animals that use O2 in the air and water for metabolism, multicellularity and larger growth

what occurred during Paleozoic era?

• marked by Cambrian explosion

• creatures with hard outer skeletons, claws, teeth and complex eyes appeared

• life invaded land as plants, amphibians and reptiles

• Paleozoic world ended in mass extinction wither 70% of land species and 90% of marine species suddenly vanished

what is Cambrian explosion?

when life in the oceans diversified abruptly

what occurred during the mesozoic age of reptiles?

• dominated by dinosaurs

• mammals and birds arose

• another mass extinction occurred because meteorite hit Yucatan Peninsula in Mexico which killed 50% of species on the planet

what occurred during Cenozoic age?

• mammals dominated planet

• late arrival of humans only present for a few tens of thousands of years

what is numerical age dating?

places specific ages on individual rocks

• ex. oldest rock on earth is 4.031 ± 0.003 billion years

what is radiometric dating?

uses the naturally occurring breakdown of radioactive elements (uranium, thorium, potassium) within minerals to determine absolute numerical date for mineral

which types of rocks does radiometric dating work best on?

minerals that crystallize from a magma or lava - when minerals solidify from a molten magma beneath the earth’s surface or from a lava spreading out from a volcano, some of them incorporate radioactive elements into atomic structure

what is radioactive decay?

• minerals solidify from molten magma and some of them incorporate radioactive elements into atomic structure

• once mineral is fully crystallized within larger body of rock, radioactive elements within begin to spontaneously decay and radioactive “clock” starts ticking

• uses naturally occurring “decay” of radioactive elements of minerals to determine a numerical date for mineral

what is zircon?

a mineral found in igneous rocks like granite

• ideal for radiometric dating since it preferentially incorporates uranium into atomic lattice and remains stable through time

what is a mineral crystal?

specific object that is dated, thus determining the time at which the mineral first crystalized from magma or lava

what is radioactive decay?

some elements are naturally radioactive meaning they undergo spontaneous continuous change which converts them to a different element

• element that decays is called the “parent” but decaying product is called the “daughter”

• results in release of heat

what is a mass spectrometer

measuring parent-to-daughter ratio on a very sensitive instrument

• operates elements according to their mass

which rocks does radiometric age dating work on?

igneous and some metamorphic rocks but also to meteorites, moon rocks and oldest rocks on earth

what is the principle of superposition?

rocks at the bottom are put there first so they are the oldest and the ones on top are youngest

what is the principle of cross cutting relation?

if one rock layer cuts across or goes through another rock layer, it must be younger than the layer it cuts

• ex. a tree falling across a stream, it landed there last so it is newer

what is principle of faunal succession?

if we find the same fossils in different layers of rock in two different places, those layers are from the same time and age

what is k-ar dating?

some rocks have minerals with elements like potassium that slowly change into other elements over millions of years. by measuring the amount of potassium and the element it changes into, called argon we can tell exactly how old the rock is

• used to date volcanic ash because it contains radioactive elements

what is radiocarbon dating?

special type of radiometric dating for things that were once alive like bones, wood or plant material

• uses carbon c-14, a radioactive form of carbon which doesn’t last long so it only works for things that are less than 50,000 years old

why can’t we use radiocarbon for older fossils?

older fossils don’t have c-14 because it decays after about 50,000 years so if something is very old radiocarbon dating will not work

how do geologist use both relative age dating and numerical age dating?

• sometimes geologist can’t use radiometric dating directly on fossil so they date igneous rocks around the fossil and relative age-dating to figure out when the fossil was alive

why is geological time scale important?

helps us know where we as humans fit into the bigger picture and reminds us that earth has been around for billions of years and life has been evolving for a long time