Geologic Time Notes:
Name:___________________
Geologic Time: The time scale used to describe the 4.6 billion years since the earth was
formed.
 Humans are very recent – only in the last 10,000 years
 The geological time is divided into Eons, Eras, Periods and Epochs
o Eon – The eon is the largest division of geologic time
o The 4 eons are:
 Hadean – 3900 to 4600 million years ago
 Earth was mostly a molton (liquid) sphere – few surviving rocks

Archean 2500 to 3900 million years ago
 The crust cooled
 Rocks formed
 Rains filled the ocean
 First fossils of life found – simple bacteria

Proterozoic – 544 to 2500 million years ago
 Fossils of cells with a nucleus are first found in this eon

Phanerozoic – 544 million years ago until the present
 Includes the evolution of most of the complex forms of life we
know – invertebrates, fish, mammals, birds, reptiles
o Precambian: The Hadean, Archean and Proterozoic are considered the eons
that make up the oldest part of Earth’s history
o Era – A span of time that includes several periods, but that is shorter than
an eon
 There are 3 eras outlined in the textbook

Paleozoic – “Age of Ancient Life”
o Continents were in different places on the earth and
had different shapes
o Climate was warmer and more humid (damp)
o Ancient life forms included: early invertebrates, fish
amphibians and reptiles including the trilobite

Mesozoic – “Age of the Dinosaurs”
o Small continents merged together to form Pangaea
o Pangaea starting to split up by the end of the era
o Rocky Mountains and Andes Mountains (SA) were
formed
o Plants – Gymnosperms were dominant: ginkgos, conifers
(eg. pine and spruce) and cycads
o Reptiles (especially dinosaurs) dominated the animal life

Cenozoic – “Age of Recent Life”
o Continents moved apart
o Alps and Himalayan Mountains formed
o Climate cooled down, several ice ages during this era
o Animals included mammals – hornless rhinoceros, Sabertoothed cat, giant bison, and wooly mammoths
o Flowering plants (like willow, fireweed, berry plants)
evolved and dominated

Period – there are 11 periods in the Phanerozoic Era
 A geologic time unit that is longer than an epoch and shorter
than an era
 Quaternary, Tertiary, Cretaceous, Jurassic, Triassic, Permian,
Carboniferous, Devonian, Silurian, Ordovician, Cambrian

Epoch – Periods are divided up into shorter Epoch
 Holocene, Pleistocene, Plinocene, Micocene, Oligocene,
Eocene, and Paleocene are Epochs of the Quaternary and
Tertiary Periods
 Scientists used fossils, rocks and rock formations to solve the puzzle of what
events happened on the earth in what order. Some puzzle pieces are still
missing.
 Fossil: The shape or remains of a prehistoric (before history) plant or animal
that has been embedded and reserved in rock.
 Pangaea: a supercontinent that existed from about 200 – 300 million years ago
and included most of the continental crust of the Earth
Dating the Earth
 Scientists use clues to determine the age of the Earth and various rocks and
fossils
 Two types of dating are Relative Dating and Absolute Dating
o Relative Dating: The chronologic (time) ordering of features, fossils or
events in the geologic time scale without reference to there exact,
absolute age – what order do things and events go in?
o Absolute Dating: Methods that help us determine the geologic age of a
fossil, rock or event – such as radiometric dating using radioactive isotopes
of certain minerals or atomics.
 There are 4 principles are used when scientists perform relative dating
o Principle of Uniformitarianism: The geologic processes (chemistry, physics,
erosion, deposition, rate of radioactive decay) that are happening to rocks
today are the same as the ones that happened past.
o Principle of Original Horizontality: Layers of sedimentary rock (strata) are
initially deposited as horizontal (level) layers of sediment (tiny bits of rocks
and organic matter)
o Principle of Superposition: The layers of sedimentary rock layers (strata)
are deposited in order. The oldest layer is at the bottom and the youngest
layer is at the top as long as the rock beds are not disturbed (bent,
twisted, uplifted, eroded)
o Principle of Cross-cutting Relationships: If a fault or intrusion cuts
through another body of rock, then it must be younger than the rock
through which it cuts or that it displaces. Basically if there is a crack or
intrusion of igneous rock (rock that was liquid) of that goes through a bunch
of layers, it must be younger than all of the layers it goes through, since
you have to form the layer first before it can be cracked or have an
intrusion.
 Key Beds: A special layer of sedimentary rock that was formed at one time and
covers a large area. It is useful for geologic correlations and relative dating
over a large area.
 The layer needs to have some easy way for scientists to recognize it
– volancanic ash, coal beds, limestone beds
 Correlation: The matching up of rocks of the same age from place to place is
called correlation
o Correlation can be done using key beds or fossils
 Finding the same type of rock in two sequences (key beds) can be
used to estimate the dates of layers above and below it
 Finding fossils in strata in 2 places far from each other can help
scientists to approximate the date that that layer was laid down or
that the two beds are a similar age
 Some fossils only existed for a relatively short and specific
time – allowing scientists to narrow down the age of rocks
based on the combination of fossils found in it
Absolute Dating:
 3 methods that can be used for absolute dating are: radiometric dating, varves
and dendochronology
o Radiometric dating: any method of finding the absolute age of fossil or
rock or rock formation using the rate of nuclear decay of naturally
occurring radioactive isotopes such as Carbon-14 and Uranium-238
 Isotope: a atom of an element (i.e. Hydrogen, Carbon, Oxygen) that have
the same number of protons, but different numbers of neutrons in the
nucleus (center) of their atoms
 Carbon-14 has 6 protons and 8 neutrons - radioactive
 Carbon-12 has 6 protons and 6 neutrons –stable
 Radioactive isotopes have more neutrons and are not stable, which
means that they break down over time releasing energy and particles
from their nuclei. The original isotope will change to a daughter isotope
as a result of the changes caused by radioactive decay
 Half-Life: The amount of time it takes for half of the radioactive
atoms to decay to their daughter isotopes.
 Radioactive minerals in Igneous Rocks, formed when molten or liquid
rock cooled and crystallized, can tell you when that rock formed
 Igneous rocks are the best kind of rocks for radiometric dating.
 Sedimentary rocks, made up of bits of worn down rocks that can be
from several places, are not good for radiometric dating
Scientists look at the ratio of radioactive element to daughter isotopes
to figure out how long it has been since the rock was formed
 If there is more daughter isotope then the rock was formed a
longer time ago
 Every half life that passes results in about half the
radioactive atoms changing into daughter isotopes
 Radiocarbon Dating: uses Carbon-14 to estimate the absolute age of
organic matter or fossils
 When a living thing is alive it eats and has Carbon-14 in about
the same proportion as the environment around it. Once it
dies, the amount of Carbon-14 starts to decrease as the
unstable isotope breaks down to Nitrogen-14. Because it is no
longer eating, no new Carbon-14 can get into it.
 The half-life of Carbon-14 is short compared to a lot of other
radioactive isotopes because of this after 60,000 years, there
is too little Carbon-14 left in a sample of a living thing to
accurately date it using this technique
 The half life of Carbon-14 is 5,730 years
 Dendrochronology
o Definition: a method of scientific dating based on the analysis of tree ring
patterns – size
 Absolute dating technique
 The age of a tree can be found by counting the number of rings in its
trunk
 Rings can tell us about fires, droughts or diseases in the past years
from the way the ring is like
 Thin rings might mean cold or dry conditions
 Think rings might mean longer growing seasons, or warmer weather
 Varves
o A varve is an annual layer of sediment deposited in a lake or fjord by rain or
meltwater from glaciers.
 Can tell us about seasonal climate (weather) changes
 Pairs of layers of sedimentary rock that are formed in a lake each year
 Summer: light coloured; made of sand, pollen and coarse
material
o Because the lake is open and wavy, fine particles stay
suspended, but pollen and other things get deposited
 Winter: dark coloured; made of clay and fine organic matter
