Fossils and the Rock Record Chapter 21. The Geologic Time Scale Section 21.1

Fossils and the Rock Record

Chapter 21

The Geologic Time Scale

Section 21.1

The Rock Record•To help in the analysis of rocks,

geologists have divided the history of Earth into time units.

•How do they do this?

•Based on the fossils within the rocks

•Geologic Time Scale - time units in Earth’s history, beginning from it’s origin 4.6 billion years ago to the present

Geologic Time

• The oldest division is at the bottom

• Each division going up is younger

• Just like the rock wall

Geologic Time

• Eon - the longest time unit and is measured in billions of years

• Archean

• Proterozoic

• Phanerozoic

Geologic Time

• Era - next longest; measured in millions to billions of years

•Defined by differences in life-forms found in rocks

• Paleozoic Era

•Mesozoic Era

• Cenozoic Era

Eras - Brief Overview(Listen...your activity includes this

information)• Paleozoic

• Includes animals like sponges, corals, snails, clams, squids, trilobites

• Fishes with backbones appear

• Sharks become abundant

• Plants, fungi, and air breathing animals move to dry land

• And then...

• MASS EXTINCTION!

Eras - Brief Overview(Listen...your activity includes this

information)• Mesozoic

• Known as “The Age of Reptiles”

• WHY?

• Dinosaurs!

• Cone-bearing plants

• Flowering plants

• And then...

• MASS EXTINCTION!

Eras - Brief Overview(Listen...your activity includes this

information)• Cenozoic

• Most information because of most amount of fossils

• “Age of Mammals”

• Why?

• Mammals, birds, insects, flowering plants

• Mastodons, sabertooth cats, giant sloths, small horses

Geologic Time• Period - defined by life-forms that were abundant or

became extinct during the time specific rocks were deposited

• Measured in tens of millions of years to hundreds of millions of years

• Cambrian

• Ordovician

• Silurian

• Devonian

• Mississippian

• Pennsylvanian

• Permian

• Triassic

• Jurassic

• Cretaceous

• Tertiary

• Quaternary

Geologic Time

• Epochs - smaller divisions of geologic time; measured in millions of years to tens of millions of years

• In the Cenozoic period:

• Paleocene

•Oligocene

Relative-Age Dating of

RocksSection 21.2

Uniformitarianism• Until the turn of the 19th century, most of

the world believed the Earth was only 6000 years old.

• About 1770, some scientists started to observe Earth’s landscapes and forces of nature.

• Uniformitarianism - states that the processes occurring on Earth today have been occurring since Earth was formed

• What are these processes?

Principles for Determining Relative Age

•Main Concept:

•Places age of rocks and the events that formed them in order

•Does not use exact dates

•Compares one event or rock layer to another

Principles for Determining Relative Age

• Major Geologic Principles

• Principle of Original Horizontality - states that sedimentary rock is deposited in horizontal or nearly horizontal layers

• Principle of Superposition - states that in an undisturbed rock sequence, the oldest rocks are at the bottom

• Because of this, we can assume the oldest layers are on the bottom of a rock wall and the younger layers are on the top

Other Means of Determining Relative Age

•Weathering, erosion, earthquakes, and volcanism make determining relative age difficult

•Why?

•Unconformity - gaps in the rock record caused by erosion and weathering

Other Means of Determining Relative Age

•Correlation of Rock Strata - the matching of outcrops (rock walls) of one geographic region to another

Absolute-Age Dating of

RocksSection 21.3

Absolute-Age Dating of Rocks

• What is one issue with relative-dating of rocks?

• Scientists have devised a method for dating very old objects using the decay rate of radioactive isotopes

• These are found in igneous and metamorphic rocks, fossils, and organic remains

• Ex: Uranium (U-238) decays into lead (Pb-206)

• The number of protons and neutrons change

Absolute-Age Dating of Rocks

•Radioactive Decay - the emission of radioactive particles and the resulting change into other elements over time

•The rate of decay remains constant

•These particles become accurate indicators of the absolute age of an object

Use of Radioactive Isotopes

• Radiometric Dating - a process that determines the ratio of the original radioactive element to the newly formed element within a given sample in a rock or fossil

• Uses the half-life of radioactive elements

• The length of time it takes for one-half of the original amount to decay

• U-238 = 4.5 billion years = Pb-206

Use of Radioactive Isotopes•Carbon-14

•Used for finding age of organic materials

•amber, humanoid bones, charcoal fragments, etc

•Half-life = 5730 years

•Becomes Nitrogen-14

•Accurate for objects up to 75,000 years old

Other Ways to Determine Age• Tree Rings

• The age of a tree can be determined by counting the number of annual rings in a cross-section

• A pair of spring and winter rings represents an annual tree ring

• Have helped to determine age of structures

Other Ways to Determine Age

•Seasonal Climatic Changes

•Glacier movement causes glacial sediment change

•These changes can help date the different nearby lakes

•Common in Alaska

Remains of Organisms

Section 21.4

Fossils•Fossils - the evidence or remains of

once-living plants or animals

•Provide evidence of past existence

•Provides evidence that populations have undergone change though time in response to changing environments

•Evolution - the change in populations as a result of environmental change

Types of Fossils

•Original Preservation - soft and hard parts of plant and animal remains that have not undergone any kind of change since the organism’s death

Types of Fossils

• Index Fossils - remains of plants or animals that can be used by geologists to correlate rock layers over large geographic areas or to date a particular rock layer

The Early Earth

Section 22.1 and 22.2

Earth’s “Birth”•For most of Earth’s 4.6-billion-year

existence, most of the life forms were unicellular

•What’s that mean?

•What evidence did these organisms leave of their existence?

•What was Earth like when these organisms lived?

How Old is Earth?• We know that Earth must be at least as old

as the oldest rocks in the crust

• Radiometric dating has determined the age of the oldest rocks on Earth is between 3.96 to 3.8 billion years old.

• BUT! the rocks that form Earth’s curst have eroded over time

• Meteorites have been dated between 4.5 and 4.7 billion years old.

• Scientists commonly agree that Earth is 4.6 billion years old

Earth’s Heat Sources

•Earth was probably extremely hot shortly after it formed

•Sources:

•radioactivity

•asteroids and meteorites

•gravitational contraction

Formation of the Atmosphere and

OceansSection 22.3

The Precambrian Atmosphere

• Atmosphere in early Earth was dominated by

• Hydrogen

• Helium

• Carbon dioxide

• Nitrogen

• Hydrogen and Helium do not have a large mass, so Earth’s gravity did not hold them in the atmosphere.

• This is why they are not a major part of the atmosphere today

The Precambrian Atmosphere

•What caused all those gases?

•Volcanic eruptions

• Produces water vapor, carbon dioxide, nitrogen, and carbon monoxide

•Chemical reactions in the atmosphere

•Methane and ammonia formed from the volcanic gases

Oxygen in the Atmosphere• So why did it take so long for life to begin?

• There was no oxygen in the atmosphere

• Volcanoes don’t give off oxygen

• Where did it come from?

• The oldest fossils are about 3.5 billion years old

• Cyanobacteria - the remains (fossils) of tiny, threadlike chlorophyll bearing filaments

• These bacteria started converting CO2 from the atmosphere into O2

Oxygen in the Atmosphere

•Oxygen Producers

• The abundance of cyanobacteria increased throughout the Precambrian Time period.

• They produced enough oxygen to supply the newly formed Earth

Oxygen in the Atmosphere• Evidence in Rocks

• Another way to test for oxygen (other than fossils)

• Looking for oxidized iron in rock layers

• Iron reacts with oxygen to form iron oxide in the rock (which makes them red)

• This PROVES there was oxygen in the atmosphere

Importance of Oxygen

•Why is oxygen so important?

•Animals require it.

•Blocks out UV rays with ozone (O3)

•Formation of Ozone

•3 oxygen molecules are bonded together when high energy UV rays split regular O2 molecules and combine them with other oxygen molecules

Formations of the Oceans• Oceans are thought to have formed from the

same process of outgassing that formed the atmosphere

• What does that mean?

• As Earth’s temperatures began to cool, the water vapor from the air began to condensate.

• For example: It began to rain and fill low-laying basins...which eventually formed oceans

Early Life on EarthSection 22.4

Origin of Life on Earth

•How old is Earth?

•Life began 3.5 billion years ago

•Life could not have begun before that because meteorites were constantly striking Earth’s surface

Precambrian Era•All life was marine and soft-

bodied

•ocean-dwelling

•The only plants were algae and fungi

•Animal life included jellyfish, corals, and worms

•Invertebrates - No backbones

Precambrian Life

The Paleozoic Era

Chapter 23

Paleozoic Era•Began 544 million years ago

•Marked with the first organisms with hard body parts

•Warm, shallow seas cover most of Earth’s surface

•Most life forms were marine

Periods of the Paleozoic•Cambrian

•Ordovician

•Silurian

•Devonian

•Mississippian

•Pennsylvanian

•Permian

Early Paleozoic Life (Cambrian Period)

•Appearance of marine animals with hard parts

•Plants: Algae, seaweed

•Animals: trilobites, jellyfish, corals, snails, sponges, worms

Early Paleozoic

Early Paleozoic Fossil

Middle Paleozoic (Ordovician thru Devonian Periods)

• Beginning of Appalachian Mountains

• Plants: algae, seaweed, fungi, mosses, ferns, early trees

• Plants start moving to land in Silurian and Devonian Periods

• Animals: First vertebrates, corals, snails, clams, primitive fish, arthropods (millipedes, scorpions), early amphibians, bony fish, lung-fish

• Some shells were 30 feet long!

Middle Paleozoic - Ordovician Period

Middle Paleozoic - Silurian Period

Middle Paleozoic - Devonian Period

Late Paleozoic (Mississippian thru Permian Periods)

•Warm seas and swampy conditions exist

•Pangea (at end of Permian)

•Drastic climate changes at end of Permian because of Pangea and rise of Appalacians

•Plants: Conifers

•Land plants grow rapidly in size and number

Late Paleozoic (Mississippian thru Permian Periods)

• Animals: Insects, reptiles

• Become abundant and are highest life form

• Increase in number and type of fish

• Because of drastic climate change, many life forms die out.

• Reptiles adapt to change better than other animals, so they become dominant in next era

Late Paleozoic - Mississippian Period

Late Paleozoic - Pennsylvanian Period

Late Paleozoic - Permian Period

Mesozoic Era

Mesozoic Era

•Began about 245 million years ago

Periods of the Mesozoic Era

•Triassic

•Jurassic

•Cretaceous

Mesozoic Era• Pangea separates

• Causes drastic climate change

• Sierra Nevada Mountains form

• Rocky mountains produced towards the end

• Plants: Conifers, flowering plants

• Animals: Dinosaurs, flying reptiles, first small mammals

Mesozoic - Triassic

Mesozoic - Jurassic

Mesozoic - Cretaceous

End of the Mesozoic Era

•Pangea breaks up and continents move to present positions

•Mass extinction - climate change

•Estimated that 75% of life forms die out

Cenozoic Era

Cenozoic Era•Began about 66 million years

ago

•Known as “The Age of Mammals”

•Climate became cooler and ice ages occurred

•Many mountain ranges throughout North and South America formed

Periods of the Cenozoic Era

•Tertiary

•Quaternary (Current)

Cenozoic Era•Plants: flowering plants, fruit-

bearing plants

•Animals: Mammals (mammoths, bison, saber-tooth cats, horses, camels), birds, insects

•Accumulation of ice at the poles led to mass extinctions

Cenozoic Era

Cenozoic Era - Primates

•Primates - mammals with opposable thumbs and forward-facing eyes

•Hominoids - a group of primates that includes Homo sapiens, great apes

•bipedal (stands on two feet)

Cenozoic Era - Homo sapiens

•Neanderthals - hominoid group; most likely a side branch of H. sapiens

•Skulls were different than modern humans (heavy brows and gap between rear teeth and jaw bone)

•Short limbs; massive, muscular bodies

•lived in caves and used tools

Cenozoic Era - Homo sapiens

•Didn’t become dominant until about 10,000 years ago

•As human populations grew, they competed for food that other animals relied on.

•They may have contributed to extinctions by overhunting.