Physical Science

Geologic Time

Observations Regarding Rock Layers

• Relative Dating – Lower layers of sedimentary rock are formed before older layers.

• Original Horizontality – layers of sediment deposited evenly with new layers on top of old layers.

• Superposition – upper layers are younger than lower layers

• Any incline is an indication of a disturbance after deposition.

Earth History

• Cross Cutting – an igneous intrusion or fault that cuts through preexisting rock. This intrusion will be younger than the rock through which it cuts.

Earth History

• Inclusion – pieces of one rock type contained within another – any inclusion is older than the rock containing it.

Earth History

• Faunal succession – Fossils contained within rock can indicate that they were in existence around the time the rock was formed. This succession of fossils gives rise to the evolutionary nature of species.

Earth History• Unconfomities – break up points of the

original sediment layers

• Angular unconformity – tilted or folded rock layers that become overlain by younger horizontal layers.

• Nonconformity – sedimentary rock that are depositeed over and eroded surface of metamorphic or intrusive igneous rock.

from yahoo images

Earth History

How do we know how old a rock layer is?

• Radiometric dating – using half life of isotopes to determine the age of layers

• Fossil found in layers – certain fossils seem to have only been around at certain times

from yahoo images

Earth History

Precambrian• 4.6 billion years to 544 million years ago• Makes up about 85 % of the earths history• Most rocks have been eroded away,

metamorphosed or recycled• Seems to have had lots of volcanoes and meteor

hits• Simple organisms existed (stomatolites, blue

green algae) • They used carbon dioxide and produced oxygen • Fossils in Australia are of jellyfish and worm

type

Paleozoic

Cambrian

Fossils of all major groups of marine organisms are found

Some organisms secreted calcium carbonate and calcium phosphate to form outer skeletons or shells

Trilobites are from this time period

Paleozoic

Ordovician

• Abundant marine life

• Appearance of vertebrates

Paleozoic

Silurian• Most of North America rose above sea

level• This caused gypsum and other deposits

to accumulate in the receding seas• Terrestrial plants flourished – in wet

lands• Air breathing scorpions and millipede

fossils are found

Paleozoic

Devonian

• Called the Age of Fishes

• Lowland forests and seed trees appear

• Lungfish appear

• First amphibian fossils appear

• These creatures could live on land but laid eggs in the water

Paleozoic

Carboniferous • Mississippian and Pennsylvanian periods • Warm climate – lush vegetation • These swamps created the coal beds that lie

under North America, Europe, and China. • Giant cockroaches and dragonflies (80cm wing

span)• First reptiles (anapsids) appear• First egg formation – life existed solely on land

Paleozoic

Permian• Reptiles existed for 200 million years• Two more reptiles – diapsids and

synapsids appear• End of this period huge extinction took

place (loss of 90% of species)• Reason is unknown – possible cooling

took place. (meteor atmosphere?) • Large amount of earth surface change

Mesozoic Era

• Three parts Triassic, Jurassic and Cretaceous.• Known as the age of reptiles• Dinosaurs are found in these layers of rock• First birds are found in the Jurassic period• End of Cretaceous (66 milion years agao) a

large extinction took place• A layer of iridium indicates a possible meteor

hit or volcano’s erupted • Effects – nuclear winter, thick dust blocking out

sun – cooler temp • Break up of land masses occurred

Cenozoic• Two periods – tertiary and quaternary

• Known as the age of mammals

• Due to the extinction, animal competition was diminished and those that survived –thrived and differentiated.

• Climate cooled and an ice age started that continues today.

• Humans show up in this era