A Trip Through Geologic Time

The Fossil Record

• fossils tell the history of life on earth

• not all organisms form fossils

• conditions necessary for fossil formation:– protected from destruction and decay– sediments bury it and become rock– hard parts

Types of Fossils• petrified fossils – mineralized copies of

original bones– minerals in water replace original matter– parts turned to stone

• molds and casts– mold - cavity in the shape of an organism– cast - minerals fill in the cavity

• trace fossils– footprints, tracks, trails, and burrows

• coprolites– tell about what an organism ate

• unchanged fossils– amber, tar pits

Interpreting Fossils

Relative Age Dating

principle of superposition-younger rock on top of older rock

-basis for most relative age dating of fossils

Disruption of Layers• folding & faulting disrupt order of layers• erosion removes layers

Rules for interpreting layers1. Rock layers are horizontal before they deform

2. A fault or igneous dike did not exist when the layers formed, and is younger than the layers it cuts across

Absolute Dating

• absolute dates expressed in years• Radiometric dating – compares amount of

radioactive isotope present to the amount of decay present

• half-life – time it takes for half of isotope to decay

Isotope Half-Life

(Years)

Used to Date

Thorium-232 14 billion Very old rocks

Potassium-40 1,300 million Old rocks & fossils in them

Carbon-14 5,730 Fossils less than 50,000 years old

The Geologic Time Scale

• Geologic Eras:– Precambrian

– Paleozoic Era

– Mesozoic Era

– Cenozoic Era

Precambrian Era The Dawn of Life

• 4600 mya - 540 mya

• Earth’s history begins

• Seas form from volcanic emissions & comet impacts.

• Continents form & grow.

• Oxygen builds up in atmosphere from photosynthesizing organisms (stromatolites)

Paleozoic Era“Old Life” - Life Comes Ashore

• 540 mya - 248 mya• Age of Invertebrates• Began with “Cambrian Explosion” of new life forms,

due to preceeding mass extinction.• Abundant sea life. First fishes appear, first amphibians

evolved from fishes, first reptiles evolved from amphibians.

• First terrestrial life (arthropods & other invertebrates) invades from sea.

• Carboniferous swamps create coal deposits.• Pangaea formed near end of era.

Mesozoic Era “Middle Life” - Age of Dinosaurs

• 248 mya - 65 mya• Era of many changes• Dinosaurs, birds, & mammals all evolved from

reptiles. Mammals avoid competition with/predeation by dinosaurs by being nocturnal & eating insects.

• Pangea broke up 200 mya.• Flowering plants became dominant plant life.

Cenozoic Era “New Life” - Age of Mammals

• 65 mya – present

• Mammals dominated.

• Primates, human ancestors, & modern humans evolved.

• Land bridge formed between N. & S. America, disrupted ocean currents & triggered Ice Ages - sheets of ice advance and recede several times.

• Alps and Himalaya Mountains form from collisions of continents.

Check These Out!Geologic Time Machine

Fossils, Rocks, and Time

Ice Age Paleoecology

USGS FAQ about Dinosaurs

Carbon-14 Dating

Paleontology Without Walls

Dinosaur Images

Paleomapping Project

The End