Transcript A Trip Through Geologic Time

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 layers
1. 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
5,730
Fossils less
than 50,000
years old
Carbon-14
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