How old is the Earth? • SCIENTISTS BELIEVE THE EARTH WAS FORMED 4.6 BILLION YEARS AGO
3.8 – 2.5 Billion Years Ago Earth’s crust was cooling and rocks and continental plates were forming. The atmosphere consisted mostly of methane and ammonia.
2.5 Billion to 543 Million Years Ago LIFE APPEARS!!! First fossil evidence of bacteria and later the first eukaryotic cells.
How can we keep up with 4.6 Billion Years of time? In order to help us work with a huge span of time – geologists have divided up the earth’s history into time periods called the Geologic Time Scale
The Geologic Time Scale Shows how life has developed on Earth over time.
You will see different versions of the Geologic Time Scale: they aren’t meant for you to memorize, but be able to use them as a tool.
Precambrian Eon • Makes up 90% of Earth’s history. • Spans from 4.6 billion to 543 million years ago
These are the eras in which life became abundant and diversified. • Paleozoic (ancient life) • Mesozoic (middle life) • Cenozoic (recent life)
For example – during the Mesozoic Era is most famous for the dinosaurs that lived then.
Cenozoic Era – Mammals are the dominant animals: especially humans. WE ARE STILL LIVING IN THE CENOZOIC ERA!
What is a fossil? A fossil is the remains or evidence of any creature or plant that once lived on the Earth.
A fossil may be: • Bones • Teeth • The impression of a plant or animal • Footprints • Burrows • Coprolite or animal feces
Limestone matrix with 34 teeth from same shark http://www.fossilmuseum.net/fishfossils/Chondrichthyes-fossils.htm
dinosaur tracks • www.scsc.k12.ar.us/TuttS/fossil_formation.htm
Turtlesaur Coprolite – droppings from a turtle-like dinosaur • www.ihs.issaquah.wednet.edu/Teachers/robles/i...
Frozen baby mammoth found in the permafrost of Siberia goofyblog.net/news-baby-woolly-mammoth-found/
How are fossils formed? • No matter which way preservation occurs it takes very special conditions. Most living things are quickly decomposed upon death. Scavengers and bacteria usually consume all but bones and shells.
When you think of the billions and billions of living things that have inhabited the earth over the last 550 million years only a very small percentage are immortalized in stone!
How do we know how old a fossil is? Relative Dating does not give the fossil’s actual age. It compares its age with fossils or rock formations around it.
Law of Superposition In a sedimentary sequence, the older beds are on the bottom and the younger beds are on the top C B A Which is the youngest? Layer A was deposited first
From relative dating, we predict that layer B is younger than A and older than C.
Index or Guide Fossils There were some species of organisms that were only around for a relatively short period of time. (evolved, then became extinct) Therefore, when we find a rock with one of these index fossils in it – we know the relative time in which that rock was formed.
Absolute dating gives us the actual date of the fossil Most absolute dating methods use radioactive isotopes – in order to know what an isotope is, we need to review atomic structure.
Radioactive Decay the spontaneous transformation of one element into another.
By losing particles from the nucleus the atom will become a different isotope or another element entirely. The original isotope is the “parent.” The new isotope is the “daughter.”
Half-Life • Half-life is the amount of time its takes for ½ of the parent isotopes to turn into the daughter isotopes.
A fossil sample contains Carbon-14. Carbon-14 has a half-life of 5730 years. If it has gone through 3 half-lives, how old is the sample?
Geologists compare the ratio of “parent” element to the “daughter” element to determine the number of years ago the rock was formed. Which would be younger? a rock with a greater amount of parent, or a greater amount of daughter? parent, because it has had less time to decay into the daughter element
From measuring the ratios of the isotopes, scientists are able to determine the amount of radioactive decay that has taken place. Knowing the time that it takes specific isotopes to decay – gives them the absolute date of the rock or fossil’s formation.