Geologic History A new way to look at time

How Can Rocks Help? Been around for a long time Record geologic history Show changes in life forms Does this give us a complete record of Earth’s history? NO Crust is constantly recycled Like a book with pages missing

Archbishop James Ussher Created a timeline of human history Related it to the history of the Earth Earth created in 4004 BCE He wasn’t right so why was he important? Earth was over 5000 years old. First person to make a timeline of Earth’s history

James Hutton Scottish physician “The key to the future is the past” Uniformitarianism Same processes happening now as historically Shaped Earth over a long period of time Not cataclysmic events (Catastrophism) Gentleman farmer

Relative Dating Sequences events Does not tell how long ago they happened Uses rock layers 3 major laws (6 total)

Original Horizontality Assumes rock layers were originally laid down horizontally. Layers can be tilted or folded as time goes on so the more wiggles the older the rock

Law of Superposition IMPORTANT Layers on the bottom are older than layers on the top

Law of Cross-Cutting Relationships IMPORTANT Anything that cuts through another layer is younger than the layers it cuts through 2 types of cross-cutters

Cross-Cutters Faults: Intrusions: Displace layers Go through layers Faults are drawn as thin lines and you can see where the layers have been moved. Intrusions are drawn as oddly shaped blobs. They “intrude” into the surrounding layers of rock. Both can cut across the other.

Law of Included Fragments IMPORTANT AKA inclusions Fragments in the rock are always older than the rock itself Pieces of rock from below found in layers of rock above

Unconformities Not a law, but important Shows a stop in deposition Erosion surface Irregular surface between layers Associated with included fragments

Let’s Practice… E G L C H M D J A N K B f Fix f to F Fix animation on B Unconformities between E and G, C and M, D and N, and F and B

Correlation of Rock Layers Rock layers in different places can be matched up Gives more information Must be near each other Carmel Formation and Navajo Sandstone Moenkopi Formation and Kaibab Limestone Missing second circle.

Radioactive Dating AKA Radiometric Dating or Numerical Dating Tells how old something is Uses radioactive isotopes Compare and contrast with relative dating.

Parent and Daughter Parent material: Daughter material: Radioactive or unstable Original material Daughter material: Non-radioactive or stable Altered or new material Parent material decays (or changes) into daughter material

Half-Life Expresses the rate of radiometric decay How long it takes for half of the parent material to turn into daughter material

How’s it Work? Uses known rates of decay Compare the ratio of parent material to daughter material Match it with the known rates of decay

Common Radioactive Isotopes Parent Daughter Half-Life (in years) Uranium-235 Lead-206 4.5 billion Uranium-238 Lead-207 713 million Thorium-232 Lead-208 14.1 billion Rubidium-87 Strontium-87 47.0 billion Potassium-40 Argon-40 1.3 billion

Tree Rings Count the rings to find the tree’s age Can show: Rainy seasons Dry seasons Problems like ash fall and forest fires

Varves Only form on the bottom of lakes Count the layers to find how old the lake was Two layers means one year

Geologic Timescale Broken into eons, eras, and periods Eons are the largest Periods are the smallest Periods are sometimes broken into epochs (but only recently)

Eons and Eras Eons: Eras: Phanerozoic: Precambrian: Paleozoic: “visible life” What does this mean? Precambrian: 88% Why isn’t it divided? Eras: Paleozoic: “early life” Mesozoic: “middle life” Cenozoic: “recent life”