We know Earth is 4.6 Billion years old BUT HOW DO WE KNOW?

How do we know Earth’s age? Most thought Earth was 6,000 years old and all geologic features had formed at the same time James Hutton theorized the same forces changing Earth’s surface now were working in the past uniformitarianism – geologic processes that occurred in the past can be explained by current geologic processes

Relative Age Layers of rock, called strata, show the sequence of events that took place relative age - age of an object in relation to the ages of other objects Relative age indicates one layer is older or younger than another layer not the rock’s age in years

What are the principles used to study Earth to figure out its age?

Law of Superposition law of superposition – layers on bottom are older than layers on top if undisturbed Study sedimentary rocks to determine relative age of rocks

Principle of Original Horizontality Principle of original horizontality - undisturbed sedimentary rocks will remain in horizontal layers Nonhorizontal layers were tilted or deformed by movements after the layers formed.

Unconformities unconformity - break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time

Types of Unconformities Nonconformity - stratified rock rests upon unstratified rock (igneous/metamorphic) Angular unconformity - boundary between a set of tilted layers and a set of horizontal layers. Disconformity – layer is uplifted w/o tilting/folding and eroded. Sediments continue to deposit.

What is the law of superposition? Agenda for Tuesday Nov 30th Finish notes Relative age activity

Cross-Cutting Relationships law of crosscutting relationships - principle that a fault or body of rock is younger than any other body of rock that it cuts through A fault is a break or crack in Earth’s crust where rocks shift their position An intrusion is a mass of igneous rock that forms when magma is injected into rock

Relative Age How old an object is in relation to other objects, not actual age If we do not know the actual age, how do we know Earth is 4.6 Billion Years Old?

Absolute Age absolute age - numeric age of an object or event

Determining Absolute Age rate of erosion – useful for geologic features that formed within the past 10,000 to 20,000 years rates of deposition – estimate the average rate of sediment deposition Use data collected over a long period of time varve - layer of sand and silt that deposited annually in a lake near ice sheets or glaciers

Radiometric Dating radiometric dating – comparing radioactive (parent) isotopes to stable (daughter) isotopes Radioactive isotopes have nuclei that emit particles and energy at a constant rate regardless of surrounding conditions

Half Life half-life – length of time it takes for half of the atoms to decay Years it takes to decay = half life The time required for half to decay is always the same

Carbon 14 Carbon dating (radiocarbon dating) is a variety of radioactive dating which is applicable only to matter which was once living Half life of carbon is 5730 years

Compare and Contrast Relative and Absolute Age. Agenda for Wednesday Dec 1st Radiometric Dating activity Relative Age of Rock Layers Review for Final

How Old are these rocks? Figure out how old the rock layers are (relative age) Which came first Remember our Laws/Principles Law of Superposition Principle of Original Horizontality 3 Unconformities Law of Cross Cutting Relationships

Radiometric Dating Problems A sample contains 20g of an isotope that has a half life of 1000 years. How much will be left after 2 half lives? 20g at 0 10 g at 1 half life 5 g at 2 half lives