Age Dating and the Oceans

Transcription

1 Age Dating and the Oceans 1. Age dating of geologic materials reveals (a) the age of the Earth, and (b) how long oceans have been on Earth. 2. Relative dating of fossils from the seafloor allows us to see age progression of the ocean floor, revealing the operation of tectonics on Earth. 3. Age dating combined with magnetism in rocks allows us to (a) date the seafloor, and as with (2) above reveal the processes by which ocean basins are made. (b) Show the movement of continents and thus tectonic plates over time. What is the age of the Earth? A million years B. 1 million years C billion years D years Age dating uses radioactive isotopes to measure time Are radioactive elements naturally occurring? A. Yes B. No Let s not get too deep into this. 1. Basically, a radioactive parent isotope decays to a radiogenic daughter. 2. We measure relative abundances of daughter and parent isotopes to calculate ages. 3. This is fraught with fascinating issues that we don t have time for in Geol

2 Half-life & Daughter- Parent ratio As time proceeds, number of Daughters grow, parents diminish. Thus the ratio of parents to daughters (D/P) is a function of time! Animate Be-11, F-23 Isochron animation What happens to the Daughter/ Parent ratio after many half-lives? A. It becomes very large B. It becomes very small So, can very young rocks be easily age dated? A. Yes B. No 2

3 We cannot age date anything we choose Not all rocks contain appropriate isotopes (composition), such as those of the ocean floor! Age must be within magnitude of half-life or there won t be enough parent or daughter isotope present to measure the D/P ratio. Many rocks are Open Systems - they loose parent or daughter during their long history. The problem with dating Earth: No rocks on the surface of Earth are as old as Earth. This is because plate tectonics destroys old rocks and creates new ones as time progresses. So, to age date Earth, we needed to be a bit creative Age of the Earth: Patterson Slope of isochron = function of time = 4.55 billion years old 3

4 Some results of age dating Geologic Time Scale Oxygen atmosphere at ~ Ga First continental crust and oceans at 4.4 Ga Planets form in 1st 0.2% of SS history More fun examples of geologic time Register roll: if 1 Ma = 1 inch, then earth history = 379 feet of paper or 126 yards. Driving time: if 1 year = inch then earth history = 71,811 miles At 75 mph, it would take 957 hours or 40 days of continuous driving. (driving at 1320 inches/second). Counting: 1 yr = 1 sec 4.55e9 sec = 7.6E7 min = 1.3E6 hr = 5.3E4 days = 144 years Above is the concept of Absolute age. What follows is the concept of relative age Relative age dating places rocks into proper sequence of events. Is one rock (event) older or younger than another? 4

5 Principle of Original Horizontality Steno: 1600s Sediment (& lava) deposited horizontally in sheets. Principle of Superposition: Steno s If sediment is deposited in layers, then Oldest layers are on bottom provided they have not been disrupted (e.g. faulted) Principle of Continuity The layers of sediments (and lavas) are continuous over great distances. Thus, can infer that they continue across eroded distances. 5

6 Principle of Fossil Succession Fossils are remnants of previously living organisms preserved in rock or sediment Smith - late 1700s noticed: Fossils occur in groups of species (assemblage) Fossil organisms succeed one another in definite order vertically through a sediment column. He correctly reasoned that one could group/correlate rock units over distance based on their common fossil assemblage i.e., Fossil assemblages are distinct in time Loss of a fossil organism from an assemblage = extinction (Curvier, early 1800s) Draw fossil succession concept Fossil succession & relative time Given that fossil assemblages change over time due to evolution and extinctions, Then, distinct fossil assemblages can be used to assign relative age of the rocks in which they are preserved. Fossils can be used to correlate sediments ages over great distances 6

7 Dating rocks dates events in Earth history This is the very complicated science of stratigraphy The results include the geologic time scale First Mammals ~ 220 Ma First Marine Mammals ~ 60 Ma Early apes ~ 24 Ma Humans (Australopithecus) diverge from apes ~ 8-5 Ma Earth History Extinction of ~90% of life Vertebrates on land ~354 Ma Fish radiate in Devonian time Plants invade land, fish develop jaws ~440 Ma Cephalopods are main marine predator until ~360 Ma Fish begin to evolve before 500 Ma First fossil ~3.5 Ga - likely Marine Time Scale compressed to 1 year 7

8 Age dating applied to oceans We will apply the concepts developed above to understanding the age of the ocean basins. In particular, these concepts show us that the age of the ocean basins changes over the surface. This will prove to us the operation of tectonics. Before we get there, I want to develop the concepts of tectonics. We will return to age dating as it becomes appropriate. Review Questions What is the age of the Universe, our solar system, the Sun, and Earth? Who first successfully age dated the Earth and how was it done? Why can radioactive decay be used to determine age? Why can t all rocks be dated by radiogenic decay methods? Important Dates: When did plate tectonics begin on Earth and thus producing oceans and continents? When did the atmosphere become oxygen rich? What is the age of The first evidence of life on Earth The first evidence of vertebrate animals (those with backbones) The first evidence of Mammals The divergence of Humans from Apes The first fishes Think about the oceans and continents that existed 1 billion years ago. How might they be different from today? Review Questions Given a sequence of sediments, how do you know the rocks at the base of the sequence are oldest (what principles apply)? Consider Fossil Succession: Who first recognized this principle? What is an index fossil How is this principle used to indicate relative age? 8