Hello Studiers of the Earth! DQ: Place these rock layers in order from oldest to youngest Today: Go Over Rock Strata Practice One More Time… Absolute Time

How about this one? OR B A A B D D E E C C

MEASURING ABSOLUTE TIME TREE RINGS EACH RING REPRESENTS A SINGLE YEAR (SPRING/FALL) THE WIDTH OF THE RING DEPENDS UPON THE TEMPERATURE AND RAINFALL VARVES GLACIAL LAKE DEPOSITS. A THICK LIGHT COLORED LAYER IN THE SUMMER AND A THIN DARK LAYER IN THE WINTER

RADIOMETRIC DATING USED TO DETERMINE AGES OF OBJECTS THAT ORIGINATED FAR BACK IN TIME. CERTAIN ROCKS CONTAIN RADIOACTIVE ISOTOPES RADIOACTIVE ISOTOPES ARE ATOMS OF ELEMENTS THAT GIVE OFF RADIATION FROM THEIR NUCLEI RADIOACTIVE DECAY IS THE PROCESS BY WHICH A RADIOACTIVE ISOTOPE CHANGES INTO A NEW STABLE ELEMENT

But Wait!!! – How does this work? Atoms always have the same number of protons but can have different numbers of neutrons Example: Carbon Always has 6 protons Can have different numbers of Neutrons Each of these is an isotope of carbon SOURCE:sjsugeology.org/.../powerpoint/Petsche_Relative_and_Absolute_Dating.

Absolute Dating: Big Ideas Some isotopes are unstable (radioactive) They decay or change into something else by losing parts of the atom Radioactive isotopes change from one form to another at a steady pace We call the original (unstable) isotope the parent isotope and the new form the daughter isotope

Absolute Dating: Big Ideas Half-Life is the time it takes for half of the original, unstable isotope of the element (parent) to change into the daughter isotope Different radioactive isotopes decay at different rates We can figure out how old a sample is by comparing the amount of parent material to the amount of daughter material and calculating the number of half-lives (half-lifes?) it takes to get that ratio

Absolute Dating: Big Ideas For radioactivity dating we use igneous rocks and minerals. The clock starts when radioactive atoms that are present in the magma get incorporated in the crystalline structure of certain minerals in the rocks. The crystals containing the parent atoms form and so we then have a “container” with parents that can begin decaying to form daughters. We can then use measure the parent-daughter ratio. This is our “atomic clock” that records the time since the rock crystallized. Figure 17.21: (a) Magma contains both radioactive and stable atoms. (b) As magma cools and begins to crystallize, some radioactive atoms are incorporated into certain minerals because they are the right size and can fit into the crystal structure. Therefore, at the time of crystallization, the mineral will contain 100% radioactive parent atoms and 0% stable daughter atoms. (c) After one half-life, 50% of the radioactive parent atoms will have decayed to stable daughter atoms. M&W4 Fig. 17.21; M&W5 Fig. 17.21 SOURCE:sjsugeology.org/.../powerpoint/Petsche_Relative_and_Absolute_Dating.

Absolute Dating: Big Ideas C14 is an isotope of carbon that forms from Nitrogen in the atmosphere. Living things consume this radioactive carbon. Once dead, no new carbon is absorbed, and C14 turns back into Nitrogen. The Half-Life of C14 is 5,730 years. This method works best for fossils younger than 50,000 years. Why? SOURCE:sjsugeology.org/.../powerpoint/Petsche_Relative_and_Absolute_Dating. M&W4 Fig. 17.24; M&W5 Fig. 17.24

Rate of Decay The composition of rock containing radioactive elements changes slowly over time. The amount of radioactive element decreases while the amount of new element increases. The rate of decay for a specific element is constant. The half-life of an element is the time it takes for half of the radioactive atoms to decay. SOURCE: teachers.mvwsd.org/.

Example Isotope A has a half life of 999 years How long will it take for a 4 gram sample of Isotope A to decay to 1 grams of A and 3 grams of daughter material? Well… After 999 years there will be 2 grams of A and 2 grams of daughter material After another 999 years, there will be 1 gram of A and 3 grams of daughter material 999 999

Another one… Radium-226 has a half-life of 1,600 years. If we start out with 8000 atoms of radium-226, how much would be left after 3,200 years?

Divisions of Geologic Time Eras are subdivided into periods...periods are subdivided into epochs. Era Period Epoch E + P = EP

Divisions of Geologic Time Geological time begins with Precambrian Time. Precambrian time covers approximately 88% of Earth’s history.

FOUR Eras… PRE-CAMBRIAN – 88% of earth’s history Paleozoic (ancient life) 544 million years ago…lasted 300 million yrs Mesozoic (middle life) 245 million years ago…lasted 180 million yrs Cenozoic (recent life) 65 million years ago…continues through present day

Today… Today we are in the Holocene Epoch of the Quaternary Period of the Cenozoic Era. Which unit is the largest? Which unit is the smallest?

Today… Today we are in the Holocene Epoch of the Quaternary Period of the Cenozoic Era. Which unit is the largest? Which unit is the smallest?