Finding the Relative and Absolute Age of Rocks Chapter 21 James Hutton, a Scottish geologist from the 1700’s, thought Earth was very old. His work lies at the foundation of UNIFORMITARIANISM – geologic processes occurring today have been occurring since Earth formed.

Relative and Absolute Ages Relative Age The age of a rock compared to the ages of rock layers. Absolute Age The age of a rock given as the number of years since the rock formed. LET’S START WITH RELATIVE AGE

Position of Rock Layers Its difficult to determine the absolute age so geologists use method to find a rock’s relative age. Use the: LAW OF SUPERPOSITION: in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Other Clues to Relative Age Clues from Igneous Rocks: 1. Lava that hardens on the surface is called an Extrusion (example – an eruption would put a layer of igneous rock on top of sedimentary rocks. Rock layers below an extrusion are always older than the extrusion.

The extrusion is in black

Now the extrusion is the youngest layer.

Clues from Igneous Rocks: 2. Magma that cools and pushes into bodies of rock and hardens is called an Intrusion An intrusion is always younger than the rock layers around and beneath it.

The intrusion (in red) is now younger than the surrounding rocks.

Draw this in your notes and label each of the parts listed below. Intrusion Extrusion Oldest rock Youngest rock Understand that these are all rocks that are millions of years old. You can use the color pencils I have provided. Which one would you draw first?

Fault: is a break in the Earth’s crust. Clues from Faults: Fault: is a break in the Earth’s crust. Forces inside the Earth cause movement of the rock on opposite sides of a fault. Fault is always younger than the rock it cuts through. Movements along faults can make it harder for geologists to determine the relative ages of rock layers. (book figure 10 B)

Footwall (block below fault) Hanging wall (block above fault)

REVIEW Geologists use the Relative and Absolute Age of rocks to determine age. Using the Law of Superposition Other clues are from Igneous rocks Extrusion Intrusion Clues from Faults

GAPS IN THE GEOLOGIC RECORD - Record of sedimentary rock layers is not always complete - Deposition slowly builds layers upon layer of sedimentary rock, BUT some of these layers may erode away, exposing an older rock surface. Unconformity – is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion.

An ANGULAR UNCONFORMITY is a disconformity between layers of different angles. The underlying layers are first tilted, then erosion scours away a new, horizontal surface. New, horizontal layers form on top

USING FOSSILS TO DATE ROCKS To date rock layers, geologists first give a relative age to a layer of rock at one location. THEN they can give the same age to matching layers of rock at other locations. This is called Correlations.

Grand Canyon (A), Chocolate Cliffs (B), Vermilion Cliffs (C), White Cliffs (D), Zion Canyon (E), Gray Cliffs (F), Pink Cliffs (G), Bryce Canyon (H)

USING FOSSILS TO DATE ROCKS Certain fossils, called Index Fossils help geologist match rock layers. INDEX FOSSILS – Fossils of widely distributed organisms that lived during only one short period.

Example of an Index Fossil: Trilobites (hard shelled animals whose bodies had three distinct parts. Trilobites evolved in shallow seas more than 500 million years ago. Over time, many types have appeared. They became extinct about 245 million years ago. They have been found in many different places.

To become a Index Fossil … a trilobite must be different in some way from other trilobites. Example – type with large eyes These large-eyed ..bites survived for a time AFTER other bites became extinct. If a geologist finds large-eyed Trilobites in a rock layer, the geologist can infer that those rocks are younger than rocks containing other types of trilobites

The World's Biggest Trilobite A team of Canadian paleontologists working along Hudson Bay in northern Manitoba has discovered the world's largest recorded complete fossil of a trilobite, a many-legged, sea-dwelling animal that lived 445 million years ago. The giant creature is more than 70 cm long (about 28 inches), 70 percent larger than the previous record holder. "This is an important and amazing find," says Bob Elias, a professor in the department of geological sciences at the University of Manitoba. "It looks like a huge bug!"

Absolute Age Dating All you need is a tiny sample of material (mineral, bone) no larger than a grain of rice. Gives us the true “age” of a fossil or rock Mainly organic tissue or igneous crystals Measure the amount of unstable isotopes that have “decayed” to figure out age

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay). Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods. Marie Curie Ernest Rutherford

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION The number of protons (the atomic number) is fixed for any element and is unique for each element but the number of neutrons in atoms of different elements can vary. Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element). Figure 3.3: Schematic representation of the isotopes of carbon. Carbon has an atomic number of 6 and an atomic mass number of 12, 13, or 14, depending on the number of neutrons in its nucleus. M&W4 Fig. 3.3; M&W5 Fig. 3.4 32

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Radioactive decay occurs when an isotope of one element is transformed into a different element by changes in the nucleus. There are three different decay mechanisms: “Parent” “Daughter” Figure 17.18: Three types of radioactive decay. (a) Alpha decay, in which an unstable parent nucleus emits 2 protons and 2 neutrons. (b) Beta decay, in which an electron is emitted from the nucleus. (c) Electron capture, in which a proton captures an electron and is thereby converted to a neutron. M&W4 Fig. 17.18; M&W5 Fig. 17.18

How can we tell age based on the number of parent isotopes? Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION 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

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION To the oldest materials ever dated by the radioactive method are found in the Jack Hills of western Australia and are tiny zircon grains contained in sandstones and conglomerates. The zircons are 4.4 billion years old. The very remote “outback” of western Australia--the Jack Hills Scanning electron microscope image of a Jack Hills zircon. Scale bar is 0.1 mm

The Half-Life of C14 is 5,730 years. 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? (end) M&W4 Fig. 17.24; M&W5 Fig. 17.24