4Position 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.
5Other 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.
8Clues from Igneous Rocks: 2. Magma that cools and pushes into bodies of rock and hardens is called an IntrusionAn intrusion is always younger than the rock layers around and beneath it.
9The intrusion (in red) is now younger than the surrounding rocks.
10Draw this in your notes and label each of the parts listed below. IntrusionExtrusionOldest rockYoungest rockUnderstand 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?
12Fault: 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)
16REVIEW Geologists use the Relative and Absolute Age of rocks to determine age.Using the Law of SuperpositionOther clues are from Igneous rocksExtrusionIntrusionClues from Faults
17GAPS 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.
19USING 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.Certain fossils, called Index Fossils help geologist match rock layers.INDEX FOSSILS – Fossils of widely distributed organisms that lived during only one short period.
24Example 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.
26To become a Index Fossil … a trilobite must be different in some way from other trilobites. Example – type with large eyesThese 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
28The 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!"
29You can use INDEX FOSSILS to match rock layers. Page 117 GFigure 13Adobe Info sheet on Trilobites