Presentation on theme: "ordering of rock layers."— Presentation transcript:
1ordering of rock layers. Standard 8-2.6Infer the relative age of rocks and fossils from index fossils and theordering of rock layers.
2Name and give 3 facts about the main 3 types of rocks. Falcon FocusName and give 3 facts about the main 3 types of rocks.
3What is the law of superposition? ESSENTIAL QUESTIONWhat is the law of superposition?
4Objective FF EQ Intro to Law of Superposition Take Notes Brief Review for Fossil QuizTake Fossil QuizClosureNote: Mosby points are due tomorrow!!!
5Finding the Relative Age of Rocks Standard – Infer the relative age of rocks and fossils from index fossils and the ordering of the rock layers.
6Extra Question Which is most likely to become a fossil? A. a skeleton in a large lakeB. a jellyfish in the oceanC. an earthworm in a damp forestD. a skeleton in a riverbed that is drying up
7Essential QuestionExplain how the law of superposition can be used to determine the relative ages of rocks.
8The relative age means the age of one object compared to the age of another object. Relative age does not tell the exact age of an object. The relative age of rocks and fossils can be determined using two basic methods: ordering of rock layers and index fossils:
9Relative and Absolute Ages Relative Age The age of a rock compared to the ages of rock layers.Ex: younger vs. olderAbsolute Age The age of a rock given as the number of years since the rock formed.Ex: 10, 20, 50, 100
10Position of Rock Layers Its difficult to determine the absolute age so geologists use methods 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.
11Other 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.
14Clues 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.
15The intrusion (in red) is now younger than the surrounding rocks.
16Draw 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?
18Fault: 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)
22REVIEW Geologists use the Relative and Absolute Age of rocks to determine age.Using the Law of SuperpositionOther clues are from Igneous rocksExtrusionIntrusionClues from Faults
23GAPS 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.
24USING 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.
28Index FossilsCertain fossils, called index fossils, can be used to help find the relative age of rock layers. To be an index fossil –an organism must have lived only during a short part of Earth’s history;many fossils of the organism must be found in rock layers;the fossil must be found over a wide area of Earth;the organism must be unique.
29The shorter time period a species lived, the better an index it is The shorter time period a species lived, the better an index it is. A key example of an organism used as an index fossil are trilobites, a group of hard-shelled animals whose body had three sections, lived in shallow seas, and became extinct about 245 million years ago. Therefore, if a trilobite is found in a particular rock layer, it can be compared with trilobites from other layers to estimate the age of the layer in which it was found.
30Fossils that are found in many rock layers, therefore living long periods of time, do not qualify as index fossils.
31Example 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.
33To 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
34The 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!"