Performance Indicator: 8.E.6A.2 Analyze and interpret data from index fossil records and the ordering of rock layers to infer the relative age of rocks and fossils.

STANDARD Standard 8.E.6 The student will demonstrate an understanding of Earth’s geologic history and its diversity of life over time.

What is the law of superposition? ESSENTIAL QUESTION What is the law of superposition?

8.E.6A.2 Analyze and interpret data from index fossil records and the ordering of rock layers to infer the relative age of rocks and fossils.

The 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: 1.ordering of rock layers 2. index fossils:

Relative and Absolute Ages Relative Age The age of a rock compared to the ages of rock layers. Ex: younger vs. older Absolute Age The age of a rock given as the number of years since the rock formed. Ex: 10, 20, 50, 100

PRACTICE

PRACTICE

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ORDERING OF ROCKS Scientists read the rock layers knowing that each layer is deposited on top of other layers. The law of superposition states that each rock layer is older than the one above it. So, the relative age of the rock or fossil in the rock is older if it is farther down in the rock layers. Relative dating can be used only when the rock layers have been preserved in their original sequence.

Position 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 and the youngest layer is at the top. Each higher layer is younger than the layers below it.

Complex layering due to intrusions and extrusions, faults, or unconformities can make dating rocks and fossils challenging.

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. It is considered a cross-cutting section.

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. It is considered a cross-cutting section. Movements along faults can make it harder for geologists to determine the relative ages of rock layers. (book figure 10 B)

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CHECK POINT WHAT IS THE DIFFERENCE BETWEEN RELATIVE AND ABSOLUTE AGE? WHAT DOES THE LAW OF SUPERPOSITION STATES HOW WOULD YOU EXPLAIN THE FOLLOWING: Extrusion Intrusion RELATIVE AGE OF ROCKS CAN BE DETERMINED BY WHAT 2 METHODS? Faults

Question Which is most likely to become a fossil?   A. a skeleton in a large lake B. a jellyfish in the ocean C. an earthworm in a damp forest D. a skeleton in a riverbed that is drying up

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.

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

Index Fossils Certain 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. ARE EXTINCT Found only in ONE rock layer Only lived for a Short period of time

TRILOBITES: A key example of an organism used as an index fossil are ___TRIOBITES___, 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.   Note: Fossils that are found in many rock layers, therefore living long periods of time, do not qualify as index fossils.

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!"

CHECK POINT 1. WHAT IS UNCONFORMITY? 2. NAME 3 CHARACTERISTICS OF INDEX FOSSILS. 3. WHAT IS THE MOST KNOWN INDEX FOSSIL?

PRACTICE TIME FINDING THE INDEX FOSSIL

1. What "fossil clues" in layers A and B indicate the kind of environment that existed when these rocks were formed? How did the environment change? 2. Which layer is the oldest? How do you know? 3. Whch of the layers fromed most recently? How do you know? 4. Why are there no fossils in layers C and E? 5. What kind of fossils are found in layer F? 6. Which layer at Site 1 might have formed at the same time as layer W as site 2? 7. What clues show an unconformity or gap in the horizontal rock layers? Which rock layers are missing? What might have happened to these rock layers? 8. Whch is older, intrusion V or layer Y? How do you know?

Radioactive element decay can also be used to tell the age of fossils and rocks. Until the middle of the last century, "older" or "younger" was the best scientists could do when assigning ages to fossils. There was no way to calculate an "absolute" age (in years) for any fossil or rock layer. But after scientists learned that the nuclear decay of radioactive elements takes place at a predictable rate, they realized that the traces of radioactive elements present in certain types of rock, such as hardened lava and tuff (formed from compacted volcanic ash), could be analyzed chemically to determine the ages, in years, of those rocks.

REVIEW

Law of Superposition: Oldest sedimentary rock layers will be on the bottom, youngest on top. Principle of Original Horizontality: When layers of sedimentary rock are first made, they are layered horizontally.

Rock Layers Can Change!  Rock layers can fold underground. Can you still figure out the order from oldest to youngest? Intrusions of igneous rock can cut through different layers, like A. The intrusion is always younger than the layers it cuts through (even if it comes from the bottom!).

A fault or crack could form in the middle of rock layers (B) A fault or crack could form in the middle of rock layers (B). If this happens, the fault is always younger than the layers it cuts through.

Task: Put the layers in order from oldest to youngest. Hints: Start at oldest first – look at the bottom layers Igneous intrusions are younger than the layers they cut through Fault lines are younger than the layers they cut through If igneous and metamorphic are side by side, metamorphic was there first

C, E, D, A, B

E, B, C, D, A

G, C, A, B, F, H, D, I, E

D, H, A, C, G, G, I B,E

H, O, B, K, L, J, A, F, M G, N, E, I, C, K

A, E, G, L, C, M, D, J, N, K, B

FIND THE INDEX FOSSIL

Performance Indicator 8.E.6A.5: Construct explanations for why most individual organisms, as well as some entire taxonomic groups of organisms, that lived in the past were never fossilized.

Because of the conditions necessary for their preservation, not all types of organisms that existed in the past have left fossils that can be retrieved.

In order for a fossil to form, the organism’s remains must not be significantly disturbed by a scavenger/decomposer or destroyed by erosion and other natural forces. Therefore, organisms or parts of organisms that make up fossils are most likely buried quickly and deeply.

Example: woolly mammoth found in ice, insects found in amber, animals found in peat bogs (A bog is a wetland that accumulates peat, a deposit of dead plant material—often mosses), mass burials from flash floods or volcanic ash falls

Soft body parts, such as skin, muscle, fat, and internal organs, deteriorate rapidly and leave no trace. Casts of such tissues are rarely found. Similarly, organisms that are soft-bodied creatures, like jellyfish, are very uncommon fossils while hard body parts (such as teeth and shells) fossilize easier.

Molds can be made of organisms Molds can be made of organisms. However, the organism must be buried in sediment after which its tissues dissolve and are replaced by dissolved minerals which make it a solid. Without the correct minerals this process cannot take place.

The fact that extremely few living things are preserved long enough after death to become fossils makes the large collections of fossils in the museums of the world quite remarkable. Taphonomy is the study of the conditions under which plants, animals, and other organisms become altered after death and sometimes preserved as fossils.

With molds, sometimes the rock has the appearance of the organism With molds, sometimes the rock has the appearance of the organism. Sometimes, all traces of the organism are lost but an external mold is formed around the body and is preserved. Sometimes an internal mold forms when material is precipitated inside an organism (ex: a marine shell or the hollow stem of a plant).