Title Relative Age and the Laws of Geologic History

M. d’Alessio, 2004 Murder Mystery

LLaw What we see in the present is a key to what happened in the past. Law of Uniformitarianism

Wind and Process Death Valley, CA Copyright Marli Miller AGI Image Bank photo ha45z2 Zion, Utah Copyright Marli Miller AGI Image Bank photo ha45jb PROCESS ANCIENT PRESENT “The present is the key to the past.” James Hutton Famous Geologist (lived ) fundamentals/10c.html

In a horizontal layer of rock, the youngest layer is on top and the oldest layer is on the bottom. Law of Superposition

Law of Original Horizontality Rock layers have been laid down horizontally.

Grand Canyon, Arizona, Copyright Larry Fellows Originally Horizontal OLDER YOUNGER Timing Layers on top of layers

Watching Layers Form Larry Ridenhour, BLM/Jennifer Loomis, TERC

Tilted Layers

USGS/Jennifer Loomis, TERCTilted limestone beds in the Mojave Desert, California Watching Layers Tilt

An unconformity is a break in the fossil record. Layers of rock are missing due to erosion. Unconformity

Olympic Coast, Washington: 4th Beach near Kalaloch Copyright Patti Bleifuss

Fold Marin Headlands, California -- Image from Roland Bürgmann

USGS/Tom Grace, TERCFolded layers in the Sangre de Cristo Range of Colorado. Watching Layers Fold

Rock layers that cut through other layers of rock are younger than the rock layers they cut through. Law of Cross Cutting Relationships

Cross Cutting Grand Canyon, Arizona -- Copyright Ramón Arrowsmith Cross Cutting MOST RECENT

Faults A fault is a break in a rock layer. A fault is always younger than the rock layer it breaks through.

Fault

Intrusion An intrusion is a layer of igneous rock that has melted through a rock layer. Intrusion

l An intrusion is always younger than the rock that it cuts through. Intrusion H is younger than rock layers B, C, D and E. So is intrusion D. Layer A Layer B Layer C Layer D Layer E

extrusion- a hardened layer of lava that has cut through a layer of rock. An extrusion is always younger than the rock it cuts through.

Geologic Time OLDER YOUNGER Million Years Old 600+ Million Years Old TIMING: RELATIVE vs. ABSOLUTE

Relative Age of Rock Paleontologists date rock using relative age and absolute age. Relative age is the age of rock compared to the rock around it. Absolute age is the exact age of rock determined by a technique called radioactive dating.

Example of Relative Age: I am younger than my brother. Rock layer B is older than rock layer C. Example of Absolute Age: I am 11 years old. Rock layer B is 300 million years old.

Index Fossils Index Fossil An index fossil helps to determine the relative age of rock. To be an index fossil, an organism: 1) must have lived for a short period of time but is now extinct and 2) must have lived in many different locations. extinct- means an organism once lived but will never live again.

fp&wcsuffix=2042&fuseaction=home.gotoWebCode&x=0&y= 0http:// fossil interactiveindex fossil interactive

A How Old is That Rock? Absolute Age

The age of a rock in years is called its absolute age. Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock. A common "parent-daughter" combination that geologists use is radioactive uranium and non-radioactive lead. As shown in the diagram above, uranium is trapped in a newly formed rock. As the rock ages, more and more of the uranium changes into lead.

M. d’Alessio, 2004 Murder Mystery

1.Which is older, layer 1 or layer 4? How do you know? 2.Which is older, layer 3 or layer1? How do you know? 3.What is the irregular surface of 4 called? 4.What happened at point 2? 5.How do geologists know what happened in the past?