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 http://www.msnucleus.org/membership/html/jh/earth/sedimentary/lesson5/sedimentary5d.html PROCESS ANCIENT PRESENT “The present is the key to the past.” James Hutton Famous Geologist (lived 1726-1797) http://www.physicalgeography.net/ 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
USGS/Jennifer Loomis, TERCTilted limestone beds in the Mojave Desert, California http://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page04.cfm. Watching Layers Tilt
An unconformity is a break in the fossil record. Layers of rock are missing due to erosion. Unconformity
Fold Marin Headlands, California -- Image from Roland Bürgmann http://www.seismo.berkeley.edu/~burgmann/EPS116/labs/marin2004/Fold%20-%20John%20R%202.JPG
USGS/Tom Grace, TERCFolded layers in the Sangre de Cristo Range of Colorado. http://www.classzone.com/books/earth_science/terc/content/investigations/es2903/es2903page05.cfm?chapter_no=investigation 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 http://activetectonics.la.asu.edu/ramon/Images/Grand_Canyon/36grandcanyon.tif.jpg 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.
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 http://www.astro.lsa.umich.edu/users/cowley/grand33.jpg 270 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.
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.
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?