PSC 121 Prince George’s Community College Reading the Rock Record

M. d’Alessio, 2004

PSC 121 Prince George’s Community College List all of the OBSERVATIONS that you saw in the picture

In Studying Earth History, geologists make use of 3 main ideas:  The rock record provides evidence of geological events and life forms of the past  Processes observed on Earth in the present also acted in the past  Earth is very old and has changed over geologic time.

PSC 121 Prince George’s Community College What Information do Strata Provide to a Geologist? To answer this question we need to look at some geologic principles. #1 Processes that occurred many millions of years ago probably happened the same way as they do today (because the science principles are the same). Uniformitarianism 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

PSC 121 Prince George’s Community College What Information do Strata Provide to a Geologist? #2 Sedimentary deposits or lava flows generally form horizontal layers.

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

PSC 121 Prince George’s Community College What Information do Strata Provide to a Geologist? #3 Unless disturbed by an unusual process, younger layers will be on top of older layers. Law of Superposition Law of Superposition youngest oldest

PSC 121 Prince George’s Community College Here are the layers in the Grand Canyon shown graphically. The Bright Angel shale is younger than the Tapeats Sandstone but older than the Muav Limestone. The Vishnu Schist is the oldest formation (made of metamorphic rocks).

Whodunit??? PSC 121 Prince George’s Community College

What Information do Strata Provide to a Geologist? #4 When an igneous intrusion or fault cuts across other rocks of any type the intrusion is younger than the layers that are cut across.

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

PSC 121 Prince George’s Community College What Information do Strata Provide to a Geologist? #5 Fossils can be used to determine the approximate age of rock layers since specific living organisms lived over identified time periods. If a particular fossil is present in a layer, the layer must be the same age as other layers containing the same fossil, no matter where the layers occur. If a particular fossil is present in a layer, the layer must be the same age as other layers containing the same fossil, no matter where the layers occur.

Principles of relative dating  Unconformity An unconformity is a break in the rock record produced by erosion and/or non- deposition of rock units

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

PSC 121 Prince George’s Community College Layers 1-4 are positioned oldest to youngest (superposition) Formation 5 is younger than 1-4 since it cuts through them Fault 6 is youngest because it cuts through 5

Let’s Practice!! In the next few slides you will try to put the layers in order from oldest to youngest using the principles we just learned about. PSC 121 Prince George’s Community College

A B C D E F G X X X Ignore the X’s A - G

F G H A - H

A - J

Tilted Layers

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

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

Formation of an angular unconformity Formation of an angular unconformity 1.Deposition 2.Folding and uplifting 3.Erosion 4. Deposition

Several unconformities are present in the Grand Canyon

PSC 121 Prince George’s Community College Why Aren’t All Strata Straight?  Geologic processes generate pressures that cause layers to become deformed.  The first type of deformation is folding. Click on image to begin animation

Fold Fold Marin Headlands, California -- Image from Roland Bürgmann

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

Syncline- folds in which the youngest rock in its center or core. -trough shaped

PSC 121 Prince George’s Community College Anticline – folds in which the oldest rock lies in the center or core -arch shaped

PSC 121 Prince George’s Community College Folds

Monocline - a step-like fold consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.folddip

PSC 121 Prince George’s Community College anticline syncline

PSC 121 Prince George’s Community College Why Aren’t All Strata Straight?  Geologic processes generate pressures that cause layers to become deformed.  The second type of deformation is faulting.

PSC 121 Prince George’s Community College Faults  Faults occur when sections of Earth’s surface shift position with respect to another  One side of the fault is designated as a footwall block and the other a hanging wall block

2. Faults – Break in the Earth’s crust

Normal Fault- Caused by tension between the plates. (Plates being pulled apart.) Hanging wall moves down relative to the footwall Reverse Fault- Caused by compression between the plates. Hanging wall moves up relative to the footwall

PSC 121 Prince George’s Community College Normal Faults

PSC 121 Prince George’s Community College Hanging wall moves down NORMAL FAULT

PSC 121 Prince George’s Community College Reverse Faults

PSC 121 Prince George’s Community College Hanging wall moves upward REVERSE FAULT

PSC 121 Prince George’s Community College Strike-Slip or Transform Faults

PSC 121 Prince George’s Community College TRANSFORM FAULTS – aerial view

San Andreas Fault  Moving two inches per year.

PSC 121 Prince George’s Community College Fault Animations

PSC 121 Prince George’s Community College Can you date the features from oldest to youngest?

Using radioactivity in dating  Reviewing basic atomic structure  Nucleus –Protons – positively charged particles with mass –Neutrons – neutral particles with mass –Electrons – negatively charged particles that orbit the nucleus

Using radioactivity in dating  Reviewing basic atomic structure  Atomic number –An element’s identifying number –Equal to the number of protons in the atom’s nucleus  Mass number –Sum of the number of protons and neutrons in an atom’s nucleus

Using radioactivity in dating  Reviewing basic atomic structure  Isotope –Variant of the same parent atom –Differs in the number of neutrons –Results in a different mass number than the parent atom

Using radioactivity in dating  Radioactivity  Spontaneous changes (decay) in the structure of atomic nuclei

PSC 121 Prince George’s Community College How Can We Determine Age More Accurately?  Radioactive elements are unstable and their nuclei will release particles or energy to stabilize.  Each radioactive element has a half-life – the time it takes for one half of the atoms in a sample to disintegrate  Some half-lives are fractions of a second while some are as much as 4.5 million years.

Using radioactivity in dating Using radioactivity in dating  Parent – an unstable radioactive isotope  Daughter product – the isotopes resulting from the decay of a parent  Half-life – the time required for one- half of the radioactive nuclei in a sample to decay

A radioactive decay curve A radioactive decay curve

PSC 121 Prince George’s Community College After 1 half-life has passed, half the atoms have changed or emitted energy. After a 2 nd half-life has passed half of the remaining atoms have also changed Radiometric Dating

Using radioactivity in dating  Radiometric dating  Principle of radioactive dating –The percentage of radioactive atoms that decay during one half-life is always the same (50 percent) –However, the actual number of atoms that decay continually decreases –Comparing the ratio of parent to daughter yields the age of the sample

Using radioactivity in dating  Radiometric dating  Useful radioactive isotopes for providing radiometric ages –Rubidium-87 –Thorium-232 –Two isotopes of uranium –Potassium-40

PSC 121 Prince George’s Community College We can use radioactive elements in the rocks to help determine age. If the rock contains a radioactive element like potassium-40, we can measure how much of this element is present. Knowing how much should be there we can determine the rock age. Determining Rock Ages

PSC 121 Prince George’s Community College Suppose a rock sample has 1/64 th of the expected amount of radioactive material. 1  1/2  1/4  1/8  1/16  1/32  1/64 This means that the sample has been through 6 half-lives. If each half-life is 100,000 years then the rock layer is 600,000 years old. Determining Rock Ages

PSC 121 Prince George’s Community College Rock Layers  Sediments and lava flows form distinct layers as they are deposited  A sedimentary layer that can be distinguished from the layer above and below is called a stratum (pl. strata)