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CM Earth Science 5

Unit 5 Mr. Wilbur/Barbis: Chapter 8/RB 6

Question	Answer
How did James Hutton’s (18th century Scottish physician & farmer) theory change the ideas of the Earth’s age?	Hutton theorized same forces changed landscape of his farm that changed the Earth’s surface in the past;developed principle of uniformitarianism used today;scientists refined this theory to say that it happened but that rates of processes vary over time.
Describe Uniformitarianism and give examples of the processes	Principle that geologic processes that occurred in the past can be explained by current geologic processes such as volcanism and erosion
What is meant by “relative age”?	The age of an object in relation to the ages of other objects.
Describe the Law of Superposition.	Law that a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers have not been disturbed.
What orientation do sedimentary rock layers have as a result of the process of deposition?	The principle of original horizontality explains that sedimentary rocks left undisturbed will remain in horizontal layers.
Features of deposition	graded bedding; cross-beds; ripple marks
Graded bedding	arrangement of layers where coarse, heavy particles are located in bottom layers
Cross-beds	when sand is deposited, sandy sediments form curbed beds at an angle to the bedding plane; as sand slides down it forms slanting layers
Ripple marks	small waves that form on the surface of sand because of the action of water or wind
Unconformity	A break in the geologic record created when rock layers are eroded or when sediments are not deposited for a long period of time
How does erosion produce an unconformity?	Erosion creates an unconformity because it wears away sections or layers, causing a break in the geologic record
3 types of unconformity	nonconformity; angular unconformity; disconformity
Nonconformity	stratified rock rests on unstratified rock
Angular unconformity	rock deposited in horizontal layers is then folded or tilted then eroded.
Disconformity	boundary between horizontal layers of old sedimentary rock and younger, overlying layers that are deposited on an eroded surface
Law of Crosscutting	Principle that a fault or body of rock is younger than any other body of rock that it cuts through
Absolute age	Numerical age of an object or event often stated in years before present. Scientists use different methods, both mechanical and chemical, to determine the numeric age of rock formations
There are mechanical and chemical means of determining absolute age, and which is better?	Chemical, because mechanical means can vary a great deal over time (faster or slower)
3 mechanical processes that don't help determine age of rocks	Rates of erosion;Rates of deposition;Varve count
Rates of erosion	good for features that formed within the past 10,000-20,000 years
Rates of deposition	using data collection over a long period of time, estimating average rate of deposition to common sedimentary rock layers
Varve count	banded layers of sand deposited annually in a lake especially near ice sheets or glaciers`
Radiometric dating	Determines the absolute age by comparing relative percentage of radioactive parent isotopes and stable (daughter) isotopes. Atoms of same element have different number of neutrons called isotopes
What is half-life?	Time required for half of a radioactive isotope to breakdown by radioactive decay to form daughter isotope.
Limitations to using radioactive isotopes to tell dates of items	Limited to 7 half-lives (not enough parent isotope for accurate measurement);Not all rocks contain them;Fresh samples of rock needs to be used to avoid contamination
3 Reasons not all radioactive isotopes can be used for geologic dating	Other isotopes are: 1) too rare 2) decay too slowly 3) decay too rapidly
Carbon-14 (radioactive)	decays over a period of 5,700 years (half-life); used to date recent organic remains
Uranium-238	decays to form led-208; half-life 4.5 billion years; used to date igneous rocks; best if sample is older than 10 million years old
Rubidium-87	decays to form strontium-87; half-life 49 billion years; used to date extremely old rocks
Potassium-40	decays to form calcium-40 or argon-40; half-life 1.3 billion years; used to dated igneous /metamorphic and sometimes sedimentary rocks; potassium is a very common element found in organisms (shells/bones) and clay
fossils	A trace or remain of living organisms found in sedimentary rocks
Paleontology	Scientific study of fossils
Why do sedimentary rocks commonly contain fossils?	Dead organisms quickly buried can become fossils such as wood, bones, shells, teeth. In rare cases, entire organisms are preserved; could also be an imprint.
Why are they not found in other types of rocks?	There is extreme heat and pressure causing melting in metamorphic and igneous is formed from lava
Why don’t all organisms become fossils?	Some animal or plant remains are destroyed before being buried
How do fossils form?	Sand or silt is blown over or washed over the animal or plant and it hardens. More layers are added to the top of this.
Mummification	formed in dry places
Tar seeps	stuck in tar
Amber	hardened tree sap; DNA record preserved
Freezing	frozen in ice
Petrification	mineral solutions such as ground water replace original organic materials
Molds/casts	shells leave open space and hardened; sedimentation filled in spaces creating cast
imprints	carbonized imprints of leaves, stems, flowers and fish made in soft mud or clay
Coprolites	fossilized feces
Gastroliths	stones in digestive system of organism
Trace fossils	fossilized mark that formed in sedimentary rock by the movement of an animal on or within soft sediment (footprint, borings, burrows) Use index fossils to give relative age of rock and rock layers. Use rock to date fossil
Index fossils	fossil used to establish age of rock layers because it is distinct, abundant, and widespread and existed for only a short span of geologic time (ammonite)
Geologic column	Order arrangement of rock layers that is based on relative ages of rocks, oldest at bottom; no single area will have all geologic layers due to erosion
Divisions of geologic time	Marked by major changes in Earth’s surface; climate and types of organisms;Used to divide geologic time into smaller units;Rocks grouped within each unit contain similar fossils, generally characteristic of fossils of a dominant life form
Eons	Largest unit of time—divided into 4; 1st 3--Precambrian;88% most of Earth’s history;Little known about it, greatly deformed; 4 billion years of change
Eras	2 or more periods
Periods	Characterized by specific fossils;Named for location in which fossils first discovered
Epochs	Divided into smaller units, age
Ages	Defined by occurrence of distinct fossils

Created by: vbarbis

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