Chapter 3 The Rock & Fossil Record Our World in One Minute!!` http://www.youtube.com/watch?v=ZSt9tm3RoUU
The Principle of Uniformitarianism • For 30 years a Scottish farmer and scientist named James Hutton studied rock formations in Scotland and England. • He was trying to answer questions like, “how do mountains form,” “how is new rock created,” and “how old is the Earth” • His observations led to the foundation of modern geology – the study of the Earth, the materials of which it is made, the structure of those materials, and the processes acting upon them
In 1788, Hutton collected his notes and wrote Theory of the Earth. • In this book he stated that the key to understanding Earth’s history was all around us. • What he meant was that the processes that we observe today, such as erosion and deposition, remain uniform, or do not change, over time. • Uniformitarianism is the idea that the same geologic processes shaping Earth today have been at work throughout Earth’s history.
Uniformitarianism Versus Catastrophism • Catastrophism is the principle that states that all geologic change occurs suddenly. • Supporters thought that Earth’s features, such as mountains, canyons, and seas, formed during rare, sudden events called catastrophes
In Hutton’s time most people thought the world was only a few thousand years old. • A few thousand years was not enough time for the gradual geologic processes that Hutton described to have shaped our planet.
A Victory for Uniformitarianism • Despite Hutton’s work, Catastrophism remained geology’s guiding principle for decades. • From 1830 – 1833 a British Geologist named Charles Lyell published three volumes, collectively titled Principles of Geology, in which he reintroduced Uniformitarianism. • Armed with Hutton’s notes and new evidence of his own, Lyell saw no reason to doubt that major geologic change happened at the same rate in the past as it happens in the present – gradually.
Modern Geology – A Happy Medium • During the late 20th century, scientists such as Stephen J. Gould challenged Lyell’s Uniformitarianism. They believed that catastrophies do, at times, play an important role in shaping Earth’s history. • Today, scientists realize that neither Uniformitarianism nor catastrophism accounts for all geologic change throughout Earth’s history
Although most geologic change is gradual and uniform, catastrophies that cause geologic change have occurred during Earth’s history • Huge craters have been found where asteroids and comets are thought to have struck Earth in the past • One being the asteroid that could have potentially killed off the dinosaurs.
Paleontology – The Study of Past Life • The study of past life is called paleontology. • Scientists who study this are called paleontologists. • The data these scientists use are called fossils. • Fossils are the remains of organisms preserved by geologic processes. Edwin Colbert was a 20th-century vertebrate paleontologist who made important contributions to the study of dinosaurs http://www.youtube.com/watch?v=g_pUwYbFgpY
Geologists try to determine the order in which events have happened during Earth’s history. • They rely on rocks and fossils to do so. • Determining whether an object or event is older or younger than other objects or events is called relative dating.
The Principle of Superposition • Layers of sedimentary rock provide a sequence of history. As you move to the bottom, the layers are older. • Superposition is the principle that states that younger rocks lie above older rocks in undisturbed sequences.
Some rock layers are disturbed by forces within the Earth; these forces can push other rocks into a sequence, tilt or fold rock layers, and break sequences into movable parts. • When rocks sequences are disturbed, or out of order, geologists can use the geologic column as a tool to help.
The Geologic Column • The geologic column is an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth, arranged from oldest to youngest. • Geologists use this column to identify the layers in puzzling rock sequences.
Disturbed Rock Layers 4 Ways Rock Layers become disturbed: • Fault: a fault is a break in the Earth’s crust along which blocks of the crust slide relative to one another • Intrusion: An intrusion is molten rock from the Earth’s interior that squeezes into existing rock and cools • Folding: Folding occurs when rock layers bend and buckle from Earth’s internal forces • Tilting: Tilting occurs when internal forces in the Earth slant rock layers.
Gaps in the Record – Unconformities • Sometimes layers of rock are missing altogether, creating a gap in the geologic record. • An unconformity is a surface that represents a missing part of the geologic column. • Unconformities also represent missing time – time that was not recorded in layers of rock.
Unconformities are created by: • Nondeposition – the stoppage of deposition when a supply of sediment is cut off • Erosion
Types of Unconformities • Disconformities • The most common type • Are found where part of a sequence of parallel rock layers is missing • Can form when a sequence of rock layers is uplifted. Younger layers at the top of the material is deposited elsewhere. At some future time, deposition resumes, and sediment buries the old erosion surface. The disconformity that results shows where erosion has taken place and rock layers are missing • Represents thousands to millions of years of missing time
Nonconformities • Are found where horizontal sedimentary rock layers lie on top of an eroded surface of older intrusive igneous or metamorphic rock • Represent millions of years of missing time
Angular Unconformities • Are found between horizontallayers of sedimentary rock and layers of rock that have been tilted or folded. • The tilted or folded layers were eroded before horizontal layers formed above them. • Represent millions of years of missing time
Rock –Layer Puzzles • Geologists often find rock-layer sequences that have been affected by more than one of the events and features mentioned in this section • Determining the order of events that led to such a sequence is like piecing together a jigsaw puzzle
Section 3 Absolute Dating: A Measure of Time
The process of establishing the age of an object by determining the number of years it has existed is called absolute dating.
Radioactive Decay • To determine absolute ages of fossils and rocks, scientists analyze isotopes of radioactive elements. • Atoms of the same element that have the same number of protons but have different numbers of neutrons are called isotopes. • Most isotopes are stable, but some are unstable and referred to as radioactive.
Radioactive isotopes tend to break down into stable isotopes of the same or other elements in a process called radioactive decay. • Radioactive decay occurs at a steady rate so it can be used to calculate an objects true age.
The Process of Dating Rocks: • An unstable radioactive isotope (the parent isotope) of one element breaks down into a stable isotope (daughter isotope) • The decay of the parent isotope into the daughter isotope can occur in one step or a series of steps, but in either case the rate of decay is constant. • Therefore, to date a rock, scientists compare the amount of parent materials with the amount of daughter material. • The more daughter material there is, the older the rock is.
Radiometric Dating • Radiometric dating is determining the absolute age of a sample, based on the ratio of parent material to daughter material • A half-life is the time that it takes one-half of a radioactive sample to decay.
Example: Let’s say a rock contains an isotope with a half-life of 10,000 years. So for this rock, in 10,000 years half of the parent material will have decayed and become daughter material. If you analyze the sample and it has equal amounts of parent and daughter material, then that rock is about 10,000 years old.
After every half-life, the amount of parent material decreases by one-half. • Example: With a 10,000 year half life, after 10,000 years you would still have ½ of the parent material. After 20,000 years, you would have ¼ of the parent material (half of ½ is ¼). After 30,ooo years you would have 1/8 of the parent material (half of ¼ is 1/8), etc.
Effigy Mounds • Imagine traveling back through time before Columbus arrived in America. • You are standing on what will be the bluffs of the Mississippi River. • You see dozens of people, Native Americans, building large mounds • They are building burial mounds, now an archaeological site called Effigy Mounds National Monument
People lived at Effigy from 2,500 to 600 years ago. • How do we know these dates????
Types of Radioactive Dating • Scientists use different radiometric dating techniques based on the estimated age of the object. The older the object is the greater half-life of an isotope you will need.
1. Potassium-Argon Method • Isotope used is Potassium-40 • Half-life of 1.3 billion years • Decays to argon and calcium, measuring argon as the daughter material • Used to date rocks older than 100,000 years
2. Uranium-Lead Method • Uranium-238 decays in a series of steps to Lead-206 • Half life is 4.5 billion years • The older the rock is the more daughter material (lead-206) there will be in the material • Can be used for rocks more than 10 million years old • Younger rocks do not contain enough daughter material to be accurately measured by this method
3. Rubidium-Strontium method • Rubidium-87 is an isotope that forms the stable daughter isotope strontium-87 • Half life is 49 billion years • Used to date rocks older than 10 million years
4. Carbon-14 Method • Carbon is found in three forms: Carbon-12, Carbon-13 and the radioactive form Carbon-14 • These carbons combine with oxygen to form the gas carbon dioxide, which is taken in by plants to perform photosynthesis • As long as a plant is alive, new carbon dioxide with a constant carbon-14 to carbon-12 ratio is continually taken in • Animals then eat the plants and contain the same ratio of carbon isotopes • When a plant or animal dies, no new carbon is taken in. • The amount of carbon-14 begins to decrease as the organism decays, and the ratio to carbon-12 to carbon-14 decreases. • This decrease can be measured in a lab • Because the half-life of carbon-14 is only 5,730 years, this dating method is used mainly for dating things that lived within the last 50,000 years
The remains or physical evidence of an organism preserved by geologic processes is called a fossil. • Fossils are most often preserved in sedimentary rock, but other materials can also preserve evidence of past life.
1. Fossils in Rocks • Usually when an organism dies, it immediately begins to decay or is eaten by another organism • Sometimes though organisms are quickly buried by sediment when they die • The sediment slows down decay. Hard parts like shells and bones are more resistant to decay than soft tissues • When sediments become rock, the hard parts of animals are much more commonly preserved than are soft tissues
2. Fossils in Amber • Hardened tree sap is called Amber • Sometimes organisms, usually types of insects, are trapped in tree sap which hardens over time. • Organisms are fully preserved inside that tree sap, DNA and all
3. Petrifacation • Petrifaction is a process in which minerals replace an organisms tissues • Permineralizationis a process in which the pore space in an organisms hard tissue (bone or wood) is filled up with mineral • Replacement is a process in which the organisms tissues are completely replaces by minerals
4. Fossils in Asphalt • There are places on Earth where asphalt wells up at the Earth’s surface in thick, sticky pools (are also called tar pits) • The La Brea asphalt deposits in Los Angeles are at least 38,000 years old • These pools have trapped and preserved many kinds of organisms from the past 38,000 years • From these fossils scientists have learned about the past environment of southern California
“Early excavations at the La Brea tar pits of central Los Angeles during the period 1913–1915 unearthed roughly a million bones from nearly a hundred sites. Photo courtesy of the George C. Page Museum. Source: Fossils discovered in the tar pits of La Brea http://www.youtube.com/watch?v=G7FK59waeo0 6:09
5. Frozen Fossils • Cold temperatures slow decay • Many types of frozen fossils are preserved from the last ice age • In October 1999, scientists recovered a complete 20,000 year old woolly mammoth frozen in the Siberian tundra – woolly mammoths became extinct approximately 10,000 years ago. http://player.discoveryeducation.com/index.cfm?guidAssetId=FDCCD32E-D697-408B-8EA3-28F781244276&blnFromSearch=1&productcode=US#
6. Trace Fossils • Any naturally preserved evidence of animal activity is called a trace fossil • Examples are tracks, burrows, coprolite • Footprints that fill with sediment and become preserved in rock reveal a lot about the animal that made them, including how big it was, how fast it moved. Parallel track ways showing dinosaurs moving in the same direction have led scientist to hypothesize that dinosaurs moved in herds • Burrows are shelters made by animals such as clams, that burry in the sediment. Burrows are preserved when they are filled in with sediment and buried quickly
Cross-section of Mammoth footprints Footprints from the Triassic Period Burrows