© 2014 Pearson Education, Inc. Geologic Time Chapter 8 Lecture Outline Natalie Bursztyn Utah State University Foundations of Earth Science Seventh Edition

© 2014 Pearson Education, Inc. Focus Question 8.1 How do catastrophism and uniformitarianism affect our understanding of Earth history?

© 2014 Pearson Education, Inc. A Brief History of Geology Mid-1600s –James Ussher stated Earth was only a few thousand years old Catastrophism –Belief that Earth’s landscapes were formed by great catastrophes –Prevalent during the 1600s and 1700s –Used to fit the rate of Earth’s processes to prevailing ideas of Earth’s age

© 2014 Pearson Education, Inc. A Brief History of Geology Late 1700s –James Hutton published Theory of the Earth Uniformitarianism –States that the physical, chemical, and biological laws that operate today have also operated in the geologic past –To understand ancient rocks, we must understand present-day processes –Geologic processes occur over extremely long periods of time

© 2014 Pearson Education, Inc. Creating a Timescale — Relative Dating Principles Efforts to determine Earth’s age during the 1800s and 1900s were unreliable Today radiometric dating allows scientists to accurately determine numerical ages for rocks representing important events in Earth’s past Relative dates are determined by placing rocks in the proper sequence of formation

© 2014 Pearson Education, Inc. Creating a Timescale — Relative Dating Principles Principle of superposition –Developed by Nicolas Steno in the mid-1600s –Studied sedimentary rock layers in Italy In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below –Also applies to lava flows and ash beds

© 2014 Pearson Education, Inc. Creating a Timescale — Relative Dating Principles

© 2014 Pearson Education, Inc. Creating a Timescale — Relative Dating Principles Principle of original horizontality –Layers of sediment are generally deposited in a horizontal position –Rock layers that are flat have not been disturbed –Folded or inclined rocks must have been disrupted after deposition

© 2014 Pearson Education, Inc. Creating a Timescale — Relative Dating Principles Principle of cross-cutting relationships –Geologic features that cut across rocks must form after the rocks they cut through –Faults, igneous intrusions

© 2014 Pearson Education, Inc. Creating a Timescale — Relative Dating Principles Inclusions –Fragments of one rock unit enclosed within another Rock that contains inclusions is younger than the rock that provided the inclusions

© 2014 Pearson Education, Inc. Unconformities Layers of rock that have been deposited without interruption are called conformable –A complete set of conformable strata for all of Earth history does not exist Interrupting the deposition of sediment creates a break in the rock record called an unconformity –Represents a period when deposition stopped, erosion occurred, and then deposition resumed –Generally uplift causes deposition to stop and subsidence causes deposition to resume

© 2014 Pearson Education, Inc. Unconformities Angular unconformity –Consists of tilted or folded sedimentary rocks overlain by younger, more flat lying strata –Deformation occurred during the time that deposition stopped

© 2014 Pearson Education, Inc. Unconformities Disconformity –A break in sedimentary rock strata representing a time when erosion occurred –Difficult to identify because layers are parallel –Evidence of erosion (buried stream channel)

© 2014 Pearson Education, Inc. Unconformities Nonconformity –Younger sedimentary rocks on top of older metamorphic or intrusive igneous rocks –Imply period of uplift of deeply buried rocks

© 2014 Pearson Education, Inc. Unconformities

© 2014 Pearson Education, Inc. Applying Relative Dating Principles

© 2014 Pearson Education, Inc. Focus Questions 8.2 What principles are used to determine the relative age of sedimentary rocks? –Superposition –Original Horizontality –Cross-cutting –Inclusions What features form when the deposition of sediment is interrupted? –Unconformities Angular unconformity Disconformity Nonconformity

© 2014 Pearson Education, Inc. Focus Questions 8.3 What are the different ways that a fossil can be preserved? Are all organisms that lived in the past preserved in the fossil record?

© 2014 Pearson Education, Inc. Fossils: Evidence of Past Life Fossils –The remains or traces of prehistoric life Paleontology –The scientific study of fossils

© 2014 Pearson Education, Inc. Types of Fossils Fossils can be preserved in many ways Some remains may not be altered at all –Teeth, bones, shells –Entire animals including flesh are not common Mammoths frozen in Arctic tundra Mummified slots in a dry cave in Nevada

© 2014 Pearson Education, Inc. Types of Fossils Permineralization –Mineral-rich groundwater permeates porous tissues –Petrified wood is permineralized with silica –“Petrified” means “turned to stone” Molds –Form where a structure buried in sediment was dissolved by groundwater –Only the outside shape and surface is preserved –If hollow spaces are filled with mineral matter, a cast is formed Carbonization –Remains are encased in sediment; pressure squeezes out all liquid and gas until only a thin residue of carbon remains –Effectively preserves leaves and delicate animals –Impressions may show considerable detail

© 2014 Pearson Education, Inc. Types of Fossils Amber –The hardened resin of ancient trees –Seals organisms from atmosphere and water –Preserves delicate organisms like insects Trace Fossils –Indirect evidence of organisms Tracks Burrows Coprolites Gastroliths

© 2014 Pearson Education, Inc. Types of Fossils

© 2014 Pearson Education, Inc. Conditions Favoring Preservation Only a very small fraction of organisms are preserved as fossils Rapid burial and hard parts favor preservation –Soft parts are eaten or decomposed –Sediment protects organisms from destruction –Shells, bones, and teeth are much more common in the fossil record Fossil record is biased

© 2014 Pearson Education, Inc. Conditions Favoring Preservation What types of organisms are most likely to be missing from, or are very rare, in the fossil record? How might this bias our picture of what life on Earth was like in the past? –Hint: Think about the organisms themselves, but also their ecological context and depositional environment.

© 2014 Pearson Education, Inc. Focus Questions 8.3 What are the different ways that a fossil can be preserved? –Unaltered –Permineralization –Molds and casts –Carbonization –Amber –Trace fossils Are all organisms that lived in the past preserved in the fossil record? –Most organisms are not fossilized –Hard parts and rapid burial increase the chance of fossilization

© 2014 Pearson Education, Inc. Focus Question 8.4 How can rocks in different areas be correlated?

© 2014 Pearson Education, Inc. Correlation of Rock Layers Correlation is matching up rocks of similar age in different regions –Reveals a more comprehensive picture of the sedimentary rock record Correlation by walking along outcropping edges is possible within limited areas –Rock layers made of distinctive material can be identified in other places –Widely separated areas require the use of fossils

© 2014 Pearson Education, Inc. Correlation of Rock Layers

© 2014 Pearson Education, Inc. Correlation of Rock Layers William Smith –1700s to 1800s –Noted that rock formations in canals contained fossils unlike the fossils in the beds above and below Distinctive fossils can be used to identify and correlate widely separated sedimentary strata Principle of fossil succession –Fossil organisms succeed one another in a definite and determinable order, therefore any time period can be recognized by its fossil content –Fossils document the evolution of life through time

© 2014 Pearson Education, Inc. Correlation of Rock Layers Index fossils –Geographically widespread and limited to a short span of geologic time –Important for correlation Fossil assemblage –Can be used when there aren’t index fossils Fossils are useful environmental indicators

© 2014 Pearson Education, Inc. Correlation of Rock Layers

© 2014 Pearson Education, Inc. Focus Question 8.4 How can rocks in different areas be correlated? –Index fossils

© 2014 Pearson Education, Inc. Focus Question 8.5 How can radioactive isotopes be used to determine numerical ages for geologic materials?

© 2014 Pearson Education, Inc. Reviewing Basic Atomic Structure Each atom is made up of protons, neutrons, and electrons –Protons have a positive charge –Electrons have a negative charge –Neutrons are neutral Elements are identified by atomic number –Number of protons in the nucleus

© 2014 Pearson Education, Inc. Reviewing Basic Atomic Structure 99.9% of an atom’s mass is in the nucleus –Electrons have almost no mass # of protons + # of neutrons in an atom = the mass number An isotope has a different number of neutrons in the nucleus –Different mass number

© 2014 Pearson Education, Inc. Dating with Radioactivity Some isotopes have unstable nuclei with bonds that are not strong enough to hold the protons and neutrons together These nuclei will break apart (decay) in a process called radioactivity

© 2014 Pearson Education, Inc. Dating with Radioactivity Three common types of radioactive decay: Alpha particle = 2 protons and 2 neutrons –Mass number reduced by 4 and atomic number decreased by 2 Beta particle = electron from the neutron –Neutron is actually a proton and electron combined –Mass number remains the same, but atomic number increases by 1 Electron capture –Captured by the nucleus and combined with a proton to form a neutron –Mass number remains the same, but atomic number decreases by 1

© 2014 Pearson Education, Inc. Dating with Radioactivity

© 2014 Pearson Education, Inc. Dating with Radioactivity Parent Isotope –Unstable radioactive isotope Daughter Product –Isotope resulting from radioactive decay

© 2014 Pearson Education, Inc. Dating with Radioactivity Radiometric dating –Reliable method of calculating ages of rocks –Rate of decay for many isotopes does not vary –Rate of decay has been precisely measured –Daughter product has been accumulating at a known rate since rocks were formed

© 2014 Pearson Education, Inc. Dating with Radioactivity Half-life –Time required for one-half of the nuclei in a sample to decay –One half-life has transpired when quantities of parent and daughter are equal (1:1 ratio) If half-life of an isotope is known and parent- daughter ratio can be measured, then age can be calculated.

© 2014 Pearson Education, Inc. Dating with Radioactivity

© 2014 Pearson Education, Inc. Dating with Radioactivity Five radioactive isotopes are important in geology: 1.Rubidium-87 2.Uranium Uranium Thorium Potassium-40 Only useful if the mineral remained in a closed system –No addition of loss of parent or daughter isotopes

© 2014 Pearson Education, Inc. Dating with Radioactivity Radiocarbon dating –Using the carbon-14 isotope to date very recent events –Half-life of carbon-14 is only 5,730 years Only useful for dating events from historic past and very recent geologic history –Carbon-14 is present in small amounts in all organisms

© 2014 Pearson Education, Inc. Dating with Radioactivity

© 2014 Pearson Education, Inc. Dating with Radioactivity Radiometric dating methods have been used to determine the age of the oldest rocks on Earth –3.5 billion year old rocks found on all continents –Oldest rocks: 4.28 billion years old (Quebec, Canada) –3.7 to 3.8 billion years old in western Greenland –3.5 to 3.7 billion years old in the Minnesota River Valley and northern Michigan –3.4 to 3.5 billion years old in southern Africa –3.4 to 3.6 billion years in western Australia

© 2014 Pearson Education, Inc. Focus Question 8.5 How can radioactive isotopes be used to determine numerical ages for geologic materials? –The rate at which parent decays to daughter product is precisely known –If the ratio of parent and daughter product can be measured, then an age for the sample can be calculated

© 2014 Pearson Education, Inc. Focus Question 8.6 What is the basic structure of the geologic timescale?

© 2014 Pearson Education, Inc. The Geologic Time Scale Geologic history divided into units of variable magnitude –Developed during the nineteenth century –Based on relative dating Eons represent the greatest span of time –Phanerozoic Eon began about 542 million years ago Eons divided into eras –Phanerozoic includes Paleozoic, Mesozoic, and Cenozoic –Bounded by profound worldwide changes in life-forms Eras divided into periods Periods divided into epochs

© 2014 Pearson Education, Inc. The Geologic Timescale

© 2014 Pearson Education, Inc. The Geologic Timescale Most detail in the geologic timescale begins at 542 million years ago 4 billion years before the Cambrian is known as the Precambrian – Divided into Archean and Proterozoic eons – Each divided into four eras – Represents 88% of geologic time

© 2014 Pearson Education, Inc. The Geologic Timescale Some “unofficial” terms are associated with the geologic timescale – Precambrian = eons and eras before the Phanerozoic – Hadean = earliest eon of Earth history (before the oldest known rocks)

© 2014 Pearson Education, Inc. Focus Question 8.6 What is the basic structure of the geologic timescale? –Geologic history is divided into eons –Eons are divided into eras –Eras are divided into periods –Periods are divided into epochs

© 2014 Pearson Education, Inc. Focus Question 8.7 What are some difficulties associated with assigning numerical ages to sedimentary rocks?

© 2014 Pearson Education, Inc. Determining Numerical Dates for Sedimentary Strata Rocks can only be radiometrically dated if all minerals formed at the same time –Works for igneous and metamorphic rocks –Sedimentary rocks contain particles of many ages

© 2014 Pearson Education, Inc. Determining Numerical Dates for Sedimentary Strata

© 2014 Pearson Education, Inc. Focus Question 8.7 What are some difficulties associated with assigning numerical ages to sedimentary rocks? –Sedimentary rocks contain particles of many ages