Geologic Time Chapter 8 Lecture Natalie Bu

Geologic Time Chapter 8 Lecture Natalie Bu

Explain the principle of uniformitarianism. Discuss how it differs from catastrophism. Focus Questions 8.1 © 2017 Pearson Education, Inc. • 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 A Brief History of Geology © 2017 Pearson Education, Inc. • 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 A Brief History of Geology © 2017 Pearson Education, Inc. • Distinguish between numerical and relative dating. • Apply relative dating principles to determine a time sequence of geologic events. Focus Questions 8.2 © 2017 Pearson Education, Inc. • 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 Creating a Timescale — Relative Dating Principles © 2017 Pearson Education, Inc. • 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 Creating a Timescale — Relative Dating Principles © 2017 Pearson Education, Inc. • 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 Creating a Timescale — Relative Dating Principles © 2017 Pearson Education, Inc. • Principle of lateral continuity – Sedimentary beds originate as continuous layers that extend in all directions – Identical strata on two sides of a canyon were continuous before the canyon was carved Creating a Timescale — Relative Dating Principles © 2017 Pearson Education, Inc. • Principle of cross-cutting relationships – Geologic features that cut across rocks must form after the rocks they cut through – Faults, igneous intrusions Creating a Timescale — Relative Dating Principles © 2017 Pearson Education, Inc. • Inclusions – Fragments of one rock unit enclosed within another • Rock that contains inclusions is younger than the rock that provided the inclusions Creating a Timescale — Relative Dating Principles © 2017 Pearson Education, Inc. • 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 Unconformities © 2017 Pearson Education, Inc. • Angular unconformity – Consists of tilted or folded sedimentary rocks overlain by younger, more flat lying strata – Deformation occurred during the time that deposition stopped Unconformities © 2017 Pearson Education, Inc. • 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) Unconformities © 2017 Pearson Education, Inc. • Nonconformity – Younger sedimentary rocks on top of older metamorphic or intrusive igneous rocks – Imply period of uplift of deeply buried rocks Unconformities © 2017 Pearson Education, Inc. Unconformities © 2017 Pearson Education, Inc. Applying Relative Dating Principles © 2017 Pearson Education, Inc. • Define fossil. • Discuss the conditions that favor the preservation of organisms as fossils. • List and describe various fossil types. Focus Questions 8.3 © 2017 Pearson Education, Inc. • Fossils – The remains or traces of prehistoric life • Paleontology – The scientific study of fossils Fossils: Evidence of Past Life © 2017 Pearson Education, Inc. Fossils: Evidence of Past Life © 2017 Pearson Education, Inc. • 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 Types of Fossils © 2017 Pearson Education, Inc. • 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 marking is preserved; no internal structure – If hollow spaces are filled with mineral matter, a cast is formed Types of Fossils © 2017 Pearson Education, Inc. • 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 • Amber – The hardened resin of ancient trees – Seals organisms from atmosphere and water – Preserves delicate organisms like insects Types of Fossils © 2017 Pearson Education, Inc. • Trace Fossils – Indirect evidence of organisms • Tracks • Burrows • Coprolites • Gastroliths Types of Fossils © 2017 Pearson Education, Inc. • 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 Conditions Favoring Preservation © 2017 Pearson Education, Inc. • 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. Conditions Favoring Preservation © 2017 Pearson Education, Inc. • Explain how rocks of similar age that are in different places can be matched up. Focus Question 8.4 © 2017 Pearson Education, Inc. • 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 Correlation of Rock Layers © 2017 Pearson Education, Inc. Correlation of Rock Layers © 2017 Pearson Education, Inc. • 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 Correlation of Rock Layers © 2017 Pearson Education, Inc. • Discuss three types of radioactive decay. • Explain how radioactive isotopes are used to determine numerical dates. Focus Questions 8.5 © 2017 Pearson Education, Inc. • 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 Reviewing Basic Atomic Structure © 2017 Pearson Education, Inc. • 99.9% of an atom’s mass is in the nucleus – Electrons have almost no mass • Number of protons + number of neutrons in an atom = the mass number • An isotope has a different number of neutrons in the nucleus – Different mass number Reviewing Basic Atomic Structure © 2017 Pearson Education, Inc. • 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 Dating with Radioactivity © 2017 Pearson Education, Inc. • 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 Dating with Radioactivity © 2017 Pearson Education, Inc. • Parent Isotope – Unstable radioactive isotope • Daughter Product – Isotope resulting from radioactive decay Dating with Radioactivity © 2017 Pearson Education, Inc. • 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 Dating with Radioactivity © 2017 Pearson Education, Inc. • 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. Dating with Radioactivity © 2017 Pearson Education, Inc. • Five radioactive isotopes are important in geology: – Rubidium-87 – Uranium-238 – Uranium-235 – Thorium-232 – Potassium-40 • Only useful if the mineral remained in a closed system – No addition of loss of parent or daughter isotopes Dating with Radioactivity © 2017 Pearson Education, Inc. • 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 3.8 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 Dating with Radioactivity © 2017 Pearson Education, Inc. • 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 Dating with Radioactivity © 2017 Pearson Education, Inc. • Distinguish among the four basic time units that make up the geologic time scale. • Explain why the time scale is considered to be a dynamic tool. Focus Questions 8.6 © 2017 Pearson Education, Inc. • 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 The Geologic Time Scale © 2017 Pearson Education, Inc. The Geologic Time Scale © 2017 Pearson Education, Inc. • Most detail in the geologic time scale begins at 542 million years ago • 4 billion years before the Cambrian is known as the Precambrian – Divided into Archean and Proterozoic eons – Together are divided into seven eras – Represents 88% of geologic time The Geologic Time Scale © 2017 Pearson Education, Inc. • Some “unofficial” terms are associated with the geologic time scale – Precambrian = eons and eras before the Phanerozoic – Hadean = earliest eon of Earth history (before the oldest known rocks) The Geologic Time Scale © 2017 Pearson Education, Inc. • Geologic time scale must be updated periodically to include changes in unit names and boundary age estimates – A few years ago, Cenozoic divided into Tertiary and Quaternary periods – Today, former Tertiary is divided into Paleogene and Neogene periods The Geologic Time Scale © 2017 Pearson Education, Inc. • Explain how reliable numerical dates are determined for layers of sedimentary rock. Focus Question 8.7 © 2017 Pearson Education, Inc. • 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 • Must be related to datable igneous masses Creating a Timescale — Relative Dating Principles © 2017 Pearson Education, Inc.

Paper For Above instruction

The study of geologic time is fundamental to understanding Earth's history and the processes that have shaped its surface over billions of years. Central to this understanding are key principles such as uniformitarianism and catastrophism, which offer contrasting perspectives on Earth's past landscape formation. Uniformitarianism, articulated by James Hutton in the late 18th century, posits that the same natural laws and processes observed today—such as erosion, sedimentation, and volcanic activity—operate identically in the past. This principle emphasizes a slow, gradual change over extensive timescales, allowing geologists to interpret ancient rocks based on current processes. Conversely, catastrophism, dominant in the 17th and 18th centuries, suggested that Earth's landscapes were formed primarily through sudden, catastrophic events like floods and volcanic eruptions. While catastrophism accounts for some abrupt geological phenomena, uniformitarianism has become the cornerstone of modern geology, providing a framework for understanding Earth's long-term evolution.

Distinguishing between relative and numerical dating forms another pillar of geologic time analysis. Relative dating involves placing rocks and events in chronological order without assigning specific ages. Principles such as superposition, original horizontality, lateral continuity, cross-cutting relationships, inclusions, and unconformities enable geologists to determine the relative sequence of geological events. For example, the principle of superposition states that in an undeformed sedimentary sequence, older layers lie beneath younger ones. Relative dating thus constructs a sequence of events, forming the basis for a geologic timescale.

Numerical dating, on the other hand, relies on radiometric techniques that measure the decay of radioactive isotopes within minerals. Isotopes like uranium-238, uranium-235, rubidium-87, thorium-232, and potassium-40 decay at known rates, characterized by their half-lives. By measuring the parent and daughter isotopes in a mineral and knowing the half-life, scientists can calculate absolute ages of rocks with high precision. These methods have revealed that Earth is approximately 4.54 billion years old, with the oldest rocks found on Earth dating back over 4 billion years.

The development of the geologic time scale encapsulates Earth's history into hierarchical units based on significant biological and geological events. Eons, such as the Precambrian and Phanerozoic, represent vast spans of time, with the Phanerozoic beginning around 542 million years ago marked by abundant fossil records. Eras, periods, and epochs further subdivide these eons, helping scientists to segment Earth's history into meaningful chapters. The introduction of radiometric dating has refined these boundaries, transforming the time scale into a dynamic and continually updated framework.

Fossil evidence plays a critical role in correlating rocks of similar age across different regions. Fossils, preserved remains or traces of ancient organisms, enable geologists to match layers from disparate geographic locations. The principle of fossil succession states that fossil organisms succeed one another in a recognizable order, making fossils invaluable for relative dating. Index fossils—widely distributed and limited to short time spans—are especially useful for correlating strata. Such fossil records have uncovered the evolution of life from primitive to complex organisms, providing insights into life's resilience and adaptability.

While many fossils are preserved through mineralization, molds, carbonization, amber entrapment, and trace fossils also record past life. Conditions favoring fossil preservation include rapid burial, hard parts like shells and bones, and environments that inhibit decay, such as cold, dry, or anoxic habitats. However, soft-bodied organisms are rare in the fossil record, biasing our understanding of ancient ecosystems. This preservation bias underscores the importance of fossil records in reconstructing Earth's history, although they provide a partial view.

Understanding rock correlation across different regions involves matching layers using distinctive features and fossil content. William Smith's pioneering work in the 18th and 19th centuries established the concept of fossil succession and the use of index fossils to correlate strata. These methods allow geologists to piece together a global picture of Earth's changing environment through time, highlighting the interconnectedness of Earth's geological and biological history.

Radiometric decay involves three primary types: alpha decay, beta decay, and electron capture. Each process alters the nucleus of an unstable isotope, transforming it into a more stable daughter isotope. These decay mechanisms underpin radiometric dating techniques, enabling precise age calculations. For example, uranium isotopes decay into lead over billions of years, a process exploited to date the oldest rocks. The concept of half-life—limiting the decay time for half of the parent isotopes—enables scientists to determine the age of rocks accurately when combined with measurements.

Geologic time is divided into hierarchical units—eons, eras, periods, and epochs—each representing different scales of Earth's history. The most extended spans are eons, with the Precambrian comprising about 88% of Earth's history. The Phanerozoic, beginning around 542 million years ago, marks the rise of abundant fossil life. These divisions are based on significant biological and geological events, such as mass extinctions and evolutionary milestones. The time scale is a dynamic tool, continually refined as new data—especially radiometric ages—provide more accurate boundaries and subdivisions.

Determining the numerical age of sedimentary layers involves integrating radiometric dates obtained from interlayered volcanic or intrusive rocks. Because sedimentary rocks aggregate material of various ages, direct radiometric dating of these layers is often challenging. Instead, geologists date nearby igneous rocks and assume