A View of Earth’s Past

Chapter 8 & 9

• law of superposition the law that a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed

relative age - the age of an object in relation to the ages of other objects

Law of Superposition

• Ripple marks - are small waves that form on the surface of sand because of the action of water or wind. – When the sand becomes sandstone, the

ripple marks may be preserved.unconformity a break in the geologic

record created when rock layers are eroded or when sediment is not deposited for a long period of time

• fault - a break or crack in Earth’s crust along which rocks shift their position

• intrusion - a mass of igneous rock that forms when magma is injected into rock.

radiometric dating a method of determining the absolutes age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope.

half-life - time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope.

• Organic remains, such as wood, bones, and shells that are less than 70,000 years old can be determined by using a method known as carbon-14 dating, or radiocarbon dating.

fossils - the trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock

paleontology - the scientific study of fossils

Fossilization methods • amber - hardened tree sap• Tar seeps• Freezing • Mummification• Petrification

• Types of fossils:– Trace fossils– Imprints– Molds & Casts– Coprolites – Gastroliths– Index fossils

Geologic map of bedrock of Ohio

• geologic column an ordered arrangement of rock layers that is based on the relative ages of the rocks and in which the oldest rocks are at the bottom

Where would you find fossils of extinct animals on a geologic column?

Scientists use geologic columns to estimate the age of rock layers that cannot be dated radiometrically

• The geologic history of Earth is marked by major changes in Earth’s surface, climate, and types of organisms.

• EonEraPeriodEpochOld PeriodsPhanerozoicEon543 mya to PresentCenozoic Era65 mya to Present NeogeneHoloceneQuaternary   1.8 mya to PresentPleistocenePlioceneTertiary   65 to 1.8 myaMiocenePaleogeneOligoceneEocenePaleoceneMesozoic Era248 mya to 65 myaCretaceous   144 mya to 65 myaJurassic   206 to 144 myaTriassic   248 to 206 myaPaleozoic Era 543 to 248 myaPermian   290 to 248 myaCarboniferous   354 to 290 myaDevonian   417 to 354 myaSilurian   443 to 417 myaOrdovician   490 to 443 myaCambrian   543 to 490 myaPrecambrian Time4,500 to 543 myaProterozoic Era 2,500 to 543 myaArchaean 3,800 to 2,500 myaHadean 4,500 to 3,800 mya

• eon - largest unit of geologic unit of time

• 4 Eras of Geologic time: – Hadean eon – Archean eon– Proterozoic eon– Phanerozoic eon

era - unit of geologic time that includes two or more periods

• Paleozoic rocks contain fossils of a wide variety of marine and terrestrial life forms.

• After the Paleozoic Era the Mesozoic Era began and lasted about 183 million years.

• Mesozoic fossils include early forms of birds and reptiles.

• The present era is the Cenozoic Era, which began 65 million years ago. Fossils of mammals are common in Cenozoic

period - a unit of geologic time that is longer than an epoch but shorter than an era

Epoch - a subdivision of geologic time that is longer than an age but shorter than a period.

evolution - an inheritable change in the characteristics within a population from one generation to the next; the development of new types of organisms from preexisting types of organisms over time

Evolution

Precambrian time the interval of time in the geologic time scale from Earth’s formation to the beginning of the Paleozoic era, from 4.6 billion to 542 million years ago.

• stromatolites - one of the few Precambrian fossils that have been discovered are. – The presence of stromatolite fossils in Precambrian

rocks indicates that shallow seas covered much of Earth during that time.

Paleozoic Era - the geologic era that followed Precambrian time and that lasted from 542 million to 251 million years ago.– Paleozoic rocks hold an abundant fossil record. The

number of plant and animal species on Earth increased dramatically at the beginning of the Paleozoic Era.

– Because of this rich fossil record, the Paleozoic Era has been divided into seven periods.

• The Cambrian Period is the first period of the Paleozoic Era.– Marine invertebrates thrived in the warm

waters that existed during this time.– The most common of the Cambrian

invertebrates were trilobites. – The second most common animals of the

Cambrian Period were the brachiopods, a group of shelled animals.

The Ordovician Period• clamlike brachiopods and cephalopod mollusks

became the dominant invertebrate life-form.• first vertebrates appeared.

The Silurian Period• During the Silurian Period, echinoderms, relatives of

modern sea stars, and corals became more common. • Near the end of this period, the earliest land plants as

well as animals evolved

The Devonian Period - the Age of Fishes– fossils of many bony fishes were discovered in

rocks of this period. – lungfish, had the ability to breathe air. Another type

of fish, Rhipidistians, were air-breathing fish that had strong fins that may have allowed them to crawl onto the land for short periods of time.

– Land plants, such as giant horsetails, ferns, and cone-bearing plants also began to develop during

The Carboniferous Period• In North America, the Carbiniferous Period

is divided into the Mississippian and Pennsylvanian Periods.

• the climate was warm, forests and swamps covered most of the world.

• Amphibians and fish continued to flourish, and the first vertebrates that were adapted to live on land appeared.

The Permian Period - marks the end of the Paleozoic Era, – a mass extinction of several life-forms occurred at

the end of this period.– the continents had joined to form Pangaea

mass extinction - large numbers of species become extinct

Mesozoic Era the geologic era that lasted from 251 million to 65.5 million years ago; the Age of Reptiles.– Earth’s surface changed dramatically during the

Mesozoic Era. Pangaea broke into smaller continents, and the climate was warm and humid.

– Lizards, turtles, snakes and dinosaurs flourished during this era.

3 periods of the Mesozoic Era : – Triassic– Jurassic– Cretaceous Periods.

• The Triassic period marked the appearance of dinosaurs. Most dinosaurs were about 4 m to 5 m long and moved very quickly.

The Jurassic Period• Two major groups of dinosaurs evolved during the

Jurassic Period: the saurischians, or “lizard-hipped” dinosaurs, and the ornithischians, or “bird-hipped” dinosaurs.

• Brontosauruses, now called Apatosauruses were saurischians. Stegosauruses and Pterosaurs were ornithischians.

The Cretaceous Period– Among the common Cretaceous dinosaurs were the

Tyrannosaurus Rex, the ankylosaurs, the ceratopsians, and the hadrosaurs.

The Cretaceous-Tertiary Mass Extinction– The Cretaceous Period ended in another mass

extinction. No dinosaur fossils have been found in rocks that formed after the Cretaceous Period.

– Many scientists accept the impact hypothesis as the explanation for the extinction of the dinosaurs. This hypothesis is that about 65 million years ago, a giant meteorite crashed into Earth.

Cenozoic Era - current geologic era, which began 65.5 million years ago; – the Age of Mammals– dramatic changes in climate have occurred.

As temperatures decreased during the ice ages, new species that were adapted to life in cooler climates appeared.

– Mammals became the dominant life-form and underwent many changes.

• The Cenozoic Era is divided into two periods: the Tertiary Period and the Quaternary Period.

Fossil evidence for Gondwanaland