Environmental Science Chapter 5 Earth Through the Ages

5.1 Written in Stone Earth’s geologic history has been identified, categorized, and charted in sequential order to produce a geologic time scale. Geologic time is measured two ways: Relative time puts events in sequence, but does not give their actual age. Absolute time, or measured time, identifies the actual date an event occurred and establishes its absolute age.

5.1 Written in Stone Superposition determines the relative ages of layers of sedimentary rock by their position. The bottom layer are deposited first, thus the oldest layers. Succeeding layers are progressively younger. Certain geologic processes, such as folding and movement along faults can cause exceptions to superposition.

5.1 Written in Stone Correlation is a process to match the rock strata in different locations to determine if they formed at the same time. Following an exposed rock layer throughout its length is called correlation by continuity. For example, ash fall is always younger than the rocks it covers.

5.1 Written in Stone Radioactive dating uses the rate of decay of naturally occurring, radioactive isotopes of elements to determine the absolute age of rocks and fossils. The rate of disintegration of these isotopes remains constant through time. The half-life is the period of time it takes for half the atoms in a radioactive sample to breakdown into simpler, stable atoms. (see figure 5-2)

5.1 Written in Stone Radioactive isotopes with long half-life are used to date most rocks due to the extreme age of the earth. Elements with a shorter half-life are used to date the remains of organic materials and young rocks. The isotope Carbon-14 is an accurate tool for dating materials up to 50,000 years of age. This is called radiocarbon dating.

5.1 Written in Stone Fossils are naturally preserved remains or impressions of living things. Index fossils are the remains of organisms that existed for a brief period of geologic time and had a wide geographic distribution. Trilobites are crablike animals, whose remains are used as index fossils. These animals existed between 500 million to 600 million years ago. Graptolites, an ancient marine organism, exited between 350 million and 450 million years ago.

5.2 Geologic Time Geologic time is divided into units called eras, periods, and epochs. Eras are the largest division of geologic time. There are four eras: Precambrian Era Paleozoic Era Mesozoic Era Cenozoic Era

5.2 Geologic Time Precambrian Era, lasted 4 billion years, 87% of geological time. Rocks from this era are found at the core of continents. Fossilized algae called stromatolites were found in Precambrian rocks dating more than 2.5 billion years old. Volcanic activity, plate movement, mountain building, formation of ocean occurred during this era. Photosynthetic organisms change the atmosphere by introducing oxygen.

5.2 Geologic Time Multimedia Timeline perspective Precambrian Explosion Fossil record Stromatolites

5.2 Geologic Time Paleozoic Era lasted for 350 million years. Periods are blocks of geologic time characterized by the appearance, disappearance, or dominance of various life-forms in the fossil record. This era is divided into six periods: 1.Cambrian Period 4.Devonian Period 2.Ordovician Period 5.Carboniferous Period 3.Silurian Period 6.Permian Period Paleozoic rocks have an abundance of fossil remains. During this era, continents moved, climates changed, and many new life forms appeared.

5.2 Geologic Time Paleozoic Era – Cambrian Period Lasted 70 million years Continents covered in many shallow seas Marine life flourishes The Precambrian explosion still occurring during the beginning of this period Invertebrates, animals without a backbone, are most common

5.2 Geologic Time Paleozoic Era – Ordovician Period Lasted 70 million years First vertebrates, animals with a backbone, appear as jawless fish called ostracoderms. Extensive mountain building and volcanic activity Large land masses form

5.2 Geologic Time Paleozoic Era – Silurian Period Lasted 35 million years Coral reefs begin to form Land colonized by plants then air breathing animals, such as spiders, millipedes, and scorpions. Armored fish with jaws develop.

5.2 Geologic Time Paleozoic Era – Devonian Period Lasted for 50 million years Bony fish and sharks diversify and multiply rapidly Lobed-finned fish and lungfish emerge First amphibians develop from lungfish Age of the Fishes Primitive scale trees populate the first forests out of swamp lands.

5.2 Geologic Time Paleozoic Era – Carboniferous Period Lasted for 65 million years Age of Amphibians Deposits peat form into coal First true land vertebrates, the reptiles, appear Giant insects common

5.2 Geologic Time Paleozoic Era – Permian Period Lasted for 55 million years A large-scale glaciation occurred in the Southern Hemisphere Conifers appear Reptiles dominate amphibians on land Therapsids combine features of reptiles and mammals Half the known animal groups go extinct Cephalopods (squids and octopi) and reptiles survive (ancestors of dinosaurs, birds, snakes, mammals)

5.2 Geologic Time Mesozoic Era lasts 160 million years Divided into Triassic, Jurassic, and Cretaceous Periods Climate was mild Age of Reptiles – dinosaurs Brachiosaurus, plant eater largest dinosaur Tyrannosaurus Rex largest carnivore First angiosperms, flowering plants appear K-T extinction all dinosaurs die out Iridium suggests a large meteorite hit earth

5.2 Geologic Time Cenozoic Era began 65 million years ago and the one we live in Ice ages are periods of world-wide glaciation. Ice ages reoccur for 600 million years Paleogene, Neogene and Quaternary Periods Age of Mammals, since mammals are the dominant, large, land-dwelling life forms. Wooly mammoths and saber-toothed cats are extinct mammals of the early Cenozoic Period

5.2 Geologic Time Earliest homo sapiens fossils on record are approximately 160,000 years old. We are currently in the Holocene epoch, which covers the last 11,000 years since the last ice age.