History of Earth and Geologic Time PDF

# HISTORY OF THE EARTH AND GEOLOGIC TIME ## Key Concepts ### History of the Earth * The Earth's history is recorded in the rocks of the crust. Scientists used assumption called uniformitarianism in order to relate what we know about present-day processes to past events - the present-day is the ke...

# HISTORY OF THE EARTH AND GEOLOGIC TIME ## Key Concepts ### History of the Earth * The Earth's history is recorded in the rocks of the crust. Scientists used assumption called uniformitarianism in order to relate what we know about present-day processes to past events - the present-day is the key to the past. * Uniformitarianism states that the natural laws we know today have been constant over the geologic past. ### Earth's Early Evolution * As materials continued to accumulate, the high velocity impact of interplanetary debris and the decay of radioactive elements caused the temperature of our planet to steadily increase. * During the period of intense heating Earth became hot enough that iron and nickel began to melt. This process occurred rapidly on the scale of geologic and produced Earth's dense iron-rich core. * The early period of heating also resulted in a magma ocean, about hundred kilometers deep. Within the magma ocean buoyant masses of molten rock rose toward the surface and eventually solidified to produce a thin, primitive crust - thus, the three major divisions of the Earth's interior a) the iron rich core, b) the thin primitive crust, and c) its thickest layer, the mantle. * The light materials - including water vapor, carbon dioxide and other gasses escaped to form primitive atmosphere and shortly thereafter the oceans. ### Geologic Time Scale * The history and evolution of Earth are described using the geologic timescale. The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. * It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history. * The Geologic Time Scale is divided into big portions of time known as Eons. Eons are further divided into Eras. Eras are divided into Periods. * The Precambrian Time (the time of hidden life) is the point of time in which our planet started to form. This is subdivided into Hadean, Archean, and Proterozoic Eon. Meanwhile, Phanerozoic Eon, living organisms like plants and animals existed. It constitutes the time of multicellular life on Earth and the evolutionary process happened. * Phanerozoic Eon is further divided into the Paleozoic Era (time of ancient life), Mesozoic Era (time of middle life), Cenozoic Era (time of recent life). ## The Precambrian time * Ranges from about 4.6 billion years ago, when the earth formed, to about 544 million years ago, when abundant microscopic life appeared. * Most of the rocks in this early part of Earth's history have been extensively eroded away, metamorphosed, obscured by overlying strata or recycled into Earth's interior. * The Earth's earliest gasses were hypothesized to be swept into space by solar wind. As the planet slowly cooled, a more sustaining atmosphere was formed. * Gasses brought to the surface by volcanic processes created both primitive atmosphere and an ocean. The first simple organisms were plants. * During mid-Precambrian, organisms such as blue green algae developed a simple version of photosynthesis. * Photosynthetic organisms require carbon dioxide to utilize the sun's energy. They keep the carbon dioxide and expel oxygen. With the release of free oxygen, a primitive ozone layer began to develop which reduced the amount of harmful ultraviolet radiation reaching the Earth. * The most common Precambrian fossils are stromatolites. These are not remains of actual organisms, rather indirect pieces of evidence of algae. Many of Precambrian fossils were preserved in hard, dense chemical sedimentary rock known as chert. * Fossils of plants date from the middle Precambrian, but fossils of animals date in the Precambrian. Towards the end of this period, fossil records revealed that diverse and complete multi-celled organisms existed. ### HADEAN EON * Greek for "beneath the Earth". It is the earliest time in Earth history and ranges from the planet's origin 4.6 billion years ago to 3.8 billion years ago. * Solar System and Planet formed. Only a few Earth rocks are known that formed during the Hadean Eon and no fossils of Hadean age are known, making it difficult to subdivide the Hadean Eon based on fossils. ### ARCHEAN EON * Greek for "ancient". There are few fossils among the rocks and they are not preserved well enough to allow for finely tuned subdivision of this eon that spanned from 3.8 to 2.5 billion years ago. * It was a time when the planet was inhospitable to life. However, it was during this time that life on earth started to appear since the oldest dated bacterial microfossil were 3.5 billion years old. * The fossil record does indicate that life began on Earth 3.2 to 3.5 billion years ago, although the exact date is uncertain. ### PROTEROZOIC EON * Greek for "earlier life". Diverse groups of fossils have been found in sedimentary rocks, 2.5 billion to 543 million years ago. * It was a time of many important geologic events: appearance of stable continents, bacterial, and archaean fossils, and eukaryotic cells. * The most complex are multicellular and have different kinds of cells arranged into tissues and organs. * Few types of shell-bearing organisms have been identified, but shelled organisms did not become abundant until the Paleozoic Era. ### PHANEROZOIC EON * Phaneros is Greek for "evident". Sedimentary rocks cover the most recent 543 million years of geologic time and contained abundant fossils. * Four changes occurred at the beginning of Phanerozoic time that greatly improved the fossil record: 1. The number of species with shells and skeletons dramatically increased. 2. The total number of individual organisms preserved as fossils increased greatly. 3. The total number of species preserved as fossils increased greatly. 4. The average sizes of individual organisms increased. ## Paleozoic Era * Began about 544 million years ago and lasted about 300 million years, during which time sea levels rose and fell worldwide, allowing shallow seas to cover the continents and marine life to flourish - from marine invertebrates to fishes, amphibians and reptiles. * The Paleozoic Era is divided into six (6) major periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. * **Cambrian Period:** Almost all marine organisms came into existence as evidence by abundant fossils. A most important event is the development of organisms having the ability to secrete calcium carbonate and calcium phosphate for the formation of shells. * **Ordovician Period:** All major group of animals that could be preserved as fossils had appeared. This period marks the earliest appearance of vertebrates - the jawless fish known as agnatha. * **Silurian Period:** The Silurian brought about the emergence of terrestrial life, the earliest being the terrestrial plants with well-developed circulatory system (vascular plants). As plants move ashore so did other terrestrial organisms. Air-breathing scorpions and millipedes were common during the period. * **Devonian Period:** This period is known as the "age of fishes". Lowland forests of seed ferns, scale trees and true ferns flourished. Sharks, insects, and bony fishes developed. Today the lung fishes and coelacanth, a "living fossil" have such internal nostrils and breathe in a similar way. The first amphibians made their appearance, although able to live on land, they need to return to water to lay their eggs. * **Carboniferous period:** Warm, moist climate conditions contributed to lash vegetation and dense swampy forests. Insects under rapid evolution led to such diverse forms of giant cockroaches and dragonflies. The evolution of the first reptiles took place with the development of the amniotic egg, a porous shell containing a membrane that provided an environment for an embryo. Ice covers large areas of the earth; swamps cover low lands; first mosses; great coal-forming forests form; seed ferns grow; winged insects appear * **Permian Period:** The reptiles were well-suited to their environment that they ruled the Earth for 200 million years. The two major groups of reptiles - diapsids and synapsids dominated this period. Diapsids gave rise to the dinosaurs. Synapsis gave rise to mammals. ## Mesozoic Era * Known as the age of reptiles, it is made up of three periods: Triassic, Jurassic, and Cretaceous. The most significant event was the rise of the dinosaurs. * A famous Jurassic deposit is the Morrison Formation, within which the world's richest storehouse of dinosaurs was preserved. True pines and redwoods appeared and rapidly spread. Flowering plants arose and their emergence accelerated the evolution of insects. A major event of this era was the breakup of Pangea. * By the end of this period, the dinosaurs and reptiles were completely wiped out. * **Triassic Period:** "Age of reptiles" begins; first mammals; corals, insects, and fishes resemble modern types. * **Jurassic Period:** The Rocky Mountains rise; first birds; palms and cone bearing tress dominant; large dinosaurs thrive; primitive mammals developed. * **Cretaceous Period:** First flowering plant; placental mammals developed; dinosaurs die out, as do many marine animals at the end of the period. ## **Cenozoic Era** * This era is known as the "age of mammals" because mammals replaced reptiles as the dominant land animal. It is also sometimes called "age of flowering plants" because angiosperms replaced gymnosperms as the dominant land plants. * Cenozoic era is made up of two periods: Tertiary and Quaternary. From oldest to youngest the periods are broken up into the Paleocene, Eocene, Oligocene, Miocene, and Pliocene for the Tertiary period, and the Pleistocene and Holocene for the Quaternary period. * Climates cooled during this era, hence the widespread glaciation. * This era also brought about the advent of humans. The lowered sea level resulted in the "land bridges" connections between land masses. One of these land bridges provided the route for the human migration from Asia to North America, also throughout the world. * **Paleogene Period:** * **Paleocene Epoch:** beginning of "age of mammals"; flowering plants and small mammals abundant; many different climates existed. * **Eocene Epoch:** fossils of "dawn horse"; grasslands and forest present; many small mammals; larger mammals such as whales, rhinoceros, and monkeys begin to develop. * **Oligocene Epoch:** fossils of primitive apes; elephants, camels, and horses developed; climate generally mild. * **Neogene Period:** * **Miocene Epoch:** many grazing animals; flowering plants and tress resemble modern types. * **Pliocene Epoch:** fossils of ancient humans near end of epoch; many birds, mammals and sea life similar to modern types; climate cools. * **Quaternary Period:** * **Pleistocene Epoch:** "The Ice Age"; modern humans present; mammoths and other animals become extinct. * **Holocene Epoch:** Humans are the dominant forms of life and civilization begins and spread.