Age of the Earth -Scientists have not found a way to measure the exact age of the Earth directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed by the processes of erosion and plate tectonics. If there are any of Earth's original rocks left, they have not yet been found. -Scientists have been able to determine the probable age of the Solar System and to calculate an age for the Earth by assuming that the Earth and the rest of the Solar System formed near the same time and are, therefore, near the same age.
The ages of Earth rocks, Moon rocks and meteorites are measured by radiometric age dating.
-Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 BYA). -An interesting feature of these ancient rocks is that they are lava flows and sediments deposited in shallow water, an indication that Earth’s history began well before these rocks were deposited.
-The oldest dated Moon rocks have ages between 4.4 and 4.5 billion years and provide a minimum age for it’s formation. -There are more than 70 meteorites, of different types, whose ages have been measured using radiometric age dating. The results show that the meteorites, and therefore the Solar System, formed approximately 5 billion years ago. Meteorite
<ul><li>-The best age for the Earth comes not from dating individual rocks but by considering the Earth, Moon, and meteorites as part of the same evolving solar system. </li></ul><ul><li>-This results in an age for the Earth of </li></ul><ul><li>4.6 billion years with an uncertainty of less than 1 percent. </li></ul>Anorthosite – Moon rock
Two Ways to Consider Time: Absolute Time: Time expressed with numbers. “ This rock is 180 million years old” Relative Time: Time expressed as a comparison. “ The granite is younger than the schist”
The history of the Earth is broken up into a set of divisions for organizing geologic time. Representing smaller and smaller units of time, the divisions are Eon, Era, Period, Epoch, and Age.
<ul><li>The time before the Phanerozoic is usually referred to as the Precambrian . </li></ul><ul><li>In any case, the Precambrian is usually divided into the three "eons" as shown. </li></ul>
<ul><li>The Phanerozoic Eon represents the time during which the majority of macroscopic organisms, as well as algae, fungi, plants and animals lived. When first suggested as a division of geologic time, the beginning of the Phanerozoic (approximately 543 million years ago) was thought to be the beginning of life. </li></ul><ul><li>In reality, this eon coincides with the appearance of animals that evolved external skeletons, like shells, and the somewhat later animals that formed internal skeletons, such as the vertebrates. </li></ul>
The Phanerozoic also consists of three major divisions...the Cenozoic, the Mesozoic, and the Paleozoic Eras. The "zoic" part of the word comes from the root "zoa", which means life. This is the same root as in the words Zoology and Zoological Park (or Zoo). "Cen" means recent, "Meso" means middle, and "Paleo" means ancient. These divisions reflect major changes in the fossil record, and each era is therefore recognized by a dominant group of animals. The Cenozoic has sometimes been called the "Age of Mammals", the Mesozoic the "Age of Dinosaurs" and the Paleozoic the "Age of Marine Life". This is an overly simple, but useful view. For instance, other groups of animals lived during the Mesozoic. In addition to the dinosaurs, animals such as mammals, turtles, crocodiles, frogs, and countless varieties of insects also lived on land. Additionally, there were many kinds of plants living in the past that no longer live today. Ancient plants went through great changes too, and not always at the same times that the animal groups changed.
The Four Eras of Geologic Time 0 MYA – The Present (Now) Cenozoic (Recent Life) Age of Mammals 65.5 MYA – Mass Extinction – 3/4 of all species wiped out – Cause: Meteorite Impact Mesozoic (Middle Life) Age of Dinosaurs (Reptiles) 251 MYA – Mass Extinction – 7/8 of all species wiped out – Cause is unknown Paleozoic (Ancient Life) Age of Marine Organisms 542 MYA – Cambrian Explosion – Dramatic increase in the Abundance, Complexity, and Diversity of Life Precambrian (Before Cambrian) Late: Age of Soft-Bodied Life Early: Age of Single-Celled Life 4600 MYA – Earth Forms
Timeline Homework Due Wednesday 1-24-12 <ul><li>Create a neat and colorful timeline showing these key four categories of geologic time. Your timeline must be based on an accurate scale (length, in cm or in. on paper per billion years) which is shown clearly on the timeline. Be sure to plot boundary ages accurately and label clearly. Do not change your scale! </li></ul><ul><li>Add pictures representing characteristic life form for each time category. Pictures must be neat, colorful and easily recognizable. </li></ul><ul><li>Identify the divisions and express why they occur where they are. (What happened?) </li></ul><ul><li>Use red textbook, Page 80-85. </li></ul>