Earth Scientists
•Geologist – a scientist who studies the Earth’s crust as well as the processes and history that shaped it •Paleontologist – a scientist that studies fossil remains found on the Earth’s surface in order to study primitive life forms such as: plants, animals, fungi, and bacteria Stratigraphy •A branch of geology dealing with the arrangement of sedimentary rock layers or strata •Geologists assume the newest rock layers are on top of the older ones, unless some type of disturbance occurs. •Called the Law of Superposition Relative Age •The strata of sedimentary rocks is important in determining their relative age. •Relative age determines the “relative” order of past events but not the absolute age. •Like saying you’re relatively younger than your grandfather. Relative Age •Strata is sometimes disturbed. •Here we see a fault (E) and an igneous intrusion (D) •See if you can determine the order of the strata in this diagram. •Determining relative age in the field is sometimes very difficult because of these disturbances. Fossils •Preserved remains or traces of animals, plants, and other organisms from the remote past •Fossil Record – History of life as documented by all fossils, preserved in sedimentary rock strata •Provides a snapshot of past events in Earth’s geological history Index Fossils •Special fossils called “index fossils” indicate to geologists the boundaries in geological time. •This is another tool to determine the age of rocks. •Characteristics of Index Fossils •Easily recognizable •Abundant •Wide geographic distribution •Live a short time Fossil Record and Rock Strata •Index fossils are used to correlate the age of the rock strata •If two different rock strata in different areas on Earth contain the same index fossils, then the strata are probably the same age. Absolute Age •Most accurate form of dating, also call radiometric dating. •Uses the decay of radioactive elements to find the absolute age of a rock or fossil •This relies on the property of half life, which is the predictable time an element takes to decay. •Geologists tend to mix and match relative age and absolute age dates to piece together a geologic history. • Like saying you are 12 years old and your grandfather is 72 (absolute) instead of you are younger than your grandfather (relative). Geologic Time Scale •All of Earth’s geological history represented on a chronological time chart •Based on rock strata and the fossil record •Broken into different time chunks, the largest being the Eon, then Era, Period, and Epoch. •m.y. stands for millions of years. This diagram is not to scale. •Look how much of the Earth’s geologic history falls into Hadean, Archean, and Proterozoic (together called Precambrian) - 88% •If this were a clock, humans show up 11:58:43pm. •Notice some of the important events which happened over 4.6 b.y. Precambrian •Broken into 3 parts Hadean, Archean, and Proterozoic •Marks the beginning of Earth approximately 4.6 b.y. •Oldest rock fragment on Earth contains a mineral, zircon, that is 4.4 m.y.(found in Australia) •Very primitive life forms: one-celled animals, bacteria, jellyfish Paleozoic Era (early life) •Marks the formation of the super continent, Pangaea •Life developed rapidly •Marine plants and invertebrates •Fish and ferns •Amphibians •Mass extinction at the end of Paleozoic Mesozoic Era (middle life) •Increased complex life forms including: •Dinosaurs •Small mammals •Birds •Conifers and flowering plants •Rocks in the Petrified Forest in AZ were deposited during this time. •Another mass extinction at end of Mesozoic. Cenozoic Era (late life) •Currently in this Era •Development of: •Large mammals •Human beings •Periods of ice ages played a role in the formation of the geological features visible on Earth today. •Precambrian Time (0-87 yards) •Paleozoic Era(87-95 yards) •Mesozoic Era (95-98.5 yards) •Cenozoic Era (98.5-100 yards) Living Organisms •Fossils of previous living organisms are used to further divide Earth’s geologic history. •Life on Earth has changed over time or evolved quite a lot in 4.6 b.y. •We live in the Cenozoic Era which is further broken down into Epochs. •In which Epoch do we live? Geologic Time Scale •During the Earth’s 4.6 billion years, it has suffered 5 major mass extinctions. •An extinction is the end of a group of organisms, usually a species. Dinosaur Extinction •Most recent extinction is called the Cretaceous-Tertiary extinction event. •Happened 65.5 m.y. ago •Climate and geologic changes are thought to have weakened the dinosaurs. •A meteor strike was probably the final blow. Comments are closed.
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Erik E. Mason
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