Transcript FOSSILS
FOSSILS PRESERVED REMAINS OR TRACES OF AN ORGANISM THAT LIVED IN THE PAST Where Fossils are Found Fossils can only be found in SEDIMENTARY ROCKS Fossils are almost never found in IGNEOUS rocks because magma is found deep within Earth where no living things exist, and lava at the surface burns organisms before fossils can form. Fossils are rarely found in metamorphic rocks because heat, pressure, and/or chemical activity that causes a rock to change, also destroys or damages the fossils. WHAT FORMS THE FOSSIL • Hard Parts of organisms such as shells and bones are more resistant to decay than soft tissue • Pore space in an organisms hard tissue • Soft tissue replaced by minerals HOW FOSSILS FORM FORMATION OF FOSSILS • MOLD FOSSIL – FORMS WHEN SEDIMENTS BURY AN ORGANISM AND THE SEDIMENTS CHANGE INTO ROCK. THE ORGANISM DECAYS LEAVING A CAVITY IN THE SHAPE OF THE ORGANISM • CAST FOSSIL – FORMS WHEN A MOLD IS FILLED WITH SAND OR MUD THAT HARDENS INTO THE SHAPE OF THE ORGANISM. MOLD AND CAST • Which is the mold and which is the cast? FORMATION OF FOSSIL • PETRIFIED FOSSIL – FORMS WHEN MINERALS SOAK INTO THE BURIED REMAINS, REPLACING THE REMAINS, AND CHANGING THEM INTO ROCK. FORMATION OF FOSSILS • PRESERVED FOSSIL – FORMS WHEN ENTIRE ORGANISMS OR PARTS OF ORGANISMS ARE PREVENTED FROM DECAYING BY BEING TRAPPED IN ROCK, ICE, TAR OR AMBER. • CARBONIZED FOSSIL – FORMS WHEN ORGANISMS OR PARTS, LIKE LEAVES,STEMS,FLOWERS,FISH ARE PRESSED BETWEEN LAYERS OF SOFT MUD OR CLAY THAT HARDENS SQUEEZING ALMOST ALL THE DECAYING ORGANISM AWAY LEAVING THE CARBON IMPRINT IN THE ROCK. FORMATION OF FOSSILS • TRACE FOSSILS – FORMS WHEN THE MUD OR SAND HARDENS TO STONE WHERE A FOOTPRINT, TRAIL OR BURROW OF AN ORGANISM. DATING ROCK LAYERS Relative Age The age of a rock compared to the ages of rock layers. Best when rock layers in original sequence. Relative Dating Absolute Age The age of a rock given as the number of years since the rock formed. Radiometric dating Principle of Original Horizontality • The principle of original horizontality states that sediments are deposited in horizontal layers that are parallel to the surface on which they were deposited. • This implies that tilted or folded layers indicate that the crust has been deformed. POSITION OF ROCK LAYERS Its difficult to determine the absolute age so geologists use method to find a rock’s relative age. Use the: LAW OF SUPERPOSITION: in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it. EXTRUSIONS Clues from Igneous Rocks: 1. Lava that hardens on the surface is called an Extrusion example – an eruption would put a layer of igneous rock on top of sedimentary rocks. Rock layers below an extrusion are always older than the extrusion. The extrusion is in black EXTRUSIONS Now the extrusion is the youngest layer INTRUSION • Magma that cools and pushes into bodies of rock and hardens is called an Intrusion • An intrusion is always younger than the rock layers around and beneath it. The intrusion (in red) is now younger than the surrounding rocks. INDEX FOSSILS Index fossils are fossils or organisms that are useful in correlating the sedimentary rocks in which they are found. TO BE AN INDEX FOSSIL • Must have lived only during a short part of earth’s history • Many fossils of the organism must be found in rock layers • The Fossil must be found over a wide are of the earth • The organism must be unique Which is the index fossil? • Index Fossils INDEX FOSSILS Trilobite Ammonite PALEOZOIC MESOZOIC Brachiopod CAMBRIAN FOSSIL CORRELATION