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EARTH’S HISTORY Creation of Sedimentary Rocks Law of Original Horizontality: All sedimentary rocks are originally deposited horizontally or flat due to gravity and erosion. Sedimentary rocks that are no longer horizontal have been titled from their original position. The sediment that forms sedimentary rocks is deposited in flat layers one on top of the other. Over time, the sediment hardens and changes into sedimentary rock. Cementation • A major chemical digenetic change in which minerals are precipitated in the pores of sediments, forming cements that bind clastic sediments and rocks. Lithification The process that converts sediments into solid rock by compaction or cementation Question Why are sedimentary rocks often formed in large, flat layers? A. Flat layers are formed whenever magma flows outward from Earth. B. Sediments find flat surfaces on Earth to collect. C. Sediments usually collect on Earth’s surface as flat deposits. D. Heat and pressure cause sediments to flatten out into sheets. Question What must occur for loose sediments to become a rock? A. minerals from water flowing through them must cement the sediments together B. magma flowing upward must cement the particles of sediment together. C. heat and pressure squeeze the sediments closely together and they begin to stick. D. sediments are magnetized by Earths magnetic field and are attracted to each other. Question How do sediments carried by a river act when they reach a lake? A. they pile up at the far end of the lake. B. they spread out on the lake bottom C. they form a small mountain when the river meets the lake. D. they dissolve in the lake water and disappear. Relative Dating Relative age of a rock is its age compared to the ages of other rocks. Doesn’t provide it absolute age, like a birthday. Law of Superposition: geologist use to determine the relative age of sedimentary rock layers. In horizontal sedimentary rock layer the oldest layer is at the bottom and each higher layer is younger than the layer below it. Relative Dating Continues Geologists also use extrusions and intrusions of igneous rocks to determine relative age Extrusions is lava flowing across Earth’s surface. Each layer added is younger then the previous. Intrusions is magma pushing into bodies of rocks. The intrusion is always younger then the body of rock. Relative Dating Continues Geologist also use faults to determine relative age. Fault is a break in Earth’s crust A fault is always younger than the rock it cuts through Relative Dating Continues Geologist use gaps in the geologic record to determine relative age. Unconformity: Where a rock layer has been eroded away leaving a gap in the geologic record hays.outcrop.org Relative Dating Continues Geologist use index fossils Index fossil Fossil must be widely distributed and represent a type of organism that existed only briefly. Help to match up relative ages over the Earth Index Fossil Activity Strata Diagram Question According to scientific assumptions, which rock layers in the Grand Canyon will usually contain older fossils? Those found: A. nearer to the surface or the top of the canyon. B. at the edge of a cliffs. C. in rock layers located half way down the canyon. D. in the rock layers located deep in the canyon. Fossil Formation Molds and Casts Mold: hollow area in sediments in the shape of an organism or par of an organism Minerals replace all or part of an organism Done with mineral solution: minerals are dissolved in water Petrified wood Trace Fossils Cast: solid copy of the shape of an organism Petrified Fossils: turned to stone Hard parts such as shells or bones Provide evidence of the activities of ancient organisms Foot prints, burrows, etc.. Preserved Remains Tar traps that preserved bones Amber/sap: protects body from decay Insects Freezing preserve the most of the organism such as bones, hair, and skin. Mold and Cast Cast Mold petrifiedwoodmuseum.org Petrified Fossils animals.howstuffworks.com Trace Fossils answers.com Butcher Fall 2008 Geo Trip Preserved Remains Frozen Mammoth: pinktentacle.com Amber: dsc.discovery.com Change Over Time Paleontologists are scientists who study fossils Fossils Record Fossils allow us to piece together what past environments may have looked like. Provides evidence about the history of life and past environments on Earth. Also shows how different groups of organisms have changed over time. Climate Changes in Earth’s surface: swamps, lakes, dry deserts, etc… Older rocks contain similar fossils, where younger rocks contain fossils similar to our time. Fossils record show us that organism have changed or evolved over time. Evolution: is the gradual change in living things over long periods of time. Extinct: organism is not living anymore. Question What evidence do scientists have that supports the idea that Earth’s surface has changed over time? A. fossils of rainforest plants in areas that are deserts today. B. rocks that came from outer space C. modern plants and animals that look like ancient organisms. D. climates in some places on Earth that are different than other places. Question In the dry and hilly region of southeast Utah, many fossil remains have been uncovered. Trilobite casts, petrified bones of alligators, and preserved dinosaur footprints, were all found in the same location, but in different rock layers. What do these fossil remains tell about the history of this region? A. this area was a gathering place for a variety of animals. B. ancient flooding in this region washed fossil remains into this area. C. the Earth’s surface and climate has changed over time. D. this region has always had hot, dry climates. Question In a region of Wyoming covered by rolling hills, small lakes, and dry gulches, a petrified shark’s tooth was found. What does this fossil indicate about the geological history of this region? A. it was once covered with a vast freshwater lake. B. it was once covered by a swamp or marshland. C. it was once covered by an ocean. D. it has always had the same geography as it has today. Question Which of the following is a typical location where a bone or shell could become petrified? A. buried in the sand of a desert. B. submerged in shallow water or mud. C. in the frozen soil of the arctic. D. covered with leaves in a tropical rainforest. Question Why do recently deposited rock layers contain fossils more like animals that are alive today? A. Rocks that are deposited over time may have moved. B. Animals cannot form fossils if they are alive today. C. Rock layers do not change quickly. D. The fossils are the most recent ancestors of today’s animals.