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Geologic Time The Foundation: Fossils and Relative Time Understanding how both are used to get a better understanding of our Earth’s past (and its future) *The Beginning: 3 Basic Rock Types* Sedimentary Rock- Rocks made by erosion of rock;( limestone, sandstone, shale and they are water soluble) Igneous Rock – rocks made by magma; granite, marble, and basalt and are water resistant) Metamorphic rock- sedimentary rocks that due to being under pressure and heat turn into more igneous nature Determining the age of rocks: 2 ways Absolute age -Actual Age (determined by Carbon dating) Relative Age -Age compared to other objects (gives you an estimate of its age; index fossils or depth provide this) *Absolute Age* It tell the actual age or how long ago an even occurred They use Radioactive dating Radioactive dating-measures the age of a material by comparing the amount of a radioactive form of an element in a rock or fossil with the amount of its decay product Precise *Relative age* It tells if something happened earlier or later than others without giving a definite date Did these Rocks fall before or after the rock layers formed? *Law of superposition* Formed by undisturbed sedimentary rock layers Older rock layers lie beneath younger rock layers: (Grand Canyon at first glance) Exceptions to the Rule: 4 Overturned Bed Angular Bed Intrusions Unconformity Law of Superposition EXCEPTIONS: Overturned bed Sedimentary Rock layers totally misplaced Commonly caused by Compression forces and found in mountains Law of Super Position EXCEPTIONS: Angular bed Horizontal rocks are younger than the tilted rocks Tilted rock caused plate-tectonic forces Law of Superposition EXCEPTIONS : Intrusion Caused by Magma Igneous rocks are younger than any sedimentary rocks they cut *Law Of Superposition EXCEPTIONS*: Unconformity Missing layer or gap in the rock record: caused by magma intrusion or erosion Makes it hard to understand how Earth has changed during a specific period of time Missing Layer destroyed by magma *2 Types of Erosion of Rocks* Mechanical Weathering : water freezes and unfreezes causing Rocks to split open from ice the expanding in the cracks; also wind blowing sand against rocks *2 Types of Erosion of Rocks: Chemical Weathering * Chemical Weathering- Acid rain dissolves away the rock Why do We Care about Rocks? Rocks provide us with a “Picture” of the Earth’s Past History Fossils *Fossils* Important to understand the history of life on Earth Older rock—simpler organisms and Younger rock-more complex organisms Shows how species have changed: EVOLUTION Shows how species are related to another or changed to environmental change *How Fossils Form* 1 Quick Burial of organism in soft sediment: Mudslide, volcanic ash, quicksand, mudpits, tarpits 2 Organism remains undisturbed for long period of time 3. Water slowly enters area soft tissue is dissloved and water slowly eats away bone/shell 4. If water flow is “fast” complete organism will be eaten away creating “hole” in the rock. Leaves basic shape of organism 5. If water eats away very slowly minerals in water can replace bone creating a Cast. Minerals recrystalize as soon as bone is dissolved creating a PERFECT COPY! 6. Fossil is then found millions of years later: FACT-Only 17 T-rex Skeletons have ever been found Ways Fossils form Fossil Process Notes Molds and casts Acids eat away the skeleton or shell and leave an impression in the rock If the mold fills with minerals it becomes a cast Tracks and trails Imprints are left in mud which later hardens Dino tracks are common Carbonization Oils leave the plant and the remaining matter becomes a layer of carbon fossilized plants Petrification Minerals in plant cells crystallize; minerals enter openings or cavities in shells or bones Common with plants Replacement Object buried in mud has its molecules replaced by minerals rare Recrystallization mineral aragonite in shells turns then into calcite Preserves the general shape of the animal Soft tissue preservation Mummification, frozen in ice Fossils are relatively you and do not last long Traps Whole organisms locked in tar pits, asphalt, amber etc… Molds Hollow area in sediment in the shape of an organism or part of organism Hard part of organism buried in sediment Water carries dissolved minerals and sediment may seep into the empty space of the mold Cast/Recrystalization Solid copy of the shape of an organism: bones replaced by minerals in water. EXTREMELY ACCURATE; VERY RARE Petrified Fossils where minerals replace all or part of an organism Turns into stone Water rich with mineral seep into spaces Carbonization Oils from organism turn into carbon imprint on rock Common in plants and Fish Trace fossils Fossilized footprint or trail *INDEX FOSSILS* Index fossilcommonly found fossil worldwide that can give you an approximate age and environment of the area then. (Trilobites)