GEO 102 1st Edition Exam 2 Study Guide Lectures 12 21 Lecture 12 September 22 Correlation Linking time equal rocks Presently many different depositional environments form simultaneously Later they will be preserved incompletely as a wide range of rocks How do you link the outcrops Rock types Geochemistry Fossils Fossil succession Fossils change in an orderly way through time Index fossils Extinction and evolution Absolute dating Absolute dating Relative dating has been around centuries and but absolute estimates of the age of the Earth varied The goal was a fixed and absolute age Radioactivity can act as a clock Decay and Half life Unstable nuclei eject particles predictably in order to achieve equilibrium alpha particles 2 protons and 2 neutrons beta particles Electron from nucleus Electron capture Forms a neutron after fusing with a proton Half Life For any given atom it can not be predicted when decay will occur However a POPULATION of atoms will decay like clockwork the time it takes for half of the population to decay one half life What do we date Only some elements have radioactive isotopes Only some isotopes have half lives that are useful Only some minerals contain these few isotopes Only some rocks contain pristine minerals There must be no non radioactive alteration of parent or daughter Closed system Lecture 13 September 24 Radio isotopes There are many isotope systems Each has important differences Half life Mineral hosts Abundances Decay pathways Origins Lecture 14 September 26 Minerals to Rocks Minerals form rocks o Some are simple with one mineral o Others are complex with many minerals Rock Types Igneous o From magma or lava Metamorphic o Altered through and or pressure Sedimentary o Reformed bits of other rocks broken down or from a solution Rocks linked to tectonics So why the differences Time Energy Distance Minerals Transitions Parent rock to sediment sediment to sedimentary rock Types of Sedimentary Rocks Clastic a sedimentary rock made from pieces of pre existing rock that weathered and eroded o Also called detrital Chemical sedimentary rock derived from minerals that precipitate from a fluid often water Lecture 15 September 29 Types of Physical Weathering Frost wedging ice gets into a crack of a rock expands and breaks the rock Unloading exfoliation Biological from tree roots and fungi Current glacial Thermal effects Types of Chemical Weathering Dissolution dropping acid o minerals It dissolves the mineral away Hydrolysis electrochemically plucking molecules of a mineral out of the mineral Oxidation oxygen displacing a portion of the molecules in metal rich rock o Clays do not form initially as clay Clay minerals only form as the residual broken down stuff of another mineral breaking down Erosion The transport of materials weathered from a rock The principle agents are o Liquid water the most common o Ice rare but very effective o Wind only small particles Entrainment This refers to the act of lifting a particle from a position of rest to be carried in a wind or current We can predict how this will happen with great precision using math Order of Transport Heavy particles need a lot of energy to lift and carry Clay needs a lot of energy to lift van der waals forces but very little to carry it Particles are lifted in the following order o Silt sand pebbles and clay last But they are dropped in a different order o Pebbles sand silt and clay Lecture 16 October 1 Life of a Rock Let s imagine how a parent rock granite becomes a series of daughter rocks shale sandstone and siltstone o Weathering o Erosion o Sorting o Rounding o Deposition different environments o Lithification o Uplift Sorting The longer time and distance a sediment is transported the more it will sort according to mass Rounding The longer time and distance a sediment is transported the more it will round according to hardness Environments of Deposition Other signs of depositional environments o Ripple marks o Cross bedding o Larger debris o Desiccation cracks o Fossils o Chemistry o Mineralogy Lecture 17 October 3 What is Ichnology Study of traces Neoichnology study of modern traces Paleoichnology study of fossil traces trace fossils ichnofossils o Examples footprints tooth marks nests burrows cocoons and trails The Holy Trinity of Ichnology Substrate whatever is preserving the trace Anatomy Behavior the living animal of the trace One trace maker can make a variety of traces Prehistory of Ichnology Ichnology as tracking is among the oldest of sciences Tracking developed and honed by indigenous peoples of Africa Australia and North America What is it good for To date They indicate certain ecosystems Show animal or plant presence Most traces are in place That s where the animal walked or where it was Cultivates and expands awareness Limits of Ichnology Trace makers not always identified Trace fossils might have been made by organisms with no modern analogue Often do not co occur with body fossils of trace makers Potential for too much data Lecture 18 October 6 Tectonics and Sediments The distribution of all these processes is patterned based on plate tectonics Uplift and long term preservation of these rocks is controlled to a great extent by plate tectonics o Think about California beaches as compared to Alabama beaches The Rocks Once sediments settle they may cement The cement often comes from water with minerals in the solution Compaction contributes to this Common Clastic Sedimentary Rocks Conglomerate o Big clasts poorly sorted Sandstone o Smaller clasts better sorting Siltstone o Tiny clasts well sorted Shale o Nearly pure clay Shale is the most common Sandstone is the toughest All are abundant in Alabama Chemical Sedimentary Rocks What happens when you leave a glass of sweet tea in the fridge for too long What about when you don t clean your bathtub often enough As a solution becomes more concentrated in a mineral it will solidify on an available surface precipitation Evaporates salt Limestone Chert Dolostone Coal Organisms make many chemical sedimentary rocks limestone chert coal Coal not really a rock plant material deposited in an anoxic environment like a swamp or bottomland forest compacted Lecture 19 October 10 Taphonomy What parts are resistant to decay o Hard tissues o Bones shells teeth nails claws wood seeds What causes decay o Physical destruction o Dissolution o Other critters eating the remains Think about storing your food Modes of Preservation Unaltered o Intact frozen