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ENVR 202 - Lec 4.docx

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McGill University
ENVR 202
Brian Leung

Class 4 – January 16 , 2013 When did the young earth become fit for life? Readings: Ch11 – pg 16and22, 26-30, 33, 102-103 What do I need for life?  Clues in rocks, evidence of life, look for conditions which enable life, narrow down certain types of rocks  Basic reqs. To sustain life, what we need and what is their purpose o Breathable oxygen – respire to produce energy, not universal o Water (liquid H20) – movement of nutrients, medium to dissolve molecules together, reactions occur b/w elements o Source of energy o Some essential elements When did the Earth become fit for life?  How long has water been around  Rock records!!! o Igneous (cooling from liquid magma, extrusive – volcanic, intrusive), sedimentary, metamorphic (heat and pressure applied) o Sedimentary rocks  Form from ions dissolved in H2O, or fragments eroded from earlier rocks  Most accumulated in water  ALSO wind o Magma – crystallization = igneous intrusive, consolidation = igneous extrusive o Ingneous intrusive .. . . .. ROCKS – proof of water  We can see the action of water, like ripples or dunes, on the surface of rocks – related to direction that water was moving or wind  Cracks – mud getting wet and then drying  In deeper ocean basins, sediment accumulates as fine mud that is compacted to shale. Submarine slumps (slides) may happen, laying down graded beds o Settle in a patterns: Graded bed: coarser grains on bottom, grain size getting smaller upwards o Short events o Tell us about ancient envr  Sole marks of an ancient river bed o We can tell which way the current used to flow – marks elongated in direction of current. Through moving objects or scraping  Sedimentary rocks are a good place to look o Rocks laid down in h2o are made of grains arranged in patterns called “sedimentary structures” o These patterns haven’t changed through time o We can recognize grains that settled in still water vs with fast currents Relative Dating  Inferring the sequence in which older to younger events recorded in rocks occurred o Superposition - in which order did these….? o Angular unconformity – Sedimentation +tilting + erosion + new sedimentation  How do you date the layers if it’s tilted – look for ripples (one sided! Flat on bottom) find upside then you know the layers parallel are younger on ripple side, V shape cracks – narrower at bottom, grain size – smaller grain = younger  The sedimentary record is incomplete, full of gaps o Sed rocks may lie flat on top of layered rocks that are folded o These are undulated in a pattern that loose sediment cannot hold o On earliest sea floor, rocks were folded, raised and eroded, later the sea was rising again. New sediment layed down o No record of the erosion, the rock record gone.  Fossils o Rock record shows succession of fossils through time o In any one area fossils can be compared, some always show a shorter biostratigraphic range than others o Fossils w/ this short range (in chart on slide) are D, T, R, O, F  These help us recognize ages of rock  Species lived at different times, some more common than others (geographically) o The ones with short ranges can give us a relative age of rock Absolute Aging  Most elements have more than one isotope, w. different masses b/c of different #’s of neutrons  Radioactive isotope o An unstable atom which eventually loses or gains protons and or neutrons o Changing the # of protoons turns any element i
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