Earth Science

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(1)CLASS NOTES. Earth Science Unit 13: Geologic History Topics: Geologic History, Relative Dating, Bedrock Correlation, Radioactive Dating 1.

(2) In the Beginning… of TIME Universe began (Big Bang) • ~13 billion years ago (bya) Solar System formed • ~4.6 bya Earth accreted (form & solidify)? • ~4.54 bya.

(3) Geologic Time Table. ESRT pg 8 & 9.

(4) Most table divisions based on life-events that have occurred in the past • Eras: divided based upon when dominant Eras: divided based upon when dominant life forms existed Cenozoic -- "Age of Mammals" Mesozoic -- " Age of Reptiles“ Paleozoic -- " Age of invertebrates". • Periods: begin/end based on beginning/end of a certain type of animal Jurassic Period began when Giant Dinosaurs were abundant..

(5) • Eons: longest/largest Periods of Time. Largest --------------------------> Smallest. Geologic Time Increments. • Eras: 2nd longest ~hundreds of millions of years (except Cenozoic) • Periods: smaller; divide Eras • Epochs: smallest; divide periods.

(6) EON•Precambrian  4600-544 mya •Phanerozoic  544-0 mya ERA•Proterozoic  •Archean  •Paleozoic 544-251 mya •Mesozoic  251-65 mya •Cenozoic  65-0 mya PERIOD•Jurassic  206-142 mya EPOCH•Holocene  0.01-0 mya (NOW) Time Scale. Millions of Years Ago (mya).

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(10) Fossil Record • Fossil: evidence of past life. – footprints, original remains of plants/animals, molds & casts. • Most in Sedimentary Rocks.

(11) 6 Types of Fossils 1. Molds 2. Casts 3. Original Remains 4. Carbon Films 5. Trace Fossils 6. Replaced Remains.

(12) Molds Hollow depressi on left in rock.

(13) Cast Fossils Minerals fill in a mold & making a copy or cast.

(14) Original Remains Unchange d/ trapped remains  Insects in amber.

(15) Carbon Film Thin carbon films that look like the organis m.

(16) Trace Fossils Evidence of past activity: burrows, tracks, trails.

(17) Replaced Remains Organic material replaced with mineral matter Ex: petrified wood, dino.

(18) Inferences from fossil evidence • Older fossils were simple in structure. As time passed life forms began to increase in size and complexity • Types of animals found in a location indicate type of past climate • Coral fossils = tropical climate, • marine fossils = shallow or deep water environment.

(19) Evolution •Simply stated: evolution = change •Simple animals become more complex.

(20) Index Fossils • Existed in a short period of time and over large area • Easily identified & abundant. Imp. orta nt. • Useful in marking a period of time • If an index fossil is found in a layer of rock, the layer must have been formed in specific period of time.

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(22) NY State Fossil. Eurypterid aka Sea Scorpion • Up to 2m in length • When did they exist?? Use ESRT • Late Ordovician ~450 mya  Late Devonian ~362 mya.

(23) Key Bed • Single rock layer with same characteristics as an index fossil – Relatively thin, widespread area, deposited rapidly, easy to identify. • Ex: volcanic eruption can produce a key bed because an eruption happens over a short period of time and usually covers a large area.

(24) Who’s got the TIME?. RELATIVE TIME. – order/sequence is known – Unsure of actual date of occurrence • Ex: trilobite fossil is older than dinosaur fossil. ABSOLUTE TIME – Actual/exact age • Ex: dinosaur fossil is 250 million yrs old.

(25) How geologists find the RELATIVE AGES of rocks using RELATIVE TIME?.

(26) I.. 5 Relative Dating Laws Law of SUPERPOSITION:. sedimentary sequence is OLDEST on BOTTOM and YOUNGEST ON TOP (if undisturbed). II. Law of Original Horizontality:. rocks are usually deposited flat and level (parallel to Earth’s surface). III. Law of Folds and Tilts: the fold/tilt is. younger than the rocks that are folded/tilted (rocks were there first so they are older). IV. Law of Cross Cutting Relationships - a. fault or igneous rock intrusion is younger than the rock layers it has broken or intruded upon(cut across). V. Law of INCLUDED FRAGMENTS - pieces of rock found buried WITHIN another rock must be OLDER (formed first) Ex: conglomerate.

(27) I. LAW OF SUPERPOSITION YOUNGEST LAYER 2. nd. OLDEST LAYER. OLDEST LAYER.

(28) II. Law of Original Horizontality III. Law of Folds and Tilts Sediments, like those on an ocean/lake bottom, are usually deposited in flat layers. If sedimentary layers are not horizontal, there is an inference the layers were deposited first, then folded/tilted later..

(29) STEP 1. STEP 2.

(30) IV. Law of Cross Cutting Occurs when magma squeezes into or between layers of preexisting rock OR… Layers of pre-existing rock are broken by a fault due to compressional or extensional forces.

(31) IV. LAW OF CROSS-CUTTING BY IGNEOUS INTRUSIONS & FAULTS. Igneous intrusions fault.

(32) Igneous Intrusions • body of igneous rock that made its way into pre-existing rock •can then force its way through cracks in the rocks and can wedge them apart.

(33) V. LAW OF INCLUDED FRAGMENTS.

(34) Missing Time in Rock Record.

(35) 1. UNCOMFORMITY • Buried surface of erosion separating two rock masses • Represents a gap in geologic time (because that layer was exposed to the atmosphere for an amount of time in order for the erosion to happen, then another layer came and buried it…).

(36) 1. a) Angular Unconformity If layers below unconformity are at a different angle than the layers above it..

(37) 2. DISCONFORMITY • When a rock layer above the unconformity is parallel to the layers below • Appears as though layers are “missing” IN THE ROCK RECORD!.

(38) Practice – What steps formed this cross section?. B H I G. F E. C. J A. Oldest . A E C F G. J. D I. H B. Youngest. D.

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(40) Line XY = UNCONFORMITY. Contact Metamorphism. Index Fossil. Igneous Intrusion.

(41) Rock • rock Correlation layers from one area matched. to layers of another area (correlated) using Index Fossils & Key Beds.

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(45) Tell me what’s going on… • • • • •. Deposition of G,F,E,D/H,C Folding/Uplift of G,F,E,D/H,C Erosion (UNCONFORMITY) Deposition B,A Erosion.

(46) 1. What is Letter A? IGNEOUS INTRUSION 2. What do the little dash marks mean? CONTACT METAMORPHISM 3. What does line X to Y represent? EROSION/UNCONFORMITY 4. What are all of the little pictures representing? INDEX FOSSILS 5. What is the name of the Index Fossil found in the LIMESTONE layer? CTENOCRINUS lived during the Devonian ( approximately 400 mya).

(47) Absolute Time •provides a specific date • Tree rings can be used to mark the passing of each season and therefore obtain a numerical age (dendrochronology) – Tree rings can be used to date events up to 3000 years ago.

(48) RADIOACTIVE DATING. • Radioactive elements decay at a known/steady rate • Amount of Radioactive element (Parent) is compared to Amount of Decay Product (Daughter) ALWAYS PARENT  DAUGTHER. C14  N14.

(49) ESRT page 1 (after this you’ve learned it all!!!).

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(51) HALF LIFE. • ½ the original UNSTABLE amount (Parent) remains after each round creating the new STABLE product (Daughter) • Always adds to 100% • So…if 100 % Parent, 0% Daughter • One Half Life  50% Parent; 50.

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(53) Understanding RADIOACTIVITY. • Radioactive element has an unstable nucleus • Over time radiation is given off from the nucleus ( Alpha, Beta, and Gamma Rays) • As the nucleus decays, it becomes a different element – Ex: Carbon–14, (radioactive) changes to Nitrogen-14 (stable).

(54) Okay… so if we have 500g of C250 g C-14 14, 250 g N-14. g C-14 In 1 half-life11,400 (5,700 years) 125 we’ll 375 g N-14 have __________ 62.5 g C-14. 17,100. 437.5 g N-14. In 2 half-lives (__________years)  31.25 g C-14 468.75 g N-14 22,800 __________ In 3 half lives (__________years). . In 4 half-lives (__________years). . ____________ ____________.

(55) Carbon-14 is good for dating things up to ~50,000 years old because it has a short half life.

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