GEOLOGIC TIME

Early Efforts of Dating the Age of the Earth came from James Ussher (mid 1600's) 4004 B. C. Georges Cuvier During the 1700 and up to the late 1800's, catastrophism(الكارثة) influenced the formulation (صياغة)of explanations (تفسيرات)about Earth. Catastrophism states that Earth's landscapes have been developed primarily by great catastrophes.

James Hutton considered The Father of Modern geology. Why James Hutton considered The Father of Modern geology. Why? In1785 in his book “theory of the Earth” put forward one of the fundamental principles of modern geology. It is the Uniformitarianism

the physical, chemical, and biological laws that operate today have also operated in the geologic past. The idea is often summarized as "The present is the key to the past."

Hutton said that processes that appear to be slow-acting could, over long spans of time, produce effects that were just as great as those resulting from sudden catastrophic events. -Not all process work as it was in the past -Evidences are: 1.Impact of meteorites 2. Many cycle of processes “mountains building”

* 1800s-1900s various attempts to determine Earth’s age Georges Buffon (mid 1700's) * 1800s-1900s various attempts to determine Earth’s age Lord Kelvin (late 1800's) 75,000 yr 20-40 m.y.

Charles Walcott (1893) John Joly (1899) 75 m.y. 90 m.y.

The two types of dates used by geologists to interpret Earth history are: (1) relative dates, which put events in their proper sequence of formation i.e., extinction of dinosaurs ~65 M.y. (2) numerical (absolute) dates, which pinpoint the time in years when an event took place.

III. Types of Geologic Dating A. Relative Dating 1. Law of Superposition 2. Law of Original Horizontality 3. Law of Cross Cutting Relationships 4. Law of Faunal Succession 5. Inclusion 6. Unconformity 7. Principle of lateral continuity

A. Relative Dating 1. Law of Superposition -Nicolaus Steno (1636-1686), Danish geologist -Recognize a sequences of events in an outcrops of sedimentary rocks: Sates that in an undeformed sedimentary rocks, each bed is older than the one above it and younger than the one below, see Grand canyon

A. Relative Dating 2. Law of Original Horizontality Steno also recognized that, most layers of sediments are deposited in an horizontal position i.e., grand canyon. 3. Law of Cross Cutting Relationships Fault , magma intrude and cut rocks such that we can assume fault or intrusion is younger than the rock affected.

Relative Dating 4. Law of Faunal Succession The principle of faunal succession, also known as the law of faunal succession, is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances. This principle, which received its name from the English geologisit William Smith.

Relative Dating 4. Inclusion are fragments of one rock unit that have been enclosed within another rock mass, therefore the rock containing inclusion is the younger of the two. 5. Unconformity Layer deposited without interruption called conformable , but all breaks in the rock records are termed unconformities ~ long period during which deposition ceased, erosion removed previously formed rocks, and then deposition resumed. -All happen during lifting, erosion, subsidence and renewed sedimentation

Disconformity: more common, Strata on either side are essentially parallel, Difficult to identify, Little evidence of erosion. b) Angular Unconformity: Tilted or folded sedimentary rocks that overlain by younger, more flat-laying strata. ~ pause of deposition followed by folding and tilting as well as erosion (recognized by James Hutton). c) Non Conformity: The break separates older metamorphic or intrusive igneous rock from younger sedimentary strata. ~To develop there must be period of uplifting and erosion of overlying rocks.

Unconformity Types Using Grand Canyon as Example

Illustration of Relative Age Principles K I H F G Older :G –F– H– B– I– E– K– C– D– A : Younger

Radiometric Dating each atom has nucleus containing +ve Protons and neutral (no charge) Neutrons and orbited by –ve electrons Atomic number : number of protons in the nucleus H=1, O= 8, U=92. 99.9 % of atoms mass found in the nucleus. Mass number : summation of Protons and Neutrons. Number of Neutrons varies, so called Isotope. i.e., Uranium U has 92 atomic number but atomic mass varies such as U234, U235, U238: same in nature and chemical reactions Radioactivity means that in some isotopes the nuclei are not are unstable , because the forces that combined P and N are weak. ~ nuclei spontaneously break apart (decay).

III. Types of Geologic Dating A. Radiometric Dating 1. Types of decay alpha decay beta decay electron capture neutron capture

III. Types of Geologic Dating alpha decay , 2P+2N atomic number reduced by 2 + mass number reduced by 4

III. Types of Geologic Dating beta decay: N=p+e, atomic number increased by one

III. Types of Geologic Dating Electron capture: electron captured by nucleus, e+P=N, proton is fewer by one atomic mass unchanged N P

Unstable (radioactive) isotope is referred to as the parent The isotopes resulting from the decay of the parent are the daughter products. 92U238 decays 8 Alpha, 6 Beta To produce stable daughter product : Lead 206, 82Pb206

Why radiometric dating reliable: because the rates of decay for many isotopes have been precisely measured and not vary under the physical conditions that exist in Earth outer layer Half-life The time required for one half of the nuclei in a sample to decay is called the half-life of the isotopes 100, 50:50, 25:75, 13:87, 6:94, 3:97 1:1 1:3 1:7 1:15 1:32 1 2 3 4 5

8 {lose 2 protons} + 6 (add 1 proton) An Example…. Parent U238 → 8 alpha + 6 beta 8 {lose 2 protons} + 6 (add 1 proton) {lose 2 neutrons} - 16 protons + 6 protons = - 10 protons - 16 neutrons - 32 atomic mass - 10 atomic number U238 – 32 = “X” 206 92 protons – 10 protons = 82 protons “X” 206 with 82 protons U238 – 32 = “X” 206 92 protons – 10 protons = 82 protons

C14 → N14 5730 years organic carbon

B. Carbon Dating C6 12 P6+N6 C6 13 P6+N7 C6 14 P6+N8 N7 14 P7+N7 N14---Neurtron capture-----C14: C6 14 C14 ---Beta decay-----N14: N7 14

Source of Errors Radiometric must be obtained in closed system Source of Errors Radiometric must be obtained in closed system. Neither addition nor loss of parent and daughter products, Ex. Some daughter like Ar in K----Ar decay may release from system since it is gas. Some radiometric materials do not decay directly into as table daughter products, Ex. U 238 produces 13 intermediate unstable daughter products before fourteenth and final product the stable Pb 206 In sedimentary rock , grains are not the same age as the rock in which they aoccur, rather, the sediment must have been weathered from rock of diverse age It is also difficult for metamorphic rock since the date may indicate a number of subsequent metamorphic phases

Geologic Time Scale -Earth history was developed in Western Europe and Great Britain in 19th and 20 century. Geologic time scale subdivide the 4.5 billion-year history of Earth. Eons: greatest expanse of time 540 MY ago is phanerozoic (visible life) Eras: paleozoic , mesozoic, cenozoic, ancient middle recent lifes Period Epoch: early, middle, and late