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Before the 1700’s, scientists thought that the Earth was only 6,000 years old. This mindset was based on biblical references. However, as scientists began to better understand the processes that shape the Earth today, they realized that these forces, such as , erosion and deposition, the realized that the Earth had to be much older. By examining layers, they began to get a better understand the Earth’s history and age. Determining geological ages

• Relative ages – placing rocks and geologic events in their proper sequence, oldest to youngest.

• Absolute dates – define the actual numerical age of a particular geologic event. For example, large dinosaurs died out 65 mya. The Lavas along Rt 22 and Rt 78 were deposited about 205 mya. Relative Age Dating assigns a non-specific age to a rock, rock layer or fossil based on its position in the Strata relative to other rocks, rock layers or fossils. Relative Age Dating is based on a list of principles or rules. First principle of

• Developed by Nicolaus Steno in 1669 • In an undeformed sequence of sedimentary or volcanic rocks the oldest rocks are at the base; the youngest are at the top -Superposition Principle of Superposition Superposition illustrated by strata in the Grand Canyon 2nd principle of relative dating

• Principle of original horizontality • Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting)

3rd principle of relative dating • Principle of cross-cutting relationships 3rd principle of relative dating • Principle of cross-cutting relationships (example 2) Cross-Cutting An is always younger than the rock layer that it has intruded or cut across. Principle of Cross-Cutting Relationships

The is youngest because it cuts across layers 1-4

Layer 1 is the oldest rock layer Key to Rocks Used in Diagrams Limestone

Igneous Metamorphic Cross-cutting Relationship with multiple overlapping intrusions Erosional Features and Faults that cut across rock layers are always younger. Example of Law of Cross-Cutting Relationships

Which came first, the rock layers or the faults? Cross-cutting Normal Fault

Inclusion The Law of Embedded Fragments, or Law of Inclusion, states that rocks that are embedded in another rock must be older than the rock in which it is found. Examples of Law of Inclusions Inclusion- Conglomerate fragments in overlying Shale Inclusion- fragments included in overlying Shale Inclusion- Shale fragments imbedded in Granite intrusion

Another method of examining the Geologic Record involved examining instances where rock layers are missing (Unconformities). The processes that would bring about the removal of these missing layers require large amounts of time. Unconformities (loss of rock record) • An unconformity is a break in the rock record produced by erosion and/or nondeposition

• Types of unconformities – Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement) with time lost – Angular unconformity – tilted rocks overlain by flat-lying rocks – Disconformity – strata on either side of the unconformity are parallel (but time is lost) Layered (a) sedimentary rocks 8_9 Nonconformity Metamorphic Igneous rock intrusive rock

(b) Younger sedimentary rocks Angular unconformity Older, folded sedimentary rocks

(c)

Disconformity

Brachiopod Trilobite (290 million years old) (490 million years old) Formation of an angular unconformity Angular Unconformity Angular Unconformity Erosional Surface Horizontal younger sediments over tilted older sediments Cambrian Tapeats sandstone over Precambrian Unkar Group

What type of unconformity is this?

Grand Canyon in Arizona Angular Unconformity Angular Unconformity, Siccar Point, Scotland

Disconformity Development of a Nonconformity

An intrusion occurs

The overburden is eroded away

Pennsylvanian sandstone over Precambrian granite is a nonconformity

Sea level rises, new sediment is deposited Nonconformity- Sedimentary Rock layers over older Igneous or Metamorphic Nonconformity in the Grand Canyon - Sediments deposited over Cross Cutting Relationships in strata Zoroaster Granite across Vishnu Schist Rock Layer Correlation

• Correlation is the matching of rock layers from one area to another. – Matching rocks in different locations due to their similar characteristics – Key Beds – Stratigraphic Matching – Using Index Fossils (fossils that lived and died in one particular geologic time) to match rock layers Correlating Rock age using Index Fossils and Stratigraphic Matching Correlation of rock layers

• Matching strata of similar ages in different regions is called correlation http://www.uwsp.edu/geo/faculty/ozsvath/images/stratigraphy.jpg Correlation of strata in southwestern United States

Sections are incomplete Match with fossils and lithology Matching Rock Layers in Africa and South America

Index Fossil Requirements

Index Fossils must – be easy to identify – have been very abundant – have lived in a wide geographic area – have existed for a short geologic time (ie: someone’s picture in a yearbook) NYS Regents Exam diagram

Absolute Age Dating Radiometric Dating- Proportion of Parent to Daughter Isotopes

To get amount of parent material for each half-life, know that after one half-life, you have ½ of parent isotope left, then double your Radioactive Dating- Half Life Half Life

• The original isotope is called the parent • The new isotope is known as the daughter isotope – Produced by radioactive decay – All parent isotopes decay to their daughter isotope at a specific and unique rate – Based on this decay rate, it takes a certain period of time for one half of the parent isotope to decay to its daughter product – Half life – the time it takes for half of the atoms in the isotope to decay Tree Ring Chronology (Dendrochronology) Comparison with known tree ring sequences Can go back 10,000+ years Based on living and fossil wood Paleoclimate information Paleohydrology Archeology. • EX: The half life of C-14 is 5,730 years – So it will take 5,730 years for half of the C-14 atoms in an object to change into N-14 atoms – So in another 5.730 years, how many atoms will be turned into N-14? • HALF LIFE • In another 5,730 years, another half of the remaining atoms will degrade to N-14, and so on. • So after 2 half lives, one forth of the original C-14 atoms remain • After 3 half lives, one eighth of the original c-14 atoms still remain • Keeping cutting in half Radiocarbon Dating

– C-14 is useful for dating bones, wood and charcoal up to 75,000 yo – Living things take in C from the environment to make their bodies – Most is C-12 but some is C-14 • The ratio of these two types in the enviro is always the same • By studying the ratio in an organism it can be compared to the ratio in the environment presently