Time and Geology Physical Geology 12/e, Chapter 8

Define the following terms: Geology textbook page 194-204 Geologic Time Terms: Definition Quick Sketch Uniformitarianism Numerical age Relative time Original Horizontality Superposition Lateral Continuity Cross-Cutting Relationships Inclusions Unconformity Disconformity Angular Unconformity Nonconformity Correlation

Zircon Article Read the article On the same paper as your terms, tell at least 5 facts from the reading that explain or describe earth’s geologic history. Whether finished or not all work is due today.

Relative vs. Numerical Age Relative age - the order of events or objects, from first (oldest) to last (youngest) Determined by applying simple principles, including original horizontality, superposition, lateral continuity, cross-cutting relationships, inclusions, unconformities, and correlation of rock units and fossils Numerical age/ Absolute Age - the age of events or objects, expressed as a number or numbers Determined using radiometric dating (determining how much radioactive decay of a specific element has occurred since a rock formed or an event occurred)

The Law of Superposition In any undisturbed sequence of strata, the oldest layer is at the bottom of the sequence, and the youngest layer is at the top of the sequence.

The Cross-Cutting Law Any feature that cuts across a body of sediment or rock is younger than the body of sediment or rock that it cuts across. NOTE: • A fracture is a crack in rock. • A fault is a fracture along which movement has occurred.

Law of Inclusion If one rock body contains fragments of another rock body it must be younger than the fragments of rock it contains. OR…The inclusions are older than the rocks which contain them.

Principles associated with Relative Dating The comparing of rock units to decipher their age relative to one another Principle of Original Horizontality Sedimentary. If layers are folded, episode of deformation must have occurred after rocks formed. Age of folding is younger than youngest deformed rocklayers are deposited in approximately horizontal sheets unit.  

Relative Age Determination Contacts - surfaces separating successive rock layers (beds) Formations - bodies of rock of considerable thickness with recognizable characteristics allowing them to be distinguished from adjacent rock layers Original horizontality - beds of sediment deposited in water are initially formed as horizontal or nearly horizontal layers

Relative Age Determination Superposition - within an undisturbed sequence of sedimentary or volcanic rocks, layers get younger from bottom to top Lateral continuity - original horizontal layer extends laterally until it tapers or thins at its edges

Relative Age Determination Cross-cutting relationships - a disrupted pattern is older than the cause of the disruption Intrusions and faults are younger than the rocks they cut through Baked contacts - contacts between igneous intrusions and surrounding rocks, where surrounding rocks have experienced contact metamorphism Inclusions - fragments embedded in host rock are older than the host rock

Unconformities Unconformity - a surface (or contact) that represents a gap in the geologic record Disconformity - an unconformity in which the contact representing missing rock layers separates beds that are parallel to each other Angular unconformity - an unconformity in which the contact separates overlying younger layers from eroded tilted or folder layers

Unconformities Nonconformity - an unconformity in which an erosional surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock Plutonic and metamorphic rocks exposed by large amounts of erosion Typically represents a large gap in the geologic record

Correlation Correlation - determining the time-equivalency of rock units Within a region, a continent, between continents Physical continuity Physically tracing a continuous exposure of a rock unit Easily done in Grand Canyon Similarity of rock types Assumes similar sequences of rocks formed at same time Can be inaccurate if very common rock types are involved Correlation by fossils Fossil species succeed one another through the layers in a predictable order (faunal succession) Similar fossil assemblages (groups of different fossil species) used

Numerical Age Dating Numerical dating - puts absolute values (e.g., millions of years) on the ages of rocks and geologic time periods Uses radioactive decay of unstable isotopes Only possible since radioactivity was discovered in 1896 Radioactive isotopes decay in predictable manner, depending on the half-life (time it takes for a given amount of radioactive isotope to be reduced by half)

Fossils Earth Science Textbook page 343-45 and Fossils Handbook page 10-15 Direct fossils Indirect fossils Petrified Replacement Mold Cast Carbonization Impression Amber Tracks Coprolites

Petrified Replacement

Mold & Cast Carbonization

Impression Amber

Tracks Coprolites

Conditions for Fossil Preservation (Earth Science Textbook page 345) A) B)

Combining Relative and Numerical Ages Radiometric dating gives numerical time brackets for events with known relative ages Individual layers may be dated directly Radiometric dating of units above and below brackets age of units in between Geologic Time Scale Divided into four Eons Hadean, Archean, Proterozoic, Phanerozoic Precambrian (all time prior to Phanerozoic) represents 87% of geologic time)

Geologic Time Scale Standard geologic time scale Worldwide relative time scale Subdivides geologic time based on fossil assemblages Divided into eons, eras, periods, and epochs Precambrian - vast amount of time prior to the Paleozoic era; few fossils preserved Paleozoic era - “old life” appearance of complex life; many fossils

Geologic Time Scale Mesozoic era - "middle life" Dinosaurs abundant on land Period ended by mass extinction Cenozoic era - "new life" Mammals and birds abundant We are currently in the Recent (Holocene) Epoch of the Quaternary Period of the Cenozoic Era Most recent ice ages occurred during the Pleistocene Epoch of the Quaternary Period

Age of the Earth Numerical dating gives absolute age for Earth of about 4.56 billion years Oldest age obtained for meteorites, believed to have been unchanged since the formation of the solar system Earth and rest of solar system very likely formed at this time Geologic (deep) time is vast A long human lifetime (100 years) represents only about 0.000002% of geologic time

Geologic Time Pre-Post Test Practice Each person needs a scratch piece of paper Number 1-20 Answers only Work as a group/table partner

Geologic Time Review Test Review

Divisions of Geologic Time Eras are subdivided into periods...periods are subdivided into epochs. Era Period Epoch E + P = EP

Today… Today we are in the Holocene Epoch of the Quaternary Period of the Cenozoic Era. Which unit is the largest? Which unit is the smallest?

Geologic Time How do geologists determine how old rocks are?   How do geologists determine how old rocks are? 1. Relative dating -- determine whether the rock is older or younger than other rocks 2. Absolute dating -- use radiometric dating techniques to determine how long ago the rock formed in the exact number of years *Not all rocks can be dated absolutely, so combinations of techniques are used.

Relative Age Dating

The Law of Superposition In any undisturbed sequence of strata, the oldest layer is at the bottom of the sequence, and the youngest layer is at the top of the sequence.

The Cross-Cutting Law Any feature that cuts across a body of sediment or rock is younger than the body of sediment or rock that it cuts across. NOTE: • A fracture is a crack in rock. • A fault is a fracture along which movement has occurred.

Law of Inclusion If one rock body contains fragments of another rock body it must be younger than the fragments of rock it contains. OR…The inclusions are older than the rocks which contain them.

2 A D C B E Superposition/Cross-Cutting

3 A B D E C Superposition/Cross Cutting

4 H H G G F C E F E D M B A C D C A B M

Numerical Age Dating Numerical dating - puts absolute values (e.g., millions of years) on the ages of rocks and geologic time periods Radioactive isotopes decay in predictable manner, depending on the half-life (time it takes for a given amount of radioactive isotope to be reduced by half)

Correlation Correlation - determining the time-equivalency of rock units Within a region, a continent, between continents Physical continuity Similarity of rock types Correlation by fossils

End of Chapter 8