Geologic History Unit HES chapters 21 – 24 Page 550 and ff

Relative dating vs. Absolute Relative dating is determining the relative order of events without dates This is done using geologic principles that rely on common sense reasoning and logic Absolute time is when you put a date or time for an event This is done using radiometric dating which relies on the fact that radioisotopes decay at constant rates.

Geologic Principles Principle of Uniformitarianism The processes occurring today are the same processes that shaped the earth in the past. Credited to James Hutton, Scottish Geologist, 1770 The present is the key to the past.

Principle of Original Horizontality Sedimentary rocks are deposited in horizontal or nearly horizontal layers.

Principal of Superposition In undisturbed rock layers, the oldest is on the bottom and each added layer is younger than the one below it.

Principle of Cross-cutting relationships An intrusion or fault is younger than any rocks it cuts across.

Principle of Inclusions Inclusions are older than a layer they are part of.

Principle of Faunal (fossil) Succession Fossils occur in a consistent vertical order in rocks all over the world. William Smith, late 1700’s England Index Fossils - Widespread distribution - Restricted, known time of existence - Common to many environments

Unconformity A gap in the rock record caused by erosion. Three types Nonconformity occurs when sedimentary rocks overlie nonsedimentary rock layers (igneous or metamorphic) Disconformity occurs when the rock layers involved are both sedimentary Angular unconformity occurs when folding makes the layers below the unconformity tilted

Unconformities

Visual of 3 types of unconformities _1.swf _1.swf

Putting it all together

Correlation of rock strata

Geologic Time Scale See back cover of textbook A record of 4.6 billion years of Earth history Eons- measured in billion of years 1. Archean = ancient 2. Proterozoic = beginning life Archaen and Proterozoic = Precambrian 3. Phanerozoic = visible life Eras – measured in hundreds of millions to billions of years 1. Paleozoic (old-life) Age of invertebrates, but fish and reptiles appear 2. Mesozoic (middle life. Age of dinosaurs, but mammals appear 3. Cenozoic (recent life), Age of mammals and flowering plants

Precambrian time – makes up 90% of geologic time is divided into Archean and Proterozoic Eons Periods – lasted tens of millions to hundreds of millions of years, defined by life forms that were abundant or became extinct, some named for geographic regions Epoch – millions of years to tens of millions of year divisions of periods

Memorize The Eras and their periods Paleozoic =Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian Mesozoic = Triassic, Jurassic, Cretaceous Cenozoic = Paleogene, Neogene, Quaternary