Visualizing Earth Science By Z. Merali and B. F. Skinner Chapter 10 – How Old is Old? The Rock Record and Deep Time

Relative Age The Geologic Column Numerical Age The Age of Earth Chapter Overview

The Age Debate: A change of perspective Catastrophism –Large scale geologic features formed in catastrophic events –The age of Earth measurable in human historical terms Uniformitarianism –Past and present geological processes similar –Discarded catastrophic formation –Earth processes can be observed to understand past –Proposed the Earth was ancient much beyond the span of human history

Relative Age Determination of sequence of geological events Use stratigraphy – Science of rock layers and strata forming processes –Law of original horizontality Beds or strata laid in water are laid horizontal

Relative Age The principle of stratigraphic superposition -Undisturbed strata have younger layers above older ones

Relative Age Principles of stratigraphy –Principle of cross cutting relationships Feature cutting through or disrupting a stratum – younger than the stratum

Unconformities: Gaps in the record Numerical age –Time elapsed since a feature formed Unconformity –A chronological gap in sequence of rock layers

Correlating Layers Fossil s – Signs of ancient living organisms Usually consist of preserved hard parts –Paleontology Study of fossils and ancient life –Fossils useful to correlate rock layers by age –Fossil succession – determine sequences in fossil records

Correlating Layers

Sequencing geologic time The geologic column –Visual, suggestive representation of geologic time –Based on succession of rock strata –Major division -- Eon –Subdivisions – Eras, Periods and Epochs –Four eons Current eon: Phanerozoic – visible life –Eras: Paleozoic, Mesozoic, Cenozoic Past: Hadean, Archean, Protrozoic

Sequencing geologic time

Periods and Epochs Eras divided into periods –Based on fossil groups –Some period boundaries marked by extinctions The Cambrian period –First of the Paleozoic era –Animals with hard shells –Period of “explosion” in bio- diversity Precambrian – the whole of geologic time before the Cambrian period Periods last for multi-million years –Subdivided into epochs Recent fossil record

Numerical Age Attempts made at estimating the age of Earth –Rate of sedimentation and thickness of strata Too much variability in estimates –Ocean salt deposit rate (Halley) Flaw in assumption – salt content not varying –Darwin’s estimate based on erosion processes Rejected on the basis of thermodynamic cooling of Earth –Discovery of radioactivity reveals internal heating of Earth –Modern estimates (> 4.5 billion years) Based on radiometric dating

Numerical Age

Radioactivity and Numerical Ages Rocks contain internal clock –Radioactive matter decays at determinable rate –Development of sensitive instruments measuring Rates of radioactive decay Concentrations of elemental substances –Enabled measurement of numerical ages of rocks

Radioactivity and Numerical Ages

Radiometric dating Radioactive atoms decay into daughter products –Half life: time taken for half the atoms in sample to decay Known half life and known initial concentrations of –Parent atoms –Daughter atoms Provide accurate estimates of age when final concentration are measured Half life

Radiometric dating Radiometric dating difficulties –Need to find Rock samples with initially discoverable parent/daughter ratios No contaminations to externally change parent daughter ratio after rock formation –Different dating techniques found for Rocks, fossils and biological material –Based on estimated age and composition

Radiometric dating

Radiometric dating and the geologic column Finding sedimentary strata with igneous inclusions –Provides an application of the radiometric dating technique to the geologic column Fossil correlated layers can be dated –using the principles of stratigraphy –Numerical dates from radiometric dating

Magnetic Polarity Dating Earth’s magnetic field periodically reverses –During rock formation Orientation of magnetized mineral grains locked in Allows determination of magnetic field polarity –Field polarity measurements along with radiometric dating Provide a timescale for more recent geologic time Used to date history and succession of hominid fossils

Modern estimates of the age of Earth Earth has volcanic, geological and biogenic activity –Pristine rock from early Earth difficult to find –Oldest sedimentary rock mineral dated to 4.4 billion years However sedimentation requires prior rock formation and erosion Earth older than oldest rock –Moon rock samples and meteorites provide clues to age of planetary formation –Oldest meteorite 4.56 billion years old

Chapter Summary Relative Age  The Age Debate  Stratigraphy: Principles and Application  The Fossil Record The Geologic Column  Sequencing Geologic Time: Eons, Eras, Epochs and Periods Numerical Age  Numerical Age: The Age of Earth Problem  Radioactivity and Radiometric dating  Magnetic Polarity Dating The Age of Earth  Modern estimates of the Age of Earth