A Brief History of Geology

A Brief History of Geology
Mid-1600s James Ussher stated Earth was only a few thousand years old Catastrophism Belief that Earth’s landscapes were formed by great catastrophes Prevalent during the 1600s and 1700s Used to fit the rate of Earth’s processes to prevailing ideas of Earth’s age

A Brief History of Geology
Late 1700s James Hutton published Theory of the Earth Uniformitarianism States that the physical, chemical, and biological laws that operate today have also operated in the geologic past To understand ancient rocks, we must understand present-day processes Geologic processes occur over extremely long periods of time

Creating a Timescale — Relative Dating Principles
Efforts to determine Earth’s age during the 1800s and 1900s were unreliable Today radiometric dating allows scientists to accurately determine numerical ages for rocks representing important events in Earth’s past Relative dates are determined by placing rocks in the proper sequence of formation

Principle of superposition Developed by Nicolas Steno in the mid-1600s Studied sedimentary rock layers in Italy In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below Also applies to lava flows and ash beds

Principle of original horizontality Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed Folded or inclined rocks must have been disrupted after deposition

Principle of lateral continuity Sedimentary beds originate as continuous layers that extend in all directions Identical strata on two sides of a canyon were continuous before the canyon was carved

Principle of cross-cutting relationships Geologic features that cut across rocks must form after the rocks they cut through Faults, igneous intrusions

Inclusions Fragments of one rock unit enclosed within another Rock that contains inclusions is younger than the rock that provided the inclusions

Unconformities Layers of rock that have been deposited without interruption are called conformable A complete set of conformable strata for all of Earth history does not exist Interrupting the deposition of sediment creates a break in the rock record called an unconformity Represents a period when deposition stopped, erosion occurred, and then deposition resumed Generally, uplift causes deposition to stop and subsidence causes deposition to resume

Unconformities Angular unconformity
Consists of tilted or folded sedimentary rocks overlain by younger, more flat lying strata Deformation occurred during the time that deposition stopped

Unconformities

Unconformities Disconformity
A break in sedimentary rock strata representing a time when erosion occurred Difficult to identify because layers are parallel Evidence of erosion (buried stream channel)

Unconformities Nonconformity
Younger sedimentary rocks on top of older metamorphic or intrusive igneous rocks Imply period of uplift of deeply buried rocks

Unconformities

Applying Relative Dating Principles

Fossils: Evidence of Past Life
The remains or traces of prehistoric life Paleontology The scientific study of fossils

Fossils: Evidence of Past Life

Types of Fossils Fossils can be preserved in many ways
Some remains may not be altered at all Teeth, bones, shells Entire animals including flesh are not common Mammoths frozen in Arctic tundra Mummified slots in a dry cave in Nevada

Types of Fossils Permineralization Molds
Mineral-rich groundwater permeates porous tissues Petrified wood is permineralized with silica “Petrified” means “turned to stone” Molds Form where a structure buried in sediment was dissolved by groundwater Only the outside shape and surface marking is preserved; no internal structure If hollow spaces are filled with mineral matter, a cast is formed

Types of Fossils Carbonization Amber
Remains are encased in sediment; pressure squeezes out all liquid and gas until only a thin residue of carbon remains Effectively preserves leaves and delicate animals Impressions may show considerable detail Amber The hardened resin of ancient trees Seals organisms from atmosphere and water Preserves delicate organisms like insects

Types of Fossils Trace Fossils Indirect evidence of organisms Tracks
Burrows Coprolites Gastroliths

Types of Fossils

Conditions Favoring Preservation
Only a very small fraction of organisms are preserved as fossils Rapid burial and hard parts favor preservation Soft parts are eaten or decomposed Sediment protects organisms from destruction Shells, bones, and teeth are much more common in the fossil record Fossil record is biased

Conditions Favoring Preservation
What types of organisms are most likely to be missing from, or are very rare, in the fossil record? How might this bias our picture of what life on Earth was like in the past? Hint: Think about the organisms themselves, but also their ecological context and depositional environment.

Correlation of Rock Layers
Correlation is matching up rocks of similar age in different regions Reveals a more comprehensive picture of the sedimentary rock record Correlation by walking along outcropping edges is possible within limited areas Rock layers made of distinctive material can be identified in other places Widely separated areas require the use of fossils

William Smith 1700s to 1800s Noted that rock formations in canals contained fossils unlike the fossils in the beds above and below Distinctive fossils can be used to identify and correlate widely separated sedimentary strata Principle of fossil succession Fossil organisms succeed one another in a definite and determinable order, therefore any time period can be recognized by its fossil content Fossils document the evolution of life through time

Index fossils Geographically widespread and limited to a short span of geologic time Important for correlation Fossil assemblage Can be used when there aren’t index fossils Fossils are useful environmental indicators

Reviewing Basic Atomic Structure
Each atom is made up of protons, neutrons, and electrons Protons have a positive charge Electrons have a negative charge Neutrons are neutral Elements are identified by atomic number Number of protons in the nucleus

Reviewing Basic Atomic Structure
99.9% of an atom’s mass is in the nucleus Electrons have almost no mass Number of protons + number of neutrons in an atom = the mass number An isotope has a different number of neutrons in the nucleus Different mass number

Dating with Radioactivity
Some isotopes have unstable nuclei with bonds that are not strong enough to hold the protons and neutrons together These nuclei will break apart (decay) in a process called radioactivity

Three common types of radioactive decay: Alpha particle = 2 protons and 2 neutrons Mass number reduced by 4 and atomic number decreased by 2 Beta particle = electron from the neutron Neutron is actually a proton and electron combined Mass number remains the same, but atomic number increases by 1 Electron capture Captured by the nucleus and combined with a proton to form a neutron Mass number remains the same, but atomic number decreases by 1

Parent Isotope Unstable radioactive isotope Daughter Product Isotope resulting from radioactive decay

Radiometric dating Reliable method of calculating ages of rocks Rate of decay for many isotopes does not vary Rate of decay has been precisely measured Daughter product has been accumulating at a known rate since rocks were formed

Half-life Time required for one-half of the nuclei in a sample to decay One half-life has transpired when quantities of parent and daughter are equal (1:1 ratio) If half-life of an isotope is known and parentdaughter ratio can be measured, then age can be calculated.

Five radioactive isotopes are important in geology: Rubidium-87 Uranium-238 Uranium-235 Thorium-232 Potassium-40 Only useful if the mineral remained in a closed system No addition of loss of parent or daughter isotopes

Radiometric dating methods have been used to determine the age of the oldest rocks on Earth 3.5-billion-year-old rocks found on all continents Oldest rocks: 4.28 billion years old (Quebec, Canada) 3.7 to 3.8 billion years old in western Greenland 3.5 to 3.7 billion years old in the Minnesota River Valley and northern Michigan 3.4 to 3.5 billion years old in southern Africa 3.4 to 3.6 billion years in western Australia

Radiocarbon dating Using the carbon-14 isotope to date very recent events Half-life of carbon-14 is only 5,730 years Only useful for dating events from historic past and very recent geologic history Carbon-14 is present in small amounts in all organisms

The Geologic Time Scale
Geologic history divided into units of variable magnitude Developed during the nineteenth century Based on relative dating Eons represent the greatest span of time Phanerozoic Eon began about 542 million years ago Eons divided into eras Phanerozoic includes Paleozoic, Mesozoic, and Cenozoic Bounded by profound worldwide changes in life-forms Eras divided into periods Periods divided into epochs

Most detail in the geologic time scale begins at 542 million years ago 4 billion years before the Cambrian is known as the Precambrian Divided into Archean and Proterozoic eons Together are divided into seven eras Represents 88% of geologic time

Some “unofficial” terms are associated with the geologic time scale Precambrian = eons and eras before the Phanerozoic Hadean = earliest eon of Earth history (before the oldest known rocks)

Geologic time scale must be updated periodically to include changes in unit names and boundary age estimates A few years ago, Cenozoic divided into Tertiary and Quaternary periods Today, former Tertiary is divided into Paleogene and Neogene periods

Determining Numerical Dates for Sedimentary Strata
Rocks can only be radiometrically dated if all minerals formed at the same time Works for igneous and metamorphic rocks Sedimentary rocks contain particles of many ages Must be related to datable igneous masses

Determining Numerical Dates for Sedimentary Strata

A Brief History of Geology

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