How Old is the Earth? 1654: Archbishop James Ussher of Ireland studied the timing of biblical events described in the Old Testament, and concluded that the the world was created on - October 23, 4004 B.C.E. at 9:00 AM. Thus, the age of the earth, based on Ussher's results, would be: = 6,007 years old. Many calculations of this type have been done by other biblical scholars, and in general, the Earth was found to be about 6,000 years old based on these types of calculations.

Current Estimate of the Age of the Earth Scientific methods of estimating the age of the earth are based on analysis of evidence found in rocks and on radiometric dating. Based on these methods, scientists have concluded that the earth is much more than 6,000 years old. Recent estimates of the age of the earth: 4.6 billion years old (4.6 billion = 4,600,000,000)

Age of the Earth

Geologic Time Geologic time is measured in millions of years. How do we know about such long periods of time? Geochronology: The science of determining the ages of rocks.

Relative Geologic Ages Nicholas Steno: Studied layers of rock called strata. Steno described how the position of strata could yield relative ages of rock units (i.e., older versus younger). Steno applied the following simple rule that has come to be called the principle of superposition: In any sequence of "undisturbed" strata of sedimentary rocks, the oldest layer is on the bottom, and successively higher rocks are successively younger. Sedimentary rocks: Rocks formed from weathered products of pre-existing rocks (sediments) that have been transported, deposited, and lithified (converted into rock).

Geologic Time Scale During the 16, 17 and 1800's geologists developed a geologic time scale based on relative ages.

Absolute Age of the Earth During the 16, 17 and 1800's geologists developed a geologic time scale based on relative ages. But: What about absolute ages? What is the absolute age of the earth? Define: Absolute age of the earth = amount of time since the earth solidified. At some point in the earth's early history, the entire planet was a mass of molten rock material. When did the earth cool and solidify into its present state?

Fossils and evolution Rates of deposition of sediments Cooling of the Earth from a molten state Radiometric Dating Methods for Estimating the Age of the Earth

Fossils and Evolution Fossils = remains of ancient life. For example, molds of ancient sea shells. Mid-1800's: Charles Lyell - Used evolution to estimate absolute ages of rocks.

Fossils and Evolution Mid-1800's: Charles Lyell - Used evolution to estimate absolute ages of rocks. To estimate ages of rocks: Need to find a natural process that leaves a tangible record in the rocks. Then: if you can estimate the amount of time associated with that process, the age of the rock can be estimated. Evolution is such a process.

Geology and Geophysics Geology is the science that deals with the history of the earth as recorded in rocks.

Uniformitarianism The Principle of Uniformitarianism = The physical, chemical and biological laws that operate today have also operated in the geologic past. Lyell compared the amount of evolution exhibited by marine fossils found in rocks of different relative ages with estimates of rate of evolution. Using estimates of rate of evolution and observations of fossil forms -- estimate age of rocks. How long a period of time would be required for the evolution observed in the rock strata to occur?

Fossils and Evolution How long a period of time would be required for the evolution observed in the rock strata to occur? Lyell calculated: Certain rocks were about 80 million years old. But, ages of older rocks were more difficult to estimate. Problems: Missing parts of fossil record. Rates of evolution difficult to estimate for older life forms. General conclusion based on this type of approach to estimating age of earth: Earth is at least 10's of millions of years old.

Fossils and evolution Rates of deposition of sediments Cooling of the Earth from a molten state Radiometric Dating Methods for Estimating the Age of the Earth

Fossils and Evolution How long a period of time would be required for the evolution observed in the rock strata to occur? General conclusion based on this type of approach to estimating age of earth: Earth is at least 10's of millions of years old.

Uniformitarianism The Principle of Uniformitarianism The physical, chemical and biological laws that operate today have also operated in the geologic past.

Age of the Earth

Fossils and evolution Rates of deposition of sediments Cooling of the Earth from a molten state Radiometric Dating Methods for Estimating the Age of the Earth

Rates of Deposition of Sediments Estimate the amount of time required to deposit a given thickness of strata. A = Age of sequence of sedimentary rocks. T = Thickness of strata. R = Rate of sediment accumulation. A = T/R Example: (100 ft)/( ft/yr) = 500,000 yr

Uniformitarianism The Principle of Uniformitarianism The physical, chemical and biological laws that operate today have also operated in the geologic past.

Rates of Deposition of Sediments TT = total thickness of sedimentary strata that has been deposited during earth history. Age of Earth = TT/R

Rates of Deposition of Sediments Because of poor estimates of rates of deposition, this method gave a very wide range of results. But, this method generally suggests that the earth is at least 10's of millions of years old, and (based on both fossil evidence and rates of deposition of sediments): Many geologists and biologists in the 1800's concluded that the earth was several hundreds of millions of years old.

Fossils and evolution Rates of deposition of sediments Cooling of the Earth from a molten state Radiometric Dating Methods for Estimating the Age of the Earth Based on fossils and evolution, and on rates of deposition of sediments: By the late 1800's many geologists and biologists thought that the earth was several hundreds of millions of years old.

Cooling of the Earth from a Molten State Late 1800's: A physicist named Lord Kelvin, used physics and mathematics to challenge geologists and biologists who, by then estimated the age of the earth to be at least 100's of millions of years old. Kelvin estimated: Earth was about 24 to 40 million years old. Many geologists and biologists of the time argued that the earth was much older.

Cooling of the Earth from a Molten State Kelvin's Analysis: 1.Assume that the Earth cooled from a molten state. 2.Assume that cooling followed ordinary laws of heat conduction and radiation (uniformitarianism). 3.Apply laws of heat conduction and radiation, and calculate the number of years it would take for the earth to cool from a hot molten mass to it's present state. Kelvin's Result: Earth is 24 to 40 million years old.

Uniformitarianism The Principle of Uniformitarianism The physical, chemical and biological laws that operate today have also operated in the geologic past.

Age of the Earth

Late 1800s Lord Kelvin: 10's of millions of years old Cooling of earth from molten state Physics and mathematics "Quantitative" arguments Geologists and Biologists: 100's of millions of years old Fossils, evolution, and rates of deposition of sediments Intuition "Qualitative" arguments

The real purpose of the scientific method is to make sure that nature hasnt misled you into thinking that you know something that you dont actually know. Robert M. Pirsig, Zen and the Art of Motorcycle Maintenance: An Inquiry into Values

Radioactivity Discovery of radioactivity in the 1890's showed that the geologists and biologists were right: Radioactivity as an internal heat source. Radiometric dating. So, earth is much older than Kelvin thought…

Fossils and evolution Rates of deposition of sediments Cooling of the Earth from a molten state Radiometric Dating Methods for Estimating the Age of the Earth

Age of the Earth

Radioactivity and Radiometric Dating