Geologic Time

The geologic time scale is based upon rock and fossil evidence The geologic time scale is based upon rock and fossil evidence. It is broken into the following divisions from largest to smallest: eon, era, period, and epoch. An interactive timeline shows how long geologic time really is.

Charles Darwin’s Theory of Evolution describes the process of change that produces new life forms over time.

Evolution is driven by the theory of natural selection, the idea that the organisms that survive to produce offspring are those that have inherited the most favorable traits for surviving in a particular environment.

These cactus plants show convergent evolution These cactus plants show convergent evolution. The one on the left is from Arizona, the one on the right is from Africa.

A fossil is evidence of earlier life that has been preserved in rock.

A fossil can be original remains (not very common), replaced remains, molds/casts, trace fossils, and carbonaceous films.

Fossils aid in relative dating of rocks.

Relative Dating is the process of placing geologic events in the process in which they occurred.

The Grand Canyon is one of the best places for observing relative time.

There are certain principles for dating rock layers.

The principle of superposition states that the oldest rock layer will be at the bottom and the youngest at the top. Which picture shows the oldest rocks?

The principle of original horizontality says that sedimentary rocks are first deposited horizontally.

Rocks can be folded, tilted, or faulted after they are deposited horizontally.

Rocks can also be intruded by magma which later cools to form rock.

The Principle of Cross Cutting Relationships says that an igneous intrusion is always younger than the rock it has intruded or cut across.

Number this diagram from oldest to youngest Number this diagram from oldest to youngest. 1 is always oldest (the first to form).

Observe this animation to help you visualize geologic processes Observe this animation to help you visualize geologic processes. Notice the unconformity, a gap in the sedimentary rock record.

An angular unconformity occurs when younger, flat strata are deposited on top of the older strata that have been tilted at an angle.

A disconformity occurs when younger, flat strata are deposited on top of the older flat strata. The older flat strata is uplifted and eroded. The layers are then re-submerged under water and a second instance of deposition occurs on top of the unconformity.

A nonconformity occurs when sedimentary layers are deposited on top of igneous or metamorphic rock.

Correlation is the matching of rock layers from one area to another Correlation is the matching of rock layers from one area to another. Correlated strata will have the same age.

Methods of Correlation Walking the outcrop Matching Rock Characteristics Using Index Fossils Climate indicating fossils Matching Key Beds Stratigraphic Matching

Index Fossils are useful to date layers because each layer contains fossils unlike those in the layer above or below.

Where is The Grand Staircase?

What is The Grand Staircase?

Pictures of The Grand Staircase Use the following images to give you a sense of what the Grand Staircase and the Colorado Plateau Look Like from South to North

Sedona, Arizona

Grand Canyon with San Francisco Peaks in the Background

Zion National Park

Bryce Canyon National Park

Relative dating helps us to figure out the relative ages of rocks, but it does not help us to figure out the absolute age of the rocks. So, what do we do?

We use Absolute Dating to put numbers on our dates.

Trees can be used to record time Trees can be used to record time. We can use tree ring history (dendrochronology) to put an actual date on a historic occurrence and to gauge past climates.

Varves, annual deposits of sediments, can be used for geologic dating purposes. Sediments deposited in glacial lakes vary with the seasons. Thick, light colored, sandy layers are deposited in spring and summer when runoff of water from a glacier is greater. Thin, dark colored, clay layers are deposited in winter. Counting annual glacial varves can help us to date items up to 15,000 years old.

Absolute dating can also be based upon the concept of radioactivity of chemical isotopes. This is also known as radiometric dating.

Radioactive decay is based upon the conversion from one isotope to another, but what is an isotope?

Remember from chemistry that an isotope is any element with more neutrons than protons.

Radioactive isotopes are unstable and want to achieve stability Radioactive isotopes are unstable and want to achieve stability. To do this, they give off radiation until they are stable. This usually involves changing from one element to another.

The parent isotope is the original element and is unstable The parent isotope is the original element and is unstable. The daughter isotope is the product of decay and is stable.

There are three types of radioactive decay Alpha Decay: Atomic # decreases by 2 and atomic mass decreases by 4 because 2 protons and 2 neutrons are expelled from the nucleus of the atom. Remember that the proton determines the atomic # and protons + neutrons determines the atomic mass.

There are three types of radioactive decay Beta Decay: Atomic # increases by 1 and atomic mass does not change because an electron is expelled from a neutron, thereby turning the neutron into a proton. Remember that the proton determines the atomic # of an atom.

There are three types of radioactive decay Electron Capture: Atomic # decreases by 1 and atomic mass does not change because an electron is added to a proton, causing a neutron to form. Remember that the proton determines the atomic # of an atom.

Radioactive decay will continue until the resulting atom is no longer radioactive. In other words, a stable isotope is formed.

Half life is the time it takes for half of the atoms of unstable parent isotope to decay to stable daughter isotope.

Graph of Half Life

In Radiometric Dating, scientists use radioactivity and half-lives of elements to measure absolute time.

Scientists measure the amounts of parent and daughter isotope in a rock to find its age.

Isotopes Used in Radiometric Dating. Parent Isotope Decay System Daughter Isotope Half-Life (years) Effective Range (years) Possible Materials for dating Carbon-14 Beta Nitrogen-14 5730 100-70,000 Once-living matter (wood, charcoal, bone) Uranium-238 Alpha and Beta Lead-206 4.5 Billion > 10 Million Uranium-bearing minerals (zircon) Rubidium-87 Strontium-87 47 Billion Micas, feldspars, metamorphic rocks Potassium-40 Beta capture Argon-40 1.3 Billion > 50,000 Micas, amphiboles, feldspars, volc. rocks

Example 1: If a scientist finds 100% parent isotope in a rock, how old is the rock if the half life of the parent isotope is 2 million years? Look at the following graph, find where the line intersects 100%, then read the value for the number of half lives on the x axis.

Graph of Half Life

Age of Rock = # of half lives X(times) half life of isotope

Age of Rock = 0 X 2,000,000 = 0 years

Example 2: If a scientist finds 25% parent isotope in a rock, how old is the rock if the half life of the parent isotope is 4 million years? Look at the following graph, find where the line intersects 25%, then read the value for the number of half lives on the x axis.

Graph of Half Life

Age of Rock = # of half lives X(times) half life of isotope

Age of Rock = 2 X 4,000,000 = 8,000,000 years!