EARTH’S HISTORY

Earth Science Unit 5

Big Idea

The application of age dating techniques provides evidence for an ancient Earth and allows for the interpretation of Earth history, which has been the basis of the design and refinement of the geologic time scale.

I Can…

identify a sequence of geologic events using relative-age dating principles.

describe how index fossils can be used to determine time sequence.

Geology’s Time Scale

Evidence for an ancient earth is concealed in rocks.

Because of Earth’s processes, some sections of Earth’s history are missing. They have been weathered and eroded; however, much remains to be studied.

Interpreting the Earth is the goal of geology.

Geology’s Time Scale

One of geology’s major accomplishments has been the development of the geologic time scale.

This time scale provided information that Earth is much older than previously thought and the surface and interior have changed many times.

Geology’s Time Scale

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

“The present is the key to the past”.

Relative Dating

Radioactivity allows us to accurately determine dates for important dates in Earth’s history.

Relative dating was the method that was used prior to radioactivity and is still used today.

Relative Dating

Relative dating placed rocks and their formation in the sequence that they formed. It cannot tell us how long ago it formed, but can only be compared to other rocks/formations.

Relative Dating

The Law of Superposition

Stated simply, the law of superposition means that in an undeformed section of sedimentary rocks each bed is older than the one above it and younger than the one below.

Relative Dating

This should have seemed obvious, but was not clearly understood until the 1660’s.

This also applies to lava flows and ash beds.

This principle can be clearly shown in the Grand Canyon.

Relative Dating

Principle of Original Horizontality

The same person who explained the law of superposition, Steno, also explained that materials are usually laid down in horizontal or flat positions.

Relative Dating

Rock layers that are flat are thought to not have been disturbed, while rock layers that are folded or inclined do not have their original horizontality.

Folded or inclined rocks must have been moved after they were formed.

Relative Dating

Principle of Cross-Cutting Relationships

When a fault cuts through layers of rock, we can assume that the fault occurred after the layers of rock were laid down if the fault cuts through the rock.

Relative Dating

Magma intrusions also can be assumed to be younger than the rock it cuts through.

However, if the fault or intrusion does not cut through the rock, it is assumed that the rock layer is younger than the fault or intrusion.

Relative Dating

Inclusions  Inclusions are pieces of rock that are

contained within another type of rock.  For example if a large area of granite

(igneous) is overlain with sedimentary rock, we know that the igneous was formed first.

Relative Dating

After weathering through the igneous may begin to show through the sedimentary. Because it is contained within the sedimentary we know it formed first.

Relative Dating

Unconformities

Rock layers that have been continuously deposited are called conformable.

Often though deposition will stop and start and not all of time will be recorded in layers of rock. This is called an unconformity.

Relative Dating

During a period of unconformity, there is no deposition, there may be uplift or subsistence and there may be plenty of erosion. This could erase years of history and uncover others.

There are three types of unconformities

Unconformities

Relative Dating

Angular conformity Most easily recognized, it shows tilted or

folded rocks and erosion.

Angular

Relative Dating

Disconformity Most common, hard to recognize.

Layers are laid down, eroded away and then new layers are deposited again. This leaves an uneven surface of the once exposed surface rock.

Disconformity

Relative Dating

Nonconformity Magma intrudes upon layers of rock;

uplift and erosion occurs exposing once lower layers of rock. Deposition occurs and then uplift. This leaves the uneven and once exposed igneous layer under sedimentary layers.

Nonconformity

Relative Dating

Relative Dating techniques are good to compare rock layers to those around it, but they do not deal with numbers. It cannot help us date the earth or compare areas that are not close to one another.

The Role of Fossils

Fossils play the same role that layers of rock do to help us place things in the correct sequential order.

They can be used as time indicators, relating rocks in one region to rocks in another region with the same fossils. Especially helpful if the animal or plant was only present on earth during a certain time period.

The Role of Fossils

Some fossils are called index fossils. Index fossils are widespread geographically and limited to a short span of Earth’s history.

Their presence can establish the age of a bed of rock, and the conditions under which it formed.

Example: thick shells vs. thin shells, corals and temperature

I Can…

determine the approximate age of a sample, when given the half-life of a radioactive substance along with the ratio of daughter to parent substances present in the sample.  

explain why C-14 can be used to date a 40,000 year old tree, but U-Pb cannot.  

describe the process of radioactive decay and explain how radioactive elements are used to date the rocks that contain them.

Dating with radioactivity

Radiometric dating allows us to actually assign dates and time periods to rocks and fossils.

Although radiometric dating deals with billions and millions of years instead of the numbers we are used to dealing with, the numbers allow us to get a grasp on how old the Earth is.

Dating with radioactivity

Reviewing Basic Atom Structure

Each atom has a nucleus containing protons and neutrons which is orbited by electrons.

Protons are positively charged, neutrons are neutral, and electrons are negatively charged.

Dating with radioactivity

The atomic number is the number of protons that are present in the nucleus.

Each element has a different number of protons and therefore a different atomic number.

 Atoms of the same element always have the same number of protons, so the atomic number remains constant.

Dating with radioactivity

The mass number comes from the mass of the nucleus. Electrons have little mass.

The number of neutrons can vary, thus changing the mass, creating an isotope and changing the mass number.

Example: uranium has 92 protons, and its atomic number is 92. But the number of neutrons can vary, creating:

Uranium-234, 235, and 238 These isotopes are in nature, look the same and

act the same in chemical reactions.

Radioactivity

Radioactivity is the process that involves weak bonds in the nucleus, allowing for spontaneous decay of the nucleus.

  There are three types of radioactive

decay: Alpha-Emission Beta-Emission Electron Capture

Radioactivity

Alpha-emission - alpha particles are emitted from the nucleus. An alpha particle is 2 protons and 2 neutrons. The mass number is reduced by 4 and the atomic number by 2.

Radioactivity

Beta-emission – an electron is given off by the nucleus (by a neutron). The mass number remains unchanged, but the atomic number increases by 1.

Radioactivity

Electron capture – an electron is captured by the nucleus, combines with a proton and creates another neutron. The mass number remains unchanged, but atomic number decreases by 1.

Radioactivity

The unstable or radioactive isotope of an element is called a parent. The isotopes that result from the decay are called daughter products.

The important result of the discovery of radioactive decay is that it provides a reliable way to measure the age of rocks that contain radioactive isotopes.

This is called radiometric dating.

Radioactivity

Radiometric dating is reliable because rates of decay for some isotopes have been measured precisely.

For example: Uranium 238 decays following this pattern:

8 alpha particles, 6 beta particles, until it decays into lead 206. Radon gas is a daughter product produced by this decay.

Half-Life

The time required for half of the nuclei in a sample isotope to decay is called the isotopes half-life.

When there is an equal amount of daughter product to parent product we know that one half-life has passed.

Half-Life

When there is one-quarter of the parent isotope left, we know that 2 half-lives have passed.

If we know the half-life, we can calculate the age of the sample.

Radiometric Dating

When atoms decay, the percentage of atoms that decay always stays the same, but the actual number of atoms is declining.

This means that the amount of daughter product increases while the parent product decreases.

Radiometric Dating

Five isotopes have proven to be reliable for aging rocks.

  Rubidium-87, Potassium-40, Uranium-

238, Uranium-235, Thorium-232

Dating with Carbon-14

To date very recent events carbon-14 is used. (By very recent: 75,000 years)

Because carbon-14 is produced in the atmosphere, becomes part of carbon dioxide, and all living things use carbon dioxide, all organisms contain carbon-14.

Dating with Carbon-14

While alive, the decaying radiocarbon is replaced, but after death it is not, allowing it to decay.

Importance of Radiometric Dating

Radiometric dating is a very complex and long process.

However, is has proven the ideas of James Hutton, Charles Darwin, and others who hypothesized that the Earth was much older than previously thought.

The Geologic Time Scale

The geologic time scale divides the Earth’s history into varying segments of time.

The major time units were created during the 1800’s.

All dates were based on relative dating until the 20th century.

The Geologic Time Scale

The Geologic Time Scale

The scale is broken down into: Eons – the largest expanses of time Eras – next largest time periods Periods – broken down out of eras Epochs – the smaller units of time, which

are not broken down, but often referred to as early, late, and middle.

The Geologic Time Scale

Details are not really present in the time scale until 540 million years ago.

We mostly refer to anything before the Phanerozoic eon as Precambrian, which represents about 88% of Earth’s history.

The Geologic Time Scale

Reliable data is mostly from the most recent dates, just like any other historical accounts.

The farther back in time scientists go, the more fragmented and unclear the record becomes.

Difficulties in Dating the Geologic Time Scale

One major problem is that not all rocks can be radiometrically dated.

Igneous and Metamorphic rocks are most useful for dating, while sedimentary rocks can rarely be dated.

Metamorphic rocks can even sometimes be unreliable because of the different rocks that can be metamorphosed together.

Difficulties in Dating the Geologic Time Scale

So how do we date sedimentary features?

By using igneous features that are nearby in intruding, or comparing the location of igneous features that are nearby to the location of sedimentary features.