# Earth’s History.

## Presentation on theme: "Earth’s History."— Presentation transcript:

Earth’s History

Activator: Student Line Up
What is the exact position of _______ in the line? What is an estimate of _______ in the line? What is the difference between exact and estimate?

Absolute vs. Relative Age (Draw a T Chart in your Notebook)
Determining the age of an object in relation to other objects “Estimate” Index Fossils, Rock layers Example: A friend tells you she has 2 brothers – one that’s older than her and one that’s younger Absolute Age Determining the actual age in years of an object “Exact” Radiometric dating, half life, radioactive decay Example: I have one brother who is 8 years old and one who is 17 years old

Absolute = exact Relative = estimate Mnemonic Memory Help
When you hear these two terms, think… Absolute = exact Relative = estimate

How old is old? Scientists use several strategies to determine age of rocks and fossils. Absolute Age: actual age of an object or how long ago an event occurred by using radioactive dating. Radioactive Dating: measures age by comparing the amount of radioactive element isotope present to how much is normally present in that material Relative Age: age of an object compared to another- “I am older than you”

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay). Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods. Marie Curie Ernest Rutherford

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
The number of protons (the atomic number) is fixed for any element and is unique for each element but the number of neutrons in atoms of different elements can vary. Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element). Figure 3.3: Schematic representation of the isotopes of carbon. Carbon has an atomic number of 6 and an atomic mass number of 12, 13, or 14, depending on the number of neutrons in its nucleus. M&W4 Fig. 3.3; M&W5 Fig. 3.4 7

GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Radioactive decay occurs when an isotope of one element is transformed into a different element by changes in the nucleus. There are three different decay mechanisms: “Parent” “Daughter” Figure 17.18: Three types of radioactive decay. (a) Alpha decay, in which an unstable parent nucleus emits 2 protons and 2 neutrons. (b) Beta decay, in which an electron is emitted from the nucleus. (c) Electron capture, in which a proton captures an electron and is thereby converted to a neutron. M&W4 Fig ; M&W5 Fig

How can we tell age based on the number of parent isotopes?
Radioactive isotopes “decay” at a particular rate. We express this rate as the “HALF-LIFE”, which is the time it takes for HALF of the parent isotopes to decay.

Absolute Age Half life; the amount of time (years) it takes for 1/2 of the radioactive isotope to decay by 1/2, indicates the age of the object.

So… Why Do We Care? Relative and Absolute Dating tell us…
The age of the Earth as well as its history When the dinosaurs roamed the earth How the Earth was and where it is going Possibly Skip this slide? Need Transition

Which layer do you think is the oldest and youngest?
Law of Superposition Says: “ The rocks at the bottom are OLDER than the rocks above”

The Position of Rock Layers
It can be difficult to determine a rocks absolute age. So… scientists use the law of superposition. According to the law of superposition, in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

GEOLOGIC TIME SCALE a series of time intervals that divides Earth’s history Each layer of rock represents specific interval of time Index fossils help determine specific period Time periods divided by specific events like mass extinctions

ROCKS TELL A STORY Rocks can tell where they were made and when
Sedimentary rocks can have fossils in them Rocks can tell when mass extinctions happened STUDY AND COMPARISON OF ROCK LAYERS OR STRATA IN THE 19TH CENTURY LED SCIENTISTS TO BELIEVE THAT A CORRELATION EXISTS BETWEEN PLACE AND TYPE OF ROCK

LAW OF SUPERPOSITION For undisturbed rocks, the oldest layer is on the bottom and the youngest is on top Supai is oldest – WHY?

PALEONTOLOGY the study of fossils  remains of ancient life
Body fossils vs. trace fossils Body = remain of organism, like bones; Trace = evidence of organism, like footprints Scientific dating Absolute dating (gives age in years) uses radiometric / radioactive dating (isotopes) Relative dating (gives age before, after, during) uses observation of rock layers Radiometric dating: use the natural radioactivity of certain elements found in rocks to help determine their absolute age- the use of half-lifes to determine the absolute age of a sample. In radioactive dating, scientists calculate the age of a sample based on the remaining radioactive isotopes. Radioactive elments decay into nonradioactive elements at a steady rate which is measured in a unit called half-life.

FOSSILS Traces and preserved remains of ancient life found within rock layers Fossils show: Biodiversity How species have changed over time Correlation between rock layers from around the world Relative ages to particular strata Evidence for the geological time scale Traces are footprints, droppings, or any other type of evidnece an organism might leave behind How fossils form: Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.

SCIENTIFIC DATING Absolute Dating: numerical dating to give rocks an actual date or date range, in number of years Relative Dating: compare how old something is in comparison to something else; used to put rocks and geological events in correct chronological order HOW? Use sedimentary rocks Use fossils Study strata

INDEX FOSSIL Fossil that defines and identifies geologic periods; often in only one layer of rock Easily recognizable Exists over a short geologic time range (found only in a few layers of rock) Wide distribution (geographic range is world-wide)

Ex/ INDEX FOSSIL: AMMONITE
Ammonite fossils are found worldwide, existed for only a very specific period of time This means ammonites are found in very specific layers of rock; Once we know the ammonites, then we can determine the age of any fossil found next an ammonite fossil. Traces are footprints, droppings, or any other type of evidnece an organism might leave behind How fossils form: Dead organisms are buried by layers of sediment, which forms new rock. Then the preserved remains may be discovered and studied.

Let’s Practice Together!
Which rock layer is the oldest? Which rock layer is the youngest? Which is older, layer M or layer R? How do you know Put them in order from oldest to youngest Which is younger R or T? How do you know?

Think*Write*Pair*Share Is dating rocks using the law of superposition a form of relative dating or absolute dating? Explain your answer

Remember… Law of Superposition is a form of relative dating
Relative dating is based on the rocks that are surrounding it, NOT the exact age We know which is older but we do not know HOW old exactly

TICKET OUT! Which rock is younger, limestone or basalt? How do you know? Which of the labeled rocks is oldest? How do you know? Which of the labeled rocks is youngest? How do you know? Name the rocks in order from oldest to youngest.

Other Clues to Relative Age
Clues From Igneous Rock Lava that cools at the surface is called an extrusion. Rock below an extrusion is always older. Magma that cools beneath the surface is called an intrusion. An intrusion is always younger than the rock layers around an beneath it.

Other Clues to Relative Age
Faults (a break in the rock) are always younger than the rock it cuts through! Unconformities: An unconformity is a gap in the geological record that can occur when erosion wears away rock layers and other rock layers form on top of the eroded surface.

Fossils Fossils are usually found in sedimentary rocks.
When an organism dies, its soft parts often decay quickly leaving only the hard parts to fossilize. Ex. Bones, Shells, Teeth, or Seeds

Using Fossils to Date Rocks!
Scientists use index fossils to match rock layers. An index fossil must be widely distributed and represent a type of organism that existed only briefly. They are useful because they tell the relative ages of the rock layers they are found in.

Fossils give us clues about. . .
How old a rock is. What life existed in the past. When big events happened. Climate in the past.

Fossils and Climate (write)
Climate=average conditions like temperature, humidity, wind, and rainfall. Fossils show us how climate changes over time.

Ice Cores (write) Ice forms layers similar to that of rock
Also follow law of superposition Contain elements that help describe climate of Earth Scientists drill down and pull ice cores up An ice core from Antarctica 3km long showed 740,000 years of history and 8 ice ages!!!