Interpreting Geologic History

What happened when

Two types of dating:

Absolute Dating: Gives the exact age of a rock or a geologic event Ex. He was born on June 26th, 1902 Dinosaurs went extinct 65.95 million years ago

(true)

Relative Dating: Tells you the order in which rocks were created or geologic events took place Ex. He was the 2nd of 4 children The dinosaurs went extinct after the trilobites but

before the wholly mammoths.

Absolute Dating: Tree rings

Tree Rings: Every year trees leave a ring in it’s trunk. These vary on size based on the climate that year. Scientists can put together tree rings from ancient trees in an unbroken line all the way to the present.

Used very often in archaeology for dating wood beams and carvings.

Technical name is dendochronology

Absolute Dating: Index fossils

Scientists can know the absolute age of rock if they are able to find certain fossils in that rock called INDEX FOSSILS

These fossils are known to have lived only during a certain period of Earth’s history, were widespread, and died out or evolved suddenly

Examples are certain kinds of shells or trilobites.

Absolute Dating:Radioactive Dating

The most reliable method of absolute dating is RADIOACTIVE DATING.

When Magma cools to form an igneous rock it begins to emit radioactive particles at a constant rate. Scientists can measure how much radioactive material is left in a sample to determine when that rock first cooled from Magma. This works accurately for rocks up to 50 billion years old (far older than the age of the Earth or even the Universe)

Relative Dating

Relative Dating tells scientists the order of geologic events and rock formations.

They use FOUR PRINCIPLES to guide their analysis. We are going to discover these principles on

our own.

Create a 3 Color Stack of “Sedimentary Rock”

The Law of Superposition

In a sequence of undisturbed rock:

THE OLDEST ROCKS WILL BE AT THE BOTTOM

Now, Bend or Break your rock

FLOW

Use “great pressure” to bend your rocks to make FOLDS.

FRACTURE

Use “great pressure” (or string) to make a FAULT

The Law of OriginalHorizontality

Rocks are almost always laid down in flat horizontal layers.

If they are found so that they are bent or otherwise not horizontal, THE FORCE THAT BENT OR BROKE THEM MUST HAVE COME AFTER they were originally laid down.

Next, use a pencil to dig a hole into your

rock and fill it up with Igneous rock.

The Law of Cross Cutting Relationships An igneous rock that INTRUDES across other rock layers

is YOUNGER than the layers it cuts across.

Unconformities

What is an unconformity?

An unconformity is AN UNDERGROUND ERODED SURFACE

Unconformities tell geologists that there was erosion and new layers started building on top

Unconformities obey the Law of Cross Cutting Relationships

An unconformity and everything above it are ALWAYS YOUNGER than what they cross-cut.

More Unconformi

ties

http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm?chapter_no=visualization

Finally, split your rock almost into 2 and separate the sides to make a

“valley”

The Law of Lateral Continuity

Layers of rock extend in all directions and are the same even if separated by a valley or fault

On the other hand, rocks above or below may be very different. THERE IS MORE VARIATION BETWEEN ROCK TYPES UP AND DOWN THAN THERE IS SIDE TO SIDE

Let’s try some puzzles…

This one has Metamorphosis!

Tricky…

A little harder…

Uber-difficult (extra credit)

The technical name: Stratigraphy

Strata: Layers

Graphy: Study

Stratigraphy: Study of layers

Used in geology, paleontology, and archaeology to infer RELATIVE AGE of artifacts, bones, or rocks.

Archaeologist

What do they do?

Excavate ruins of ancient cultures systematically and scientifically

Work for developers and construction companies that are required by law to check for artifacts before building

Forensic archaeologists work for the police to solve “cold cases”

Excavation can be grueling, slow and tedious. However, archaeologists can find incredible things and make amazing discoveries.

How do you become one?

To become an archaeologist you need to receive a bachelors degree in geology, anthropology, history, or archeology (if offered).

Almost all archaeologists have a Masters, or even a PhD.

Many college students work on archaeological digs during the summer in places all around the world.

Archaeology is a hard career to find a job in, perseverance is definitely required. They are paid between $37,000-55,000.