The Rock and Fossil Record

Studying the rock and fossil record helps us understand Earth’s history

and the history of life on Earth.

The Principle of Superposition(Law of Superposition)

• Which layer is the oldest?

Answer:• In Sedimentary rock, the oldest

is on the bottom the youngest in on the top, this is Superposition. The rock’s age is based on the position of the rock.

The Principle of Superposition• Applies mainly to sedimentary rock

layers!!!

• This principle states that the younger rocks are located above the older rocks. The rock layer must be undisturbed!

• Sketch The Drawing!

Disturbing Forces

• Forces within Earth can disturb the rock layers (rock layers are also know as rock sequences).

• Examples of Events (plate boundaries converging, diverging, transforming)

1. Folding 2. Tilting

• Examples of Features that Disturb:1. Faults (found along all plate

boundaries)2. Intrusion (potential volcanic activity)

Fault (Disturbed Rock Layer)

Intrusion (Disturbed Rock Layer)

Unconformities• 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.

• Just remember these are usually as a result of erosion and nondeposition (Oops!! The sediments did not get deposited!!)

The Geologic Column• Simply think of the Grand Canyon:

• An arrangement of rock layers in which the oldest rock layers are at the bottom

Dating Rock Layers

• 2 Ways to Measure the Age of Rocks:1. Relative Dating (Age)

2. Absolute Dating (Age)

Relative Age The relative age of a rock is its age

compared to other rocks. Use words like: “older or younger”

(For example--think of your relatives and your age:

I am younger than my cousin Scott, older than my sister Sandy and older than my sister Rhonda. My age compared to my relatives.)

Now you do an example of relative age!!

Relative Dating: “When Igneous Rock Meets Sedimentary Rock”

REMEMBER: 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

Can you find the intrusion of igneous rock?

Absolute Age

• The absolute age of a rock is the number of years since the rock was formed.

• (mya—million years ago)

FossilsWhy is the study of fossils so important?

Consider the following?The Beartooth Range of Montana is just north of Yellowstone. In this location you will findBeartooth Butte. Partway down from its 10,500 foot (3200 m) summit, within the dark red zone of rocks, geologists have found fish fossils as old as 400 million years.

How is this possible? (Give a good scientific hypothesis.)

One Answer:

Plate tectonics push up the rock making a mountain, weathering and erosion move the dirt and expose the marine fossil.

Fossils can give us clues of what Earth’s surface looked like in the past.

• http://www.yellowstonetreasures.com/fish_fossils.htm• http://www.yellowstonetreasures.com/fish_fossils.htm

Fast Fossil Facts (now say that three times in your HEAD!)

Fossils are the preserved remains of past life on Earth.

Why are all dead organisms not preserved as fossils?

Why are all dead organisms not preserved as fossils?

1. Hard parts of organisms are most likely to be fossilized.

2. Conditions must be perfect to preserve the fossil.

In other words….When an organism dies, its soft

parts often decay quickly leaving only the hard parts to fossilize.

Ex. Bones, Shells, Teeth, or Seeds

*Fossils are found in rocks deposited in the environment in which they lived.

Fossils provide clues to the past

Fossils give clues of past climate and surface changes.

Examples: Fossils of palm leaves in Antarctica could mean that Antarctica was once in a warm tropical area. (climate)

Marine fossils found on mountain tops or land well above sea level. (surface change)

Revisiting the Rock Cycle :.

1. Fossils are generally most abundant in marine sedimentary rocks.

2. Fossils generally are not found in igneous and metamorphic rocks.

3. Most fossils found in sedimentary rock.

Fossil Types:

1. Trace fossil: provide evidence of the activities of ancient organisms.

Examples: footprints, animal trails, or animal burrows.

• 2. Mold: a hollow area in sediment in the shape of an organism or part of an organism.

• 3. Cast: a copy of the shape of an organism.

Other Types Petrified Fossils: fossils in which

minerals replace all or part of the organism. Ex: petrified wood

Carbon Films: an extremely thin coating of carbon on rock that forms when materials that make up an organism become gases and escape leaving only carbon behind.

Preserved Remains are formed when an organism is preserved with little or no change. {For example when organisms become preserved in tar, amber (tree sap), and freezing.}

Index Fossils

• An index fossil must be widely distributed and represent a type of organism that existed only briefly.

• One example of an index fossil is a trilobite.

• Trilobites were a group of hard-shelled animals whose bodies had three distinct parts.

• They evolved in shallow seas more than 500 million years ago.

Links

• http://youtu.be/3rkGu0BItKM

• http://youtu.be/TcZtMFnyj1M (<iframe width="420" height="315" src="http://www.youtube.com/embed/TcZtMFnyj1M" frameborder="0" allowfullscreen></iframe>)

• http://youtu.be/dkELENdZukI