Ch.6 Earth’s History

Who’s got the TIME?

• RELATIVE: order/sequence known, but not the actual date of occurrence. “Time Line”

• ABSOLUTE: actual date determined by radioactive decay

“Clocks in Rocks”

First Things First…or…“How’d that get there?”

• In the 17th C., Nicolas Steno made an important observation:

"Sediments are usually deposited in horizontal layers."

He called this

“ORIGINAL HORIZONTALITY”

Finding Relative TimeThe LAW of...

• SUPERPOSITION: a sedimentary sequence will be OLDEST on BOTTOM (if undisturbed).

• CROSS-CUTTING: an igneous intrusion is younger than rock it has intruded (cut across).

• INCLUDED FRAGMENTS: pieces of rock found IN another rock must be OLDER (formed first).

• FOLDS/TILTS: younger than rocks themselves

Superposition- youngest

to

oldest

Grand Canyon

IGNEOUS INTRUSION:

• Occurs when magma squeezes into or between layers of pre-existing rock.

• Surrounding pre-existing rocks will undergo contact metamorphism

Intrusions

contact metamorphism on all sidescontact metamorphism on all sides

Extrusions

no contact metamorphism on topno contact metamorphism on top

Is “H ” an intrusion or extrusion?

How can you tell?

Is “H ” an intrusion or extrusion?

How can you tell?

H is an intrusion.Contact

metamorphism on top

H is an intrusion.Contact

metamorphism on top

Cross Cutting

Igneous Intrusion - Cross Cutting

Which is older:

F or S

How do you know?

Included Fragments

Included Fragments

Folds/Tilts

Folds/Tilts

What is an unconformity? What is an unconformity? • a buried erosional surface

a part of the rock record

is missing

Rocks above unconformity are younger – rocks below older

a part of the rock record

is missing

Rocks above unconformity are younger – rocks below older

How does it complicate the relative dating of rock layers?

How does it complicate the relative dating of rock layers?

Upper Silurian Carbonates

Tilted Ordovician Shales and Sandstones

Taconic Unconformity

unconformity

What processes could lead to an unconformity? What processes could lead to an unconformity?

• weathering , erosion & deposition

4 steps produce an unconformity

1. Uplift – area of crust uplifted above sea level (deposition – under water)

2. Erosion – some time after

3. Submergence (subsidence) below sea level

4. Deposition – new sediments deposited on top of the buried eroded surface

Practice: what happened here?

Using the diagram to the left, identify where the unconformity is located by drawing an arrow and writing the word “unconformity” next to it.

The unconformities shown in the cross section represent

1. buried erosional surfaces 2. locations of index fossils 3. volcanic ash deposits 4. boundaries between oceanic and continental crust

The unconformities shown in the cross section represent

1. Letter X2. Letter E3. Letter Q4. Letter Y

Index fossils Index fossils - any animal or plant that is characteristic

of a particular span of geologic time or environment.

2 criteria must be met

• Life form lived over a wide geographic area – horizontal distribution

• Life form existed for a short period of time – short vertical distribution

- any animal or plant that is characteristic of a particular span of geologic time or environment.

Index Fossils

• lived over a large geographic area (large horizontal distribution)

• lived for a short period of time (small vertical distribution)

• What characteristics must fossils have in order to be

good index fossils?

Applying Principles of Relative Dating to Determine Geologic

History of an Area

• The process of matching rocks or geologic events occurring at different locations of the same age is called

CORRELATION

In the diagram below, a geologist has matched up rock layers based on index fossils.

Where is there an Unconformity?

Where is there an Unconformity?

Correlation of rock layers often relies upon fossils

• William Smith (late l700’s) noted that rock layers in widely separated areas could be identified and correlated by their distinctive fossil content

• This led to the "principle of fossil succession“

• Fossils succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content

D. Interpreting Fossils

1. Fossils indicate that many different kinds of life forms existed at different times in Earth’s history.

2. When fossils are arranged according to age, they show that

Certain living things have changed or evolved over time.

3. Fossils indicate how

For example, fossils of marine organisms can be found in rocks that are presently high above sea level.

the Earth’s surface has changed.

4. Fossils give clues to

Since coral today live in the warm waters of equatorial regions, between 30° N and 30° S latitude, we can infer that

Earth’s past climate.

N.Y.S. had a warmer climate in the past.

5. Fossils tell about the

appearance and activities of past life.

For example, fossil teeth give clues about the kind of food the animal ate.

Eurypterus

NY State Fossil

Silurian index

fossil

OTHER METHODS OF CORRELATION

• Layers of bedrock exposed (outcrops) on either sides of river valleys/excavations

“walking the outcrop”• Rock similarities

• Volcanic ash – large eruption – widely distributed – represents a small time interval

How to Use the Geologic History of NYS in the ESRT

• ESRT p.8

• ESRT p.9