Determining the age of rocks (1)

Types of Rock Sedimentary rock forms from sediment sediment- solid particles of rock produced by weathering and erosion by water and wind * Igneous rock forms from volcanic lava flows *

Determining the Age of

Rocks

Relative Age of Rocks

Types of Rock

 Sedimentary rock forms from sediment (sediment- solid particles of rock

produced by weathering and erosion by water and wind)

* Igneous rock forms from volcanic lava flows

* Metamorphic rock- forms from intense heat and pressure

Relative Dating of

Rocks

 The relative age of rocks determines the age of rock layers as younger or

older, but does not give the exact age

 The principle of uniformitarianism

states that forces that shaped the Earth

in the past continues to shape the earth today (volcanoes and weathering and erosion by wind and water)

Sedimentary Rocks

 Sedimentary rocks formed from sediment deposited millions of years ago

Sedimentary rocks are deposited in

horizontal layers (principle of original horizontality)

 Sedimentary rock is deposited in

horizontal layers over geologic time with the oldest layer on bottom and the

youngest layer at the top (principle of

superposition)

Sedimentary Rock Layers

 Remember

-sedimentary rock

layers are horizontal

with the oldest at the

bottom and the

young at the top

unless the layers are

disturbed by a fault

or igneous intrusion

Faults and Igneous

Intrusions

 Sometimes sedimentary rock layers are disturbed by geological forces A crack

in the rock layer is called a fault When igneous rock (volcanic lava) intrudes or cuts through layers of sedimentary rock it

is called an igneous intrusion.

 The principle of cross-cutting

relationships states that a fault or

igneous intrusion is always younger

than the rock it cuts across

Fault (Crack) in Sedimentary rock

Here is a picture of a

fault or crack in the

rock

Igneous Intrusion in Rock Layers

igneous intrusions

(lava) are always

younger than the rock

layers they cut

through Note the

igneous intrusions

(volcanic rock) cutting

through the rock layer

Relative Age of Rock

 The Relative Age of the rock can be

determined by the sequence of the rock

layers using the Principle of

Superposition (oldest on the bottom and youngest at the top)

 If the sedimentary rock layer has been

disturbed by a fault or igneous intrusion,

the fault or intrusion is always younger

than the rock layer they cut through

(cross-cutting)

SWBAT: Describe what an index

fossil is:

Fossils

 Certain fossils called Index fossils help geologists determine the Relative Age of rocks

 To be a useful Index fossil:

- the fossil must be widely distributed

different geographic areas

- and represent an organism that

existed only briefly

Index fossils

 Trilobites were a group of hard shelled animals that evolved in shallow sea more than 500 million years ago (trilobites

became extinct at the end of the

Paleozoic Era)

 Trilobites are useful as index fossils

because they were widely distributed

geographically and represent an

organism that existed briefly

Example of an index fossil

(trilobite) in layers of

Continental Drift

Theory

 All continents were once joined together

in a single landmass (supercontinent)

called Pangea

 Fossil evidence supports the

Continental Drift Theory. Fossils from

a fernlike plant Glossopteris have been found in Africa, South America, Australia, and Antarctica The seeds could not have traveled across the span of the oceans

Absolute Age of Rocks

 To determine the Relative Age of Rocks

geologists use the Principles of Superposition, Cross-Cutting Relationships, and Index fossils.

 To determine the Absolute Age (exact age) of

rock, geologists use Radiometric Dating.

 How did geologist determine the Earth was 4.6

billion years old?