mr. lafranca's earth science class - notepack 18 aim: how...

Notepack 18 AiM: How can we tell the age of rock layers?

Do now: Which Jenga piece was placed first to build this tower? Support your answer with

complete sentences.

How old is the Earth?

• The Earth is said to be 4.54 billion years old.

• Although there was no living things to witness the birth of this planet, scientists can use several techniques to estimate the Earth’s age.

Geological Time • Geological Time refers to time as it relates to the

Earth.

• There are 2 types of time use when studying the Earth:

– Relative age

• Time based on comparison to another object.

• An objects age falls with in a range rather than an exact date.

• Example - This rock is older than that rock

– Absolute age

• Time based with a number.

• More accurate

• Example - This rock is 4 million years old

• When the Earth formed, its surface consisted of molted rock.

• As the Earth cooled, the outer surface began to solidify and form the crust.

• Once water and the atmosphere formed, weathering and erosion of surface rock took place.

• This weathering allowed sedimentary rocks to form.

• The constant movement of crustal plates and volcanic activity helped form metamorphic rock.

Geologic Time – Relative Time

• Over time, these newly formed rock began to pile on top of each other, creating layers.

• Scientist use these layers, to help them determine the age of rock by comparing the layer’s position relative to the other layer of rock.

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Geologic Time – Relative Time (cont)

Example of Relative Age Dating and Correlation

• Principle of Superposition states that the thickness of the ground always start from the layers of rock on the bottom and ends with the layers of rock on top.

• This means the layer of rock closest to the top is younger than those layers of rock that is deep in the ground. (Oldest on bottom, youngest on top)

Principle of Superposition

In undisturbed sedimentary rock or lava flows:

Principle of Superposition uperposition

Top = youngest layer

In undisturbed sedimentary rock or lava flows

Bottom = oldest layer

Relative Age Dating

Oldest layers at bottom

The law of cross-cutting relationships

•Any feature that cuts across a rock or layers of rock is younger than the rock or layer that it cuts.

•These features include:

•Magma pushing through cracks in the layers

•Faults or cracks that break through a layer of rock.

Lava

Lava

•As the lava and magma cool, they form igneous rock. • But because the lava and magma was very hot, some of the rock that they touch change and become metamorphic rock.

Metamorphic Rock

Fault line

Fault

Metamorphic

Rock

Magma

Lava

Example of Cross-Cutting

Intrusion - igneous rock that forms when magma

squeezes between existing rock and hardens

Intrusion - igneous rock that forms when magma squeezes between existing rock and hardens

Examples of Cross-Cutting

Faults, joints, tilts, and veins also follow the law of cross--cutting relationships

Principle of Original Horizontality

• The Principle of Original Horizontality states that Sedimentary layers are deposited in horizontal sheets.

• If rock layers are found folded or up lifted, than the event that caused this deformation must have occurred after rocks formed.

• Folding occurs when rocks are pressed under tremendous pressure. (tectonic plate movement.)

Unconformity • As new layers of rock form,

the upper most layer is exposed to water, wind and climate.

• As a result, these rocks may be exposed to weathering and erosion.

• In areas where weathering is extreme, new rock layers can form as the results of new depositation occurs.

• The irregular profile is known as unconformity.

Some Problems with Unconformity

• Because of folding and uplifting, rock layers are moved out from their original position; causing scientist to make inaccurate dating.

• Due to weathering and erosion, some layers of rock could have worn down; making those layers vanish from the geological timeline.

Correlation • Correlation is determining that rock layers

from different area locations may have formed at the same time based on comparing the sequence of rock layers or the fossils that may be found in the rock layers.

A

B

C

D

E

F

A

B

D

E

F

Two Rock formations hundred of miles away from each other

Correlation

Correlation is determining that rocks are the same formation (may mean rocks are the same age)

Illustration of Principle of Faunal Succession

Unconformity Types Using Grand Canyon as Example

Put Rock Layers in order from oldest to youngest. Use your ESRT.

Contact

metamorphism