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CLASS

Agenda

TODAY• Sequencing Time• Rock Record Notes• Relative Age

activity• Hand back tests

NEXT TIME• Absolute age lab• Geologic Time• QUIZ

Sequencing Time

• How do scientists create time scales?

We know Earth is 4.6 Billion years old

BUT HOW DO WE KNOW?

How do we know Earth’s age?

• Earth is about 4.6 billion years old– First proposed by James Hutton in the 18th century

• Hutton theorized the same forces changing Earth’s surface now were working in the past– volcanism and erosion

• Hutton’s ideas raised questions about Earth’s age– Most thought Earth was 6,000 years old – all Earth’s geologic features had formed at the same

time

Uniformitarianism

• Hutton’s ideas about uniformitarianism encouraged other scientists to learn more about Earth’s history

• uniformitarianism a principle that geologic processes that occurred in the past can be explained by current geologic processes

Relative Age

• One way to learn about Earth’s past is to determine the order in which rock layers and other rock structures formed

• You can determine the order in which rock layers formed by using a few basic principles– Layers of rock, called strata, show the sequence of events

that took place

• Once they know the order, a relative age can be determined for each layer

Relative Age

• relative age - age of an object in relation to the ages of other objects

• Relative age indicates one layer is older or younger than another layer not the rock’s age in years

What are the principles used to study Earth to figure out its age?

Law of Superposition• law of superposition – layers on bottom are older than

layers above it and younger than layers below it if they are not disturbed

• Scientists study layers in sedimentary rocks to determine relative age of rocks– Layers form when new sediments are deposited on top of

old layers of sediment.

• As sediments accumulate, they harden into layers called beds– Boundary between two beds is called a bedding plane

Principle of Original Horizontality

• Sedimentary rock generally forms in horizontal layers

• Principle of original horizontality - undisturbed sedimentary rocks will remain in horizontal layers

• Nonhorizontal layers were tilted or deformed by movements after the layers formed.

Graded Bedding• Tectonic forces push older layers on top of younger ones

– look for clues to the original position of the layers

• Look at particle sizes– Larger particles are in the bottom layers.

• Graded bedding-arrangement where coarse or heavy particles are in the bottom layers

• Larger particles in top layers = overturned by tectonic forces

http://upload.wikimedia.org/wikipedia/commons/3/3a/Gradierte_Schichtung_EN.jpg

Cross Beds

• Cross beds - sandy sediment form curved beds at an angle to the bedding plane.

• The tops often erode so it appears to be curved at the bottom and cut off at the top– Study the shape of the cross-beds to determine

the original position of the layers

Ripple Marks

• Ripple marks - small waves form on the surface of sand because of the action of water or wind– Sand becomes sandstone and the ripple marks are

preserved

• The orientation of the ripple marks to determine the original position of the layers.– Undisturbed layers = ripple marks point upward

Unconformities• unconformity - break in the geologic record created

when rock layers are eroded or when sediment is not deposited for a long period of time

• Movements of Earth’s crust can lift up rock layers exposing them to erosion.

• Sediments are deposited & new rock layers form

• The missing layers create a break in the geologic record

Types of Unconformities

• Nonconformity - stratified rock rests upon unstratified rock (igneous/metamorphic)

• Angular unconformity - boundary between a set of tilted layers and a set of horizontal layers.

• Disconformity – layer is uplifted w/o tilting/folding and eroded. Sediments continue to deposit.

Cross-Cutting Relationships

• law of crosscutting relationships - principle that a fault or body of rock is younger than any other body of rock that it cuts through

• A fault is a break or crack in Earth’s crust where rocks shift their position

• An intrusion is a mass of igneous rock that forms when magma is injected into rock