Geologic Time. What is Geology? Geology is the study of the Earth’s form and composition and the changes it has undergone.

Geologic Time

What is Geology?

Geology is the study of the Earth’s form and composition and the changes it has undergone.

Why do we care?1. Rock record provides evidence of

geological evens and life forms of the past.

2. Processes observed on Earth in the present also acted in the past.

3. Earth is very old and has changed over time.

Is the Earth changing now?Yes! Although many features of the Earth

may seem to be unchanging over our lifetime, they are still changing, but on time scales of hundreds, thousands or even millions of years.

This image shows Mount Hood in Oregon USA at the same time in late summer in 1985 and 2002.

How old is the Earth?

How old is the Earth?The Earth is 4.6

billion years old.The sun is 5 billion

years old.The universe is

13.7 billion years old

How do we know? Relative Dating

Absolute Dating

Relative DatingRelative Dating identifies which rock units

formed first, second, third and so on.

Relative dating determines the sequence in which events occurred, but not how long ago they occurred.

Relative DatingLaw of Superposition – each rock layer is

older than the one above it and younger than the one below it.

Usually applies to sedimentary rock, but can also be applied to lava flows and layers of ash from volcanic rock.

Relative DatingPrinciple of Original Horizontality – layers of

sediments are generally deposited in horizontal position.

Relative DatingPrinciple of Cross-Cutting – when a fault

cuts through rock layers, or when magma intrudes other rocks and hardens, then the fault or intrusion is younger than the rocks around it.

http://www.schools.utah.gov/curr/science/core/8thgrd/sciber8/geology/html/rockages.htm

Absolute DatingAbsolute Dating identifies the exact age of

a rock.

Scientists use radioactive decay to determine the age of the rock.

Radioactive DecayDuring radioactive decay, unstable atomic

nuclei spontaneously break apart, or decay, releasing energy.

Radioactive Decay

What is an isotope?

Radioactive isotopes decay until they are stable.

Ex. Uranium-238 decays over time to form lead-206.

Half LifeHalf life is the amount of time necessary for

one half of the nuclei in a ample to decay to its stable isotope.

Radiometric DatingRadiometric dating, also called radioactive

decay dating, is a way of calculating the absolute ages of rock that contain radioactive isotopes.

Practice Problems1. The half life of nickel is 100 years. If you

had 94 grams of nickel, how much would be left after 200 years?

1 half life = 100 years = 47 grams2 half lives = 200 years = 23.5 grams

Practice Problems2. Sodium-24 has a half life of 15 hours. How much

of a 440 gram sample of sodum-24 remains after 60 hours?

1 half life = 15 hours = 220 grams2 half lives = 30 hours = 110 grams3 half lives = 45 hours = 55 grams4 half lives = 60 hours = 27.5 grams

Practice Problems3. Iron-59 has a half life of 45 days. A murder

weapon was found in a lake and the original weapon was reported to have contained 1,200 grams of iron. After 6 months (180 days), the original sample decayed. How much iron-59 was left?

1 half life = 45 days = 600 grams2 half lives = 90 days = 300 grams3 half lives = 135 days = 150 grams4 half lives = 180 days = 75 grams

KEY TERMSEon – the longest

division of time (2)Era – a well defined

time span Period – a unit of

geological time during which a system of rocks formed

Epoch – a portion of a geologic time period that marks an event

http://www.enchantedlearning.com/subjects/Geologictime.html

This website is linked on your class page.

GEOLOGIC TIME SCALE