Lecture5 lawsofgeology

Announcements

I will be writing PS #1 over the weekend, and posting it by

Monday… It will be due Feb 9th.

Clarification: The angle of Earth’s tilt does not change

during the year!

Views of the Earth from the Sun’s perspective

It only looks that way from the sun’s frame of

reference.

The axis is always parallel to itself.

Quiz 2!

2. Sun Angle: insolation curves

2. Sun Angle: insolation curves

WinterWinter

SummerSummer

Autum

n

Autum

nSpr

ing

Spr

ing

WinterWinter

Insolation

23.5o axis tilt

0o axis tilt

An Equation for the Sun Angle

On an equinox

Declination = 0°

An Equation for the Sun Angle

On an equinox

Declination = 0°

At the equatorLat =Sun angle =

0°90°

An Equation for the Sun Angle

On an equinox

Declination = 0°

At the north poleLat =Sun angle =

90°N0°

An Equation for the Sun Angle

On an equinox

Declination = 0°

At some latitude xLat =Sun angle =

x°N90°- x°

x° 90°- x°

But then we have to adjust for declination!

Equator is sub-solar point

North pole tilted toward the sun

North pole tilted away from the sun

Equator is sub-solar point

Summer

Winter

Fall

Spring

But then we have to adjust for declination!

Sun Angle = 90°-x°

Sun Angle = 90°-x°

Sun Angle = 90°-x°-23.5°

Sun Angle = 90°-x°+23.5°Summer

Winter

Fall

Spring

Final Formula

Sun Angle at Noon =

90° - Latitude° + declination°

Summer

Note: this formula is for the angle with the southern horizon, so it makes the most sense to only use it in the Northern Hemisphere

Lithosphere: solid Earth

Geologic time: dating2 types2. Relative Dating = sequential age

- based on positions of rocks relative to each other

- e.g., rock layer B is older than A and younger than C

2. Absolute Dating = actual age- based on radiometric dating- e.g., rock layer B is 50 million years

old

Dating using Geological Laws

1. Law of superposition- younger rocks are deposited on top of older

rocks

younger

young

oldest

older

old

Grand Canyon Layers

1. Law of original horizontality- sedimentary rocks & lava are originally laid

down horizontally

Roughly horizontal

Dating using Geological Laws

A roadcut photo that I stole… WV maybe?

1. Law of cross-cutting relations- rocks that intrude into other rocks are younger

than their host rocks - faults are younger than the rocks they cut

through- folds are younger than the rocks they bend

Dating using Geological Laws

Roadcut near Moab, UT

A

BC

D

Events:A depositedB depositedC depositedD depositedFaultingErosion of Surface XErosion of Canyon

But in what order?

Order of Events:D depositedC depositedB depositedFaultingErosion of Surface XA depositedErosion of Canyon

X

Events:A depositedB depositedC depositedD depositedE depositedF depositedG depositedFoldingErosion of Surface X

Order of Events:G depositedF depositedE depositedD depositedC depositedFoldingErosion of Surface XB depositedA deposited

AB

CD

X

EF G

Events:A depositedB depositedC depositedD depositedE depositedF depositedFoldingFaultingErosion of Surface XErosion of Surface Y

Order of Events:F depositedE depositedD depositedC depositedFoldingErosion of Surface YB depositedFaultingErosion of Surface XA deposited

A

BC

D

X

EF

Y

Events:Cooling of Granite ADeposition of Sandstones B, E, & IDeposition of Shales C & HDeposition of Limestone D & FCooling of Basalt G Folding (Tilting)FaultingErosion of surfaces X, Y, ZFormation of mesa

Order of Events:1 . A 9 . E2 . X 10 . F3 . B 11 . Z4 . C 12 . G5 . D 13 . H6 . fold/tilt 14 . I7 . Y 15 . mesa8 . fault