TodayToday – 2/9 – 2/9

• Weather report

• Review

• The magnetic field and the sea floor!

• Workshop

• Review session: Mon 5-7, Chavez 110

Last Time

Continental drift rejected despite good evidence because no mechanism conceivable

Seismic profiling reveals internal structure of the Earth

Convecting outer core powers geodynamo, creating Earth’s magnetic field

Earth’s Magnetic FieldEarth’s Magnetic Field Diverts solar wind, which would Diverts solar wind, which would

destroy ozone layer and ruin destroy ozone layer and ruin electronicselectronics

Guides migrating animals from Guides migrating animals from bacteria to bees to sea birds to bacteria to bees to sea birds to toothed whalestoothed whales

Earth’s Magnetic Field

Reverses it polarity occasionally!

Interacts with solar storms to produce the aurora borealis

Leaves its signature in magnetic rocks like magnetite

““Halloween” Solar StormHalloween” Solar Storm

www.nasa.gov

Solar StormSolar Storm

www.nasa.gov

Aurora from SpaceAurora from Space

www.nasa.gov

Magnetism in Minerals

Each electron has “spin”, i.e. acts as a tiny bar magnetElectrons pair up in orbitals around nuclei; the electrons in a pair have opposite polarityLone electrons in an orbital impart a net magnetic moment to an atomIf the magnetic atoms in a mineral are all oriented identically, the mineral is magnetic

Magnetite

The crystal structure of magnetite allows magnetic iron atoms to retain their magnetic orientationAbove “Curie point”, 580 °C, atoms too energetic to be permanently magnetizedBelow Curie point, atoms line up with Earth’s magnetic field, the mineral becomes permanently magnetized and records the orientation of Earth’s magnetic field at the time the magnetite cooled through the Curie pointEarly 1900’s scientists recognized that magnetism in Earth’s rocks comes in two flavors: same polarity as today or opposite

Old Assumptions re Ocean Floor

4 Ga

Flat and featureless

Deeply buried in sediment

Mid-Ocean Ridges (MORs)

Naval survey, mid-1800’s

Sonar developed 1906 for icebergs, adapted for submarine detection WWI

Mid-ocean ridges: giant mountain chains running down the center of the oceans

After WWII, Atlantic sea floor sediment found to be a thin veneer

Magnetic Striping of the Sea Floor

Magnetometers developed WWII for submarine detectionUsed to explore the magnetic properties of the sea floor during the 1950’sMagnetic striping: found alternating bands of reversed magnetic polarity in the rocks running parallel to the MOR’s!Pattern symmetrical about the MOR’s!

Sea Floor Spreading!

1968 – Glomar Challenger drills the ocean floor across the S Atlantic from S America to AfricaRetrieved rocks show that ages decrease to zero at the MAR, then increase, so new oceanic crust is being created at the MAR!The core has revealed to us the mechanism for continental drift!

Age of the Atlantic Sea FloorAge of the Atlantic Sea Floor

www.noaa.gov

Age of the Sea FloorAge of the Sea Floor

www.pubs.usgs.gov

pubs.usgs.gov

Workshop

Distance = rate time, d = rt

t = d/r

r = d/t

Rate of sea floor spreading = 2 distance from rock to ridge / age of rock

ExampleExample

90 km

atlas.geo.cornell.edu

Back in the Lab

Isotopic analysis reveals that the rock is 3 Ma

What is the rate of sea floor spreading?

Clear Unclear