EOSC Lecture 22 Magnetic Fields

Magnetic field

• either caused by quantum effects (property of the element, e.g. unpaired electrons) leading to material properties or electromagnetic - moving electrical charges (current) or both - electrically conductive, naturally magnetic

• if electrons are unpaired, there can be magnetic property to that metal, the most common is iron

• magnetic metal has special properties

• magnetic field → S, N →, same orientation repels

• not uniform

Detecting magnetic anomalies

magnetometers can be handheld or airborne, and can detect magnetic minerals or rocks below the surface

Geophysical exploration

looking at rock and physical properties

Earth’s magnetic field

• Earth’s magnetic poles line up close, but not too close with the geographic poles, as the magnetic field direction moves with time, and the two hemispheres don’t have to match

Declination

• the angle between lines connecting a point on the Earth’s surface to the geographic pole and the magnetic pole

• if poles perfectly align declination would be 0 degrees celsius

• varies over the surface, but not perfectly in a systematic way

Inclination

the angle between the magnetic field’s force line and the surface of the Earth

• field is parallel to surface at the equator, and perpendicular at the pole

• varies over the surface, but not perfectly in a systematic way

Origin of magnetic field in the outer core

• recorded pattern of magnetic field suggests it originates deep inside the Earth, not the mantle

• however, magnets only function at relatively low temperatures (<770 degrees celsius - Curie temp for iron) point above which material is no longer magnetic as it is too hot to be magnetic

• therefore electric current, but not simple answer, geodynamo theory

Electromagnetism

moving electrical charge makes a magnetic field, current on = things stick, current off = things fall off

Geodynamo theory

• convection of electrically conductive molten iron in the outer core (liquid) which can convect

• electrical current generates the magentic field

• call this a dynamo

• for this “core dynamo” to exist the fluid must convect vigorously

• temperature gradient causes convection

• this is why the magnetic poles don’t perfectly line up to the geographic poles and why the field is a little wonky and not related to materials, but chaotic interactions in the core that can vary

Where does electrical current come from?

moving electrical charges

• part of the motion is from the convection in the liquid outer core - from temperature differences

Coriolis effect - the Earth is also rotating

• the liquid metallic outer core (and the electrical charges in it) are convection and rotating at the same time

• together this generates helical motion

• helical convection creates geodynamo of magnetic field of the Earth

Magnetic field requirements

1. a liquid and electrically conductive outer core

2. a strong temperature gradient in that outer core to generate convection

3. fast enough rotation to cause currents oriented by Coriolis forces

Paleomagnetism

study of ancient magnetic fields

• day to day or even year to year, no variations

• short time scale variation - over decades it wanders

• long time scale variation - deep time shows major changes in the rock record

• one line of evidence for plate tectonics

• also how we are reconstructing the continental movements of Earth’s history

Polar wander

over short timescales (decades) we can observe the movement of the Earth’s geomagnetic North Pole

• up to 10s of km per year, significant movement

• now heading to Russia really quickly

Variations in deep time

1. geomagnetic reversal - poles flip

• weakens before reversal, lots of prelude to the reversal, it takes a long time

• not systematic, all different time periods, no uniform regression

1. apparent polar wander

• found ancient rocks measure the magnetic direction, declination, and inclination of older rocks

• reconstruct where old poles were, you need rocks from different continents → how continents would have moved with pole wander

• rocks of different ages gives different polar positions

• the older the rock, the further the calculated polar position is from the present position

• and rocks of the same age but on different continents give different polar positions

• always point to magnetic north, move position of continents,m poles have wandered, not polarity shifted, but not perfect as it depends on how many rocks of what age range etc. are collected from each continent

Magnetic striping

• changes in polarity are recorded in rocks that crystallize from magma

• polarity recorded on the sea floor can be correlated to dated rocks on land

• this is one way we know that divergent boundaries are spreading

• magma/lava is not magnetic

• when lava cools minerals crystallize, when it cools below 700 degrees celsius it takes on the magnetic properties of the Earth at that time → locked in

• magnetic minerals take on an average magnetization that is aligned with the Earth’s magnetic field

Consequences of a changing magnetic field

• reconstructions not possible without it, oceanic crust is very young and way less accurate when you go back in time

• mafic basalt is important, doesn’t exist past 300Ma in the oceanic crust

• if we didn’t have a magnetic field, the sun would burn us alive, it allows for navigation, electricity to function, bad for technology

• when it changes, it will affect navigation for many species, but no mass extinctions occur with reversals, as they happen very slowly

Aurora

manifestation of the magnetic field in the solar wind - protects us from the sun