# Presentation: 497218

## Slide 1

## Magnetic Field of the Earth

**Fig. 19.11**

**Magnetic Field of the Earth**

## Earth's magnetic field

- magnetic poles are nearly coincident with the spin axis (*i.e.*, the geographic poles).
-  A freely suspended magnet will point down at N. Pole and up at south pole.

## Earth's magnetic field

*declination*:  horizontal angle between    magnetic N and true N
*inclination*:  angle made with vertical

## Magnetic Field of the Earth

**Fig. 19.11**

**Magnetic Field of the Earth**

## Magnetic Field of a Bar Magnet

**Magnetic Field of a Bar Magnet**

**rubbish**

## The Earth's magnetic field

- Since the geothermal gradient in the Earth is ≈ 25°C/km, nothing can be permanently magnetized below about 30 km.
- Another explanation is needed.

## Self-exciting dynamo

- A dynamo produces electric current by moving a conductor in a magnetic field and *vise versa.*  (*i.e.*, an electric current in a conductor produces a magnetic field.

## Evidence of a Possible Reversal of the Earth’s Magnetic field

## Paleomagnetism

- Use of the Earth's magnetic field to investigate past plate motions
- Permanent record of the direction of the Earth’s magnetic field at the time the rock was formed
- May not be the same as the present magnetic field

## Use of magnetism in geology

- some Fe, Mn, Cr, Co minerals are magnetic
- but only below the Curie point 550°C
- lava solidifies at 1100°C

## Self-exciting dynamo

- outer core convection guided by earth's rotation
- A "stray" magnetic field (e.g.from the Sun) interacts with the moving iron in the core to produce an electric current
- this creates a local magnetic field—
- this creates an electric current
- this creates a local magnetic field etc

## Self-exciting dynamo

- The theory has this going for it:
- • It is plausible.
- • It predicts that the magnetic and geographic poles should be nearly coincident.
- • The polarity is arbitrary.
- • The magnetic poles move slowly.

## Magnetic reversals

- The polarity of the Earth's magnetic field has changed thousands of times in the Phanerozoic (the last major reversal was about 700,000 years ago).
- These reversals appear to be abrupt  (probably last 1000 years or so).

## Magnetic reversals

- A period of time when magnetism is *dominantly* of one polarity is called a magnetic epoch.
- We call north polarity *normal* and south polarity *reversed.*

## Self-reversal theory

- First suggested that it was the rocks that had changed, not the magnetic field
- By dating the age of the rocks (usually by K–Ar) it has been shown that *all* rocks of a particular age have the same magnetic signature.

## Recording the Magnetic Field in Newly Deposited Sediment

**Fig. 19.13**

**Recording the Magnetic Field in Newly Deposited Sediment**

## Lavas Recording Reversals in Earth’s Magnetic Field

**Fig. 19.14**

## Magnetic reversals

- because reversals are  random they provide a key to determining age

**numbering of reversals based on epochs**
**C is a chron    C1 toC34   (121 million years)**
**C34 cretaceous magnetic quiet 82-121 million years**
**Mesozoic chrons numbered M1 125 milion years to **
**M29  157 million years..**

## The GeomagneticTime Scale

- from ocean floor basalt
- from sediment
- from lava flows
- good record of geomagnetic reversals to ≈ 60 Ma.

## magnetics

- Total field vector B has vertical component Z and horizontal component H.
- Inclination, I=tan-1 Z/H
- tan I =2 tan (latitude)
- Declination of H measured relative to true north
- B =25000nT at equator        70,000 nT at poles
- Earth’s field varies 2nT per km.
- fluctuates 1000 nT in magnetic storm (solar radiation)
- proton magnetometer (or flux gate) can detect 1 nT in 50knT
- Magnetic anomaly on sea surface is about 100nT

## paleomagnetism

- atoms in *paramagentic* minerals contain an odd number of electrons.
- these act as tiny magnets (dipoles) because of the spin of the electrons.
*Induced magnetism* -when placed in a weak magnetic field the atomic dipoles align parallel to the field.
- ferromagnetic materials have many unpaired electrons which couple together to form magnetic domains unless temperature is too high - the Curie temperature
- below Curie temperature the magnetic field is retained as ‘permanent’ or* natural remanent magnetism    NRM*
*Thermoremanent magnetism* in  Igneous Rocks    TRM
*Detrital remanent magnetism* in sedimentary rocks  DRM
- The above are termed primary: Secondary remanent magnetism occurs after rock is formed

## Secondary Remanent magnetism

- Isothermal remanent magnetism IRM  Strong field eg.lightning strike
- Chemical remanent magnetism  CRM (formation of new minerals)
- Viscous Remanent Magnetism  VRM expose a rock to a field for a long time
- VRM adds noise to other magnetic signals in rocks, but can be removed by exposing the rock to an AC field of increasing strength.

## Paleolatitude from rock cores

- only declination can be derived from a core (no longitude information)
- tan (latitude) ={tan(inclination)}/2
- if paleo pole not aligned with current north pole - then polar wander.
- true polar wander or apparent polar wander (relative to continent)? Does pole or continent move
- half the cores indicated a reversed pole position

## motions on a sphere - Euler poles

- Euler’s fixed point theorem  “The most general displacement of a rigid body with a fixed point is equivalent to a rotation about an axis through that fixed point”
- if a tectonic plate is the rigid body, and the center of the earth is chosen as the fixed point then “Every displacement from one  position to another on the surface of the earth can be regarded as a rotation about a suitably chosen axis passing through the center of the earth.”

## Euler poles

- a *rotation axis* intersects the earth in two locations and passes through the center of the earth.
- the intersection of the *rotation axis* with the earth’s surface is termed the rotation pole.
- sign convention is that rotation is positive looking out from the center of the earth. (positive rotation is thus anticlockwise looking down from outsde the earth).  (the two poles thus have opposite rotations)
- Euler poles are determined from the strike of transform faults and spreading centers, from earthquake slip vectors, and  for instantaneous rotations from space geodesy.

## relative velocites at plate boundaries

- velocity at plate boundary increases with distance from the rotation pole
- u=asin∆ where a=radius of the earth,   is the angular distance between the rotation pole and the point, and  is the angular velocity
- Velocity needs ∆ to be calculated first.
- Azimuth of the velocity at the plate boundary follows using sine rule.