EARTH’S MAGNETIC FIELDAND IT’S REVERSAL

AN UNDERGRADUATE SEMINAR PAPER (GEOL 411)

PRESENTED

BY:1) ALI HASSANA (U10GL1063)

2) EGBA MOSES CLETUS (U10GL1046)

MAY, 2014

TABLE OF CONTENTS1. INTRODUCTION

2. DEFINITION OF TERMINOLOGIES

3. CAUSES OF EARTH’S MAGNETIC FIELD

4. VARIATIONS IN THE EARTH’S MAGNETIC FIELD

5. EARTH’S MAGNETIC REVERSAL AND ITS

EFFECTS

6. APPLICATIONS OF EARTH’S MAGNETISM

INTRODUCTIONA region of magnetic force—a magnetic field—surrounds Earth and the Earth itself is a “Great Magnet”

A magnetic field is the magnetic influence of electric and magnetic materials.

The magnetic field at any given point is specified to have both direction and a magnitude (or strength); as such it is a vector field.

The invisible lines of magnetic force surrounding Earth deflect magnetized objects, such as compass needles, that are free to move.

Because the compass is important in navigation, Earth’s magnetism has been observed for centuries.

INTRODUCTION…The field has north and south magnetic poles, one near the geographic North Pole, the other near the geographic South Pole.

It has long been known that the magnetic poles are displaced about 11.5° from the geographic poles (about which Earth rotates).

Due to its two poles, Earth’s magnetic field is called dipolar; and because the field is not 100% dipolar, the magnetic poles appear to be moving slowly around the geographic poles.

The strength of the magnetic field is greatest at the magnetic poles, where magnetic lines of force appear to leave and enter Earth vertically.

INTRODUCTION…More recently, geophysical studies have been directed towards the source of Earth’s magnetism.

The rate at which the poles’ changes position, together with the strength of Earth’s magnetic field, strongly suggest that Earth’s magnetic field is generated within the liquid outer core, rather than within the solid rock of the crust or the mantle (Fig. 1).

The aim of this presentation is to review and outline: Earth’s magnetic field; its reversal; its origin and applications.

Fig. 1. Internal Structure of the EarthEarth is made up of a series of layers that formed early in the planet’s history, as heavier material gravitated toward the center and lighter material floated to the surface. The dense, solid, inner core of iron is surrounded by a liquid, iron, outer core.

DEFINITION OF TERMINOLOGIESMagnetic field: A region of magnetic force.True north: Also geodetic north is the direction along the earth’s surface towards the geographic North Pole.

Magnetic north: Is the direction a compass point towards.Grid north: Is the direction northwards along the grid lines of a map projection.

Magnetometer: A magnetometer is an instrument used to measure the strength of Earth’s magnetic field.

Anomaly: The strength of Earth’s magnetic field varies from place to place. As with gravity, a deviation from average readings is called an anomaly.

Magnetic Declination: This is the angle between true north and the horizontal component of the earth magnetic field.

DEFINITION OF TERMINOLOGIES…Magnetic Inclination: This is the angle between horizontal and the direction of the total field.Normal polarity: When the north seeking end of the compass needle points towards the present north magnetic pole.Reversed polarity: When the north seeking end of the compass needle points southward.Curie Point: The temperature at which a mineral loses it magnetism (580 degrees for magnetite).

CAUSES OF EARTH’S MAGNETIC FIELDThe most widely accepted hypothesis suggests that the core behaves like a self sustaining dynamo, a device that converts mechanical energy into a magnetic energy.

The driving force of this system is the earth

rotation and the unequal distribution of heat in the earth interior which propels the highly conductive iron in the outer core.

Magnetic field is created by electric currents within the liquid outer core (Fig. 2).

As the iron churns in the outer core, it interacts with the earth’s magnetic field; this interaction generates an electric current.

Metals are good conductors of electricity, whereas silicate rock is generally a poor electrical conductor. Indirectly, this is evidence that the core is metallic.

Fig. 2. Absolute plate motions and true polar wander in the absence of hotspot tracks

VARIATIONS IN THE EARTH’S MAGNETIC FIELDThe earth’s magnetic field is not stable as changes happen incessantly e.g. the magnetic poles move from year to year.

Parallel with the studies of the magnetic field in space, new techniques for collecting and interpreting geological data (magnetic matter in volcanic mountains) have given new information about the earth’s crust and interior.

At temperatures above the Curie point (ca.1000K), matter loses its ferromagnetic properties and can no longer retain magnetization.

It is clear that the magnetic field has switched orientation many times in the past 4-5 million years, which means that the compass needle has turned 180° (Fig. 3).

Existing measurements indicate that a polarity shift happens such that the field decreases slowly towards zero, and then rebuilds with opposite polarity.

But we have no satisfying explanation for this polarity shift.

FIG. 3. Fluctuations in the Earths’ magnetic field (brad.ac.uk)It is clear that the magnetic field has switched orientation many times in the past 4-5 million years, which means that the compass needle has turned 180°.

EARTH’S MAGNETIC REVERSALThe Earth’s magnetic field is not stable and there are evidences it has periodically reversed its polarity in the past.

Such a change in the polarity of the magnetic field is a magnetic reversal.

During a time of normal polarity, magnetic lines of force leave earth near the geographic South Pole and re-enter near the geographic North Pole.

This orientation is called “normal” polarity because it is the same as the present polarity.

During a magnetic reversal, the north magnetic pole and the south magnetic pole exchange positions (Fig. 4).

Fig. 4 Lava Flow showing a transform fault (map.unomaha.edu)Natural Remnant Magnetization (NRM). Many periods of normal and reverse magnetization are recorded in continental lava flows. They are worldwide events. Since lava flows can be dated isotopically, the time of these reversals in Earth’s past can be determined.

Earth’s Magnetic Reversals…

EARTH’S MAGNETIC REVERSALS ANDITS EFFECTS

Many rocks contain a record of the strength and direction of the magnetic field at the time the rocks formed (Fig. 4).

When the mineral magnetite, for example, is crystallizing in a cooling lava flow, the iron atoms within the crystals respond to Earth’s magnetic field and form magnetic alignments that “point” toward the north magnetic pole.

As the lava cools slowly below the Curie point (580°C for magnetite), this magnetic record is permanently trapped in the rock.

This magnetic record is retained and when studied, reveals the direction of Earth’s magnetic field at the time the lava cooled.

EFFECTS OF EARTH’S MAGNETISMWhen the magnetic field is at its usual strength, it shields Earth from these rays, but when the field collapses, this shielding is lost. Cosmic radiation affects organisms.

Extinction of some species and the appearance of new species by mutation have been correlated with some magnetic reversals. However, far more reversals than mass extinctions have occurred.

Navigation becomes difficult to achieve.

APPLICATIONS OF EARTH’S MAGNETISM Paleomagnetism, which is the study of the intensity and orientation of the earth’s magnetic field as preserved in the magnetic orientation of certain minerals found in rocks formed throughout geological time.

Plate Tectonics which can be defined as the gradual movements and formation of continent as described by plate tectonics, which is the folding and faulting of earth crust leading to the formation of newer ones.

Magnetic Stratigraphy: Local sequences can be compared with a globally compiled record of magnetic reversals, at least for the late Mesozoic and Cenozoic.

Applications of Earth’s Magnetism cont...

CONCLUSIONThe earth’s magnetic field is not stable. Changes happen incessantly. Among others, the magnetic poles move from year to year. Details about how the magnetic field develops and is maintained are still unknown.

“Magnusmagnes ipse est globus terrestris”

William M. Gilbert (1600)