28.4 - The Earth and magnetic fields
The Earth acts like a huge magnet with an accompanying magnetic field. Compasses
take advantage of this field: Their needles are mounted so that they can rotate freely
and align with it. Once a compass has established the north and south directions, other
directions can be established relative to them.The Earth’s magnetic south pole is located near its geographic North Pole. This
means that the north pole of the magnet in a compass points north, since the south pole
of the planetary magnet attracts it. The geographic poles lie on the planet’s axis of
rotation.Scientists still do not know exactly how our planet produces its magnetic field, though
the prevalent theory is that Earth’s magnetism arises from processes that occur in its
core. Scientists have arrived at this conclusion because of the fundamental differences
between the cores of the Earth and the Moon. The Earth has a molten metallic core and
a magnetic field, while the Moon has a solid core and no magnetic field. Many scientists
believe the large-scale liquid flow that occurs inside the Earth’s core causes its
magnetic field.
While scientists are still puzzled by the “why” of Earth’s magnetic field, they do know
that the locations of its magnetic north and south poles change. The magnetic south pole is currently about 1900 km away from the geographic
North Pole of the Earth. The magnetic north pole resides in the ocean south of Australia, not even in the continent of Antarctica! These
positions are not fixed; shifts of several degrees in their locations have been measured over the last century. The poles do more than wander,
they also reverse their orientation. The last switch occurred a few hundred thousand years ago, when magnetic north became magnetic south,
and vice versa.
Scientists deduce this change in orientation by analyzing parts of the ocean floor that originate as magma (molten rock) emerging from cracks
in the Earth’s crust. Over a period of millions of years, vast quantities of solidified magma have been deposited on the ocean floor. As magma
congeals at any point in time, the iron it contains “records” the orientation of the Earth’s magnetic field. New magma flows force earlier deposits
apart, which separates them and enables scientists to establish their sequence. By examining different sections of the rock, scientists can
deduce the direction of the magnetic field at various times throughout Earth’s history.
The current strength of Earth’s magnetic field is about 5×10í^5 teslas, and that strength is decreasing at about 0.07 percent per year. If it
continues to weaken at this rate, it will be reduced to only 1 percent of its present value in 6,500 years. Since the magnetic field helps to shield
our biosphere from cosmic rays and charged particles from the Sun, this could be a matter of concern.
Scientists theorize that the poles “flip” alignment after the field passes through a state with zero magnitude. Because there are no signs of
massive mutations in the fossil record that date from the period of the last “flip,” perhaps the results are not as severe as one might fear. No
one knows exactly how long the zero-field condition exists, whether five years, 50 years, or 1000 years. As the NASA website above has
stated, “Stay tuned...”The Earth and magnetic fields
Earth is huge magnet
·North Pole § south pole of magnet
·South Pole § north pole of magnet28.5 - Physics at work: compasses and the Earth
A magnetized compass needle aligns with the Earth’s
magnetic north and south poles. The needle’s north
pole, sometimes distinguished by a bright color as in
the photograph above, points toward the Earth’s
magnetic south pole. However, the Earth’s magnetic
poles are not at the same locations as its geographic
poles. For instance, in 1831, British explorer James
Clark Ross located the magnetic south pole off the
coast of Canada (remember, the magnetic south pole
corresponds to the geographic North Pole). He tried
to duplicate this feat for the magnetic north pole near
Antarctica, but failed due to weather and ice.
Nonetheless, many regions of that continent (such as
the Ross Sea) are named after him.
The angle between the directions to the magnetic and
geographic poles is called the magnetic declination.
Many topographic maps provide the local angle of declination so that hikers and others can compensate for it and orient their compasses to
geographic north. The first sketch on the right shows the declination for Seattle, Washington: It is approximately 20°.
Scientists and navigators also found that compasses could be used to estimate latitude (north/south position on the planet). Although today’s
compasses are designed to move in a horizontal plane, the needles in some early instruments were allowed to rotate freely in all directions
(like a bar magnet suspended by a string tied around its middle). When carried to a magnetic pole of the Earth, the magnetic needles of these
compasses would point straight up or down; conversely, when near the equator, they would point almost horizontally. By measuring the angle
of the needle from the horizontal, navigators could estimate their latitude.In mountains near Seattle, Washington, the magnetic declination is about 20°.(^508) Copyright 2000-2007 Kinetic Books Co. Chapter 28