observing from orbit, this difference is negligible. In practicality, the radius of
Earth is usually rounded and taken to be 6,400 km. The second nonspherical com-
ponent of the planet is its topography above sea level. The oceans that constitute
most of the planetary surface present an almost-spherical shape to the atmosphere.
The largest exceptions are two lunar-caused tidal bulges rhythmically increasing
and decreasing sea level approximately twice every 24hours. Most land on the
planet is within a thousand meters of sea level, but some very high elevations exist.
The tallest is Mt. Everest (Sagamartha) in Nepal and is 8.8 km above sea level and
the lowest is the Dead Sea in Israel at minus 4 km. Underneath the ocean there is
an average depth of 3.8 km with the deepest being 10.9 km at the Challenger Deep
in the long, narrow extent of the Mariana Trench in the southwest Pacific Ocean.
The difference in elevation from highest to lowest points on Earth approximates
13.6 km and is dwarfed by the radii of the Earth. This is not to say that these topo-
graphic variations are inconsequential. For instance, altitude above sea level is a
major control of climate experienced at a location. So, too, varying physical and
biological features can be found with depth into the oceans.
The interior of Earth is of intense interest to the geographer because interior
processes have a profound effect on exterior landforms and the human geography
of the surface. Far from being a solid mass, Earth’s interior is differentiated over
depth and is quite active. How do we know this is the case? First, there are dra-
matic surface illustrations of activity through volcanoes and earthquakes. Second,106 Earth
Terrestrial Magnetism
Earth has a magnetic field and it has been studied scientifically since the early 19th century.
Terrestrial magnetism (geomagnetism) is caused by giant, sluggish convection currents in
the molten, iron-rich outer core of Earth. This magnetism has some irregularities so it is
not equally strong everywhere.Nonetheless, it surrounds Earth out to several thousand
kilometers and is known as the magnetosphere. The magnetosphere is “teardrop” shaped
with its long end away from the sun shielding the planet from the particles and energy of
the solar wind. The magnetic field is a dipole because it intersects in two places with Earth’s
surface. One place is near the magnetic north pole near the northern end of Earth’s axis; it
has a counterpart in the Southern Hemisphere. Compasses point to magnetic north pole
rather than the geographic North Pole, which is the north end axis of Earth’s rotation. The
angular difference between a magnetic pole and a geographic pole is called declination, and
it can be over 40°in some places. The magnetic poles systematically shift. For instance, the
north magnetic pole is currently moving at 60kilometersperyear(36miperyear)from
northern Canada toward Russia. Additionally, the North and South poles reverse with a
periodicity averaging 300,000 years.