Download Oceanic-Continental Boundary

Magnetic Reversals
In the 1950s, ocean-going research vessels recorded puzzling data based on the
magnetism of the ocean floor. It was determined that the rock of the ocean floor
had alternating bands of embedded iron oxides that pointed north and south. Thus,
in 1963, the theory of the reversal of the earth's magnetic field was proposed and it
has been a fundamental of earth science since.
1) Ship moves across view, towing
magnetometer
2) magnetic stripes the magnetometer 'sees'
then appear superimposed on ocean floor.
Scientists believe that the earth's magnetism is created by slow movements in the liquid
outer core, caused by the rotation of the earth. The generation of the earth's magnetic field
is a continuous, but variable, process that causes change in not only the intensity of the
magnetic field, but also causes the Magnetic North Pole to move as well as the reversal of
the earth's entire magnetic field.
Lava, which hardens into rock, contains grains of iron oxides that point toward the magnetic
pole as the rock solidifies. Thus, these grains are permanent records of the location of the
earth's magnetic field. As new crust is created on the ocean floor (such as at the Mid-Atlantic
ridge), the new crust solidifies, with its iron oxide acting like miniature compass needles.
Scientists have matched the magnetic bands on either side of the Mid-Atlantic ridge out to
the edges of the ocean. To determine the distance between the Americas and Europe and
Africa at any point since Pangea, one need only to "roll back" the oceanic crust to the
appropriate matching magnetic bands on either side of the ridge. Magnetic reversals helped
to prove the theory of plate tectonics and continental drift.
The earth's magnetic field has reversed approximately 170 times over the last 100 million
years. The intensity of the magnetic field has been decreasing over time since it has been
measured and some scientists expect that at the current rate of decline, there may be
another magnetic reversal in approximately 2000 years. That should be enough time to
replace all of our compasses.
Scientists have long known that the magnetic pole moves. James Ross located the
pole for the first time in 1831 after an exhausting arctic journey during which his ship
got stuck in the ice for four years. No one returned until the next century. In 1904,
Roald Amundsen found the pole again and discovered that it had moved--at least 50
km since the days of Ross.
The pole kept going during the 20th century, north at an average
speed of 10 km per year, lately accelerating "to 40 km (25 miles)
per year. At this rate it will exit North America and reach Siberia
in a few decades.
A National Geographic article in 2005 said that
the pole has moved 685 miles (1,100
kilometers) over the over the past century
Globally the magnetic field has weakened 10%
since the 19th century.
As remarkable as these changes sound,
"they're mild compared to what Earth's
magnetic field has done in the past," says
University of California professor Gary
Glatzmaier.
Sometimes the field completely flips. The
north and the south poles swap places. Such
reversals, recorded in the magnetism of
ancient rocks (sea-floor spreading), are
unpredictable. They come at irregular intervals
averaging about 300,000 years; the last one
was 780,000 years ago. Are we overdue for
another? No one knows.
According to Glatzmaier, the ongoing
10% decline doesn't mean that a reversal
is imminent. "The field is increasing or
decreasing all the time," he says. "We
know this from studies of the
paleomagnetic record." Earth's presentday magnetic field is, in fact, much
stronger than normal.
To understand what's happening we have to take a trip ... to the
center of the Earth where the magnetic field is produced.
At the heart of our planet lies a solid iron ball, about as hot as the
surface of the sun. Researchers call it "the inner core." It's really a
world within a world. The inner core is 70% as wide as the moon. It
spins at its own rate, as much as 0.2° of longitude per year faster
than the Earth above it, and it has its own ocean: a very deep layer of
liquid iron known as "the outer core."
Earth's magnetic field comes from this ocean
of iron, which is an electrically conducting fluid
in constant motion. Sitting atop the hot inner
core, the liquid outer core seethes and roils
like water in a pan on a hot stove. The outer
core also has "hurricanes"--whirlpools
powered by the Coriolis forces of Earth's
rotation. These complex motions generate our
planet's magnetism through a process called
the dynamo effect.
So, what happens during a magnetic flip?
Reversals take a few thousand years to
complete, and during that time--contrary to
popular belief--the magnetic field does not
vanish, it just gets more complicated.
Magnetic lines of force near Earth's surface
become twisted and tangled, and magnetic
poles pop up in unaccustomed places.
Convergent Boundary
Oceanic-Continental Boundary
Two plates are moving towards each other,
depending on the type of crust that is colliding When oceanic crust collides with continental
crust the oceanic crust is subducted (sinks)
mountain ranges or volcanoes may form
under the continental crust in a subduction
zone and becomes magma again.
Oceanic-Oceanic Boundary
When oceanic crust collides with oceanic crust
one plate also sinks under the other. As this
happens a deep trench is formed on the sea
bed. Magma rising up from the subduction
zone leads to the formation of volcanoes.
Given enough time (millions of years) the
submarine volcanoes grow large enough to rise
above sea level and become new volcanic
islands.
Continental-Continental Boundary
When continental crust collides with
continental crust neither is significantly more
dense than the other one so they are both
pushed up and/or sideways. This folding and
buckling creates fold mountains.
The Causes of Plate Tectonic Movement
Ridge Push: The crust is higher at mid-ocean ridges this causes the oceanic lithosphere to
slide down the hill, due to gravity, and push the rest of the crust
Slab Pull: The oceanic plate that is sinking in the subduction zone is very dense, as it sinks it
pulls the rest of the tectonic plate with it
Convection: Hot rock in the earth rises since it is less dense and cooler rock near
the surface sinks (denser). This movement of rocks causes the oceanic lithosphere
to move sideways, away from the mid-ocean ridge.