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Helena Bonici Geography 20/06/2011
12EA2 Plate Tectonics
Plates Separate - Divergent
The Mid-Ocean Ridge and rift valleys, such as the one that runs
through eastern Africa, occur along boundaries where plates are
spreading apart. New oceanic crust is created as the plates separate
and molten rock rises up from the mantle and fills the space. The
earthquakes and the volcanic eruptions along the Mid-Ocean Ridges
are a direct result of this process.
Plates Collide - Convergent
When two plates carrying continents collide, the continental crust
buckles and rocks pile up, creating towering mountain ranges. The
Himalayas were born when the Indian subcontinent smashed into
Asia 45 million years ago. The Himalayas are still rising today as the
two plates continue to collide. The Appalachian Mountains and Alps
also formed in this way.
When an ocean plate collides with another ocean plate or with a plate
carrying continents, one plate will bend and slide under the other. This
process is called subduction. A deep ocean trench forms at this
subduction boundary. As the subducting plate plunges deep into the
mantle, it gets so hot it melts the surrounding rock. The molten rock
rises through the crust and erupts at the surface of the overriding plate.
The result is either a volcanic mountain range such as the Cascades and
Andes, or chains of islands called island arcs such as Japan and the
Plates Slide - Transform
Plates grinding past each other in opposite directions create faults called transform faults. Powerful earthquakes often strike along
these boundaries. The San Andreas Fault is a transform plate boundary that separates the North American Plate from the Pacific
Plate. This fault system is largely responsible for the devastating earthquakes in Los Angeles and San Francisco.
The surface of the Earth is broken into rigid plates. These plates are 100 to 120 kilometres thick and include the crust and a
small part of the upper mantle. Many plates contain both continental and oceanic crust. The plates sit on top of a softer, more
plastic layer of the mantle called the asthenosphere.
All of the plates are in constant motion, some moving faster than others. They move in different directions at about five
centimetres each year on average.
The most accepted theory of why the plates move is that convection currents in the asthenosphere drag them along.
Convection currents occur when one part of a fluid is warmer than another part.
Similarly, columns of hot mantle material deep down in the Earth rise towards the surface. When the mantle material reaches
the Earth's thin lithosphere, it moves along the surface away from the column of hot material. It then cools and sinks back
down into the deep mantle.
Most of the earthquakes and volcanoes correspond to the boundaries of plates.
The Earth is a magnet is because it has an inner core that is made of solid iron and nickel, two very magnetic materials, and an
outer core that is more liquid. The two parts of the Earth's core are both in motion, but they do not move in exactly the same
way. The Earth's core sets up a very strong magnetic field for planet Earth.
When hot lava erupts on the ocean floor and cools it not only crystallizes, but it also becomes magnetic. This is because the
magma that seeps from the Earth's mantle has a lot of iron in it. The magnetic crystals in the cooled lava have become little
magnets, all pointing in the direction of the magnetic field of the Earth. The more of these little magnetic crystals there are in a
rock, the stronger the magnetic signal of the rock is.
The lava erupted on the seafloor is the most magnetic because it has the smallest crystals. The crystals have no time to grow
any larger because the cold, near freezing seawater at the bottom of the ocean makes the lavas cool very fast.