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the earth is a remarkable piece of real
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estate
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and a busy one at that from
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ever-changing weather patterns to the
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hustle and bustle of city streets
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our planet is in a constant state of
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motion
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however if we take a look just below the
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surface we find that things are always
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moving below our feet as well
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it's called plate tectonics and it's how
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the continents were formed and reformed
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and reformed
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but before we get to that let's dive
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underground to learn what our planet
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looks like from the inside out
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our earth is made up of layers and there
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are two types of layers we need to learn
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about in order to understand plate
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tectonics
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the compositional or chemical layers and
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the mechanical layers
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we could spend an entire session on what
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makes both of these types of layers
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unique
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but for this lesson we're simply going
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to provide an overview
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there are three primary compositional
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layers to the earth's surface
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and these are probably the ones you're
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most familiar with the crust
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the mantle and the core it's important
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to understand that the compositional
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layers refer to the materials or
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elements the earth is made of however if
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we take a look at how the inside of the
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earth behaves
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we can divide them even further into
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mechanical layers
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starting from the outside working in the
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lithosphere is the outermost layer of
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the earth which contains the entire
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crust
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in the topmost layer of the mantle the
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asthenosphere contains the same
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materials as the top layers of the
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mantle
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but the pressure in the heat are
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increased it's not exactly a liquid but
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it does move like heated rock
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so you could say it's somewhat fluid the
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mesosphere is next
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at this level the pressure is really
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starting to build which restricts the
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molecule's ability to move thus
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making it very very rigid past the
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mesosphere is the liquid outer core
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extremely high temperatures melt the
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metals contained in this layer into
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liquid form
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think flowing rivers of lava and you'll
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have a pretty good idea of what the
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outer core
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is like and finally we reach the solid
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inner core
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just like the liquid outer core the
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inner core has extremely high
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temperatures
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and is composed of mostly metals which
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might lead you to ask
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why isn't it liquid as well the answer
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is pressure
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there's so much pressure at this depth
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that even though the temperatures are
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beyond the melting points of the metals
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contained in the inner core
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the high pressure essentially squishes
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everything down into a solid
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now that we know a bit more about the
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composition of the earth let's move on
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to plate tectonics
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let's start with the history in the
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early 1900s a scientist named alfred
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wegener
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noticed that the coastlines of several
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continents look like they fit together
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similar to puzzle pieces he believed
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that around 200 million years ago
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all of the continents were joined
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together in one large supercontinent
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that he called pangaea
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which is greek for all the earth over
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the course of millions of years pangaea
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broke apart into the continents we know
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today
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a process he called continental drift
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although his theories explained quite a
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bit about the origins of the earth
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scientists at the time weren't exactly
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impressed and initially dismissed his
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ideas
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fast forward to today and wegener's
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concepts are the basis of the modern day
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plate tectonics theory
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so what exactly is the plate tectonics
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theory well it states that the earth's
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outer mechanical layer
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the lithosphere is divided into large
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continent-sized plates that are
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constantly
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moving how fast are they moving pretty
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slowly actually
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around one to two inches per year which
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is why it was so hard for scientists a
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hundred years ago to wrap their heads
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around the idea
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so what changed their minds well a few
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natural discoveries in the 20th century
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made wegener's theories seem more
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plausible for example
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in the 1950s and 60s scientists
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discovered heated magma rising up
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through cracks in the oceanic crust
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called dikes which is how new rock or
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new land is created
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this realization that our planet is
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always reforming and reshaping itself
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from beneath
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lint credibility to wegener's theories
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point wegener
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not long after scientists started
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plotting the location of earthquakes and
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volcanoes around the world and observed
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that the location of those events
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followed a similar pattern to the
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outline of the plates wegener proposed
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and then there was the fossil evidence
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fossils of tropical animal and plant
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species have been found in africa and
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other places on earth that are less than
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tropical
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one plausible explanation was that
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africa was once part of a larger
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continent that was home to these
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tropical plants and animals
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remember pangaea so in light of all this
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evidence the scientific community
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started thinking maybe this wegener
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fellow was
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actually onto something which led to the
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development of our modern day plate
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tectonics theory
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so now that we know the history let's
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dive a bit deeper into how it all works
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according to the plate tectonics theory
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these massive lithospheric plates are
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all moving in different ways
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and how they interact with one another
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can have a huge impact on the earth
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where these plates meet are called
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boundaries and there are three kinds
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convergent boundaries occur when two
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plates are moving towards one another
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transform boundaries occur when two
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plates are sliding past one another
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and divergent boundaries occur when two
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plates are moving away from one another
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let's take a closer look at what happens
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at each of these boundaries
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as we said convergent boundaries are
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when plates are moving towards one
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another
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when the two plates collide a couple
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things can happen one of the plates will
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dive under the other plate
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it's usually the heavier denser crust
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that dives under the lighter crust
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this creates what is called a subduction
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zone and the deeper under the earth's
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surface that plate goes the more
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pressure it creates
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that pressure coupled with a high heat
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causes the crust to melt forming magma
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the magma presses up towards the surface
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and voila you've got a volcano
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an example of this is the ring of fire
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which is an active ring of volcanoes
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that encircles the pacific ocean
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the other thing that can happen when two
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plates press into one another at a
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convergent boundary
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is the rock above the boundary will be
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lifted up or folded
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in foreign mountains ever hear of the
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himalayas that's an example of two
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tectonic plates colliding
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and because the plates continue to move
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the peaks in this mountain range
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continue to grow
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for example mount everest which
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currently measures around 29 000 feet
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grows around an inch every year so as
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you could imagine it takes thousands of
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years for these mountains to form
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but although movement at convergent
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boundaries is usually gradual
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as pressure underground builds the
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impact above ground can be quick and
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violent
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fast movement or slipping of the land
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can result in earthquakes either above
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ground or under the ocean
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and convergent boundaries aren't the
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only type of boundary that can result in
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earthquakes
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at transform boundaries the earth's
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plates are sliding past each other in
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opposite directions
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which creates a crack or fault in the
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earth's crust
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as the plates try to move they rub
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against one another resulting in the
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building up of pressure
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if the plates are stuck for a long
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period of time the pressure will
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continue to build at the fault line
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until
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eventually it releases resulting in an
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earthquake the san andreas fault system
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is one of the largest transformed
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boundaries in the world
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which is why that area has more than its
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fair share of seismic activity
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that just leaves divergent boundaries
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like we mentioned
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before divergent boundaries occur when
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plates are moving away from one another
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either the space between the two plates
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widens and becomes a large crack or rift
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such as the east african great rift
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valley that runs from lebanon to
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mozambique
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or if the space between the two plates
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is under the ocean where the crust is
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thinner
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magma oozes up from the earth's mantle
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and fills the space
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this is called sea floor spreading the
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mid-atlantic ridge is an example of
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where seafloor spreading has occurred
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the crust under the ocean pulled apart
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allowing magma to fill the space between
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creating the tallest and longest
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mountain chain in the world
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so in review the plate tectonics theory
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is based on concepts first proposed by
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alfred wegener in the early 20th century
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it states that the earth's outer
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mechanical layer the lithosphere
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is divided into large continent-sized
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plates that are always moving
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leading many scientists to believe the
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continents we know today were once part
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of a supercontinent called pangea
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these plates move one to two inches per
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year where they meet
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is called boundaries and there are three
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different types convergent boundaries
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where plates collide
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transform boundaries where plates slide
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past one another
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and divergent boundaries where they move
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apart volcanoes and mountains form at
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convergent boundaries
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earthquakes and tsunamis can occur at
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both convergent and transform boundaries
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fissures and large cracks in the surface
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occur when two plates move apart at a
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divergent boundary
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and seafloor spreading occurs at
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divergent boundaries located at the
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ocean floor
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so that completes our lesson on plate
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tectonics as always feel free to use the
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scrubbing bar at the bottom
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to go back and revisit any portion of
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this lesson
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you
