Theory of Plate Tectonics Video Lecture - Social Studies (SST) Class 9

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