Have you ever noticed that South America and Africa fit together like
puzzle pieces?
Why is that so?
Is it mere coincidence?
Alfred Wegener asked himself this very question over 100 years ago.
To answer it, he combined two different geoscientific observations on
the theory of plate tectonics.
Firstly, that Mesozourus fossils as well as those of other animals and
plants were found on both continents.
And also the fact that there were found in similar geological rock
formations made of clay and chalk.
Secondly, that the continental outlines of Africa and South America
fit together like puzzle pieces.
From these observations, Wegener concluded that the surface of the
earth consists of individual plates and that these plates are moving
constantly.
This means that Africa and South America used to be part of a single
contiguous landmass.
Today we know that these lithospheric plates are swimming on a more
liquid layer under them, the asthenosphere.
Hello and welcome.
In this video, I will be showing you the three typical types of plate
boundaries.
You will learn how the global distribution of earthquakes and
volcanoes makes it possible to distinguish various plate boundaries
and the direction in which lithospheric plates are moving at these
boundaries.
The lithosphere is a mostly solid layer approximately 100 km thick.
It consists of the earth's crust as well as the uppermost section of
the earth's mantle.
After Wegener presented his idea that the lithospheric plates were
moving about on the earth's surface, new measurement methods over the
following years yielded additional observations.
These underscore and refine the theory of plate tectonics.
It is a good example of how the combination of various observations
can lead to a better understanding of what goes on beneath the earth's
surface.
Two such observations are the occurrence of volcanoes and earthquakes
on earth.
Both make it possible for us to better understand the processes going
on inside the earth.
Shown here is a global earthquake distribution map.
Large circles indicate strong earthquakes and small ones indicate
weaker earthquakes.
From the map, it is clear that earthquakes are clustered along defined
lines.
Furthermore, the distribution of volcanoes also follows a specific
pattern.
Large symbols on the map indicate clusters of volcanoes.
However, if we superimpose both distributions, we see that there are
differences.
What differences are there in the global distribution of seismicity
and volcanoes?
For example, while Japan experiences both volcanic and seismic
activity, the foot of the Himalayas and California experience
earthquakes, but there are no volcanoes there.
Hence, not all plate boundaries appear to be the same.
But how do they differ from each other?
We primarily categorize plate boundaries according to the direction in
which adjacent plates move towards each other.
We distinguish between a total of three types of plate boundaries.
Firstly, transform faults, where two plates slide past each other
horizontally.
Secondly, convergent boundaries at which two plates move towards each
other.
Thirdly, divergent boundaries, where two plates are pushed apart from
each other.
Where do seismicity or volcanism occur at plate boundaries?
Let me demonstrate this to you via five examples from locations across
the globe.
The first place we are looking at is California, where earthquakes
regularly occur due to a transform fault between the North American
plate and the Pacific plate.
What you see here is a fence that has been shifted horizontally by
about one meter after the 1906 San Francisco earthquake, which
occurred along the San Andreas fault.
Generally, volcanism does not occur along transform plate boundaries.
We distinguish between two different types of convergent boundaries.
In the first, an oceanic plate moves beneath another plate.
An example of this can be seen in Japan.
Here, the older, denser Pacific plate is pushed under the younger
Eurasian plate, which is visible in this section of stone near Kobe.
A catastrophic earthquake occurred here in 1995.
When such a subduction occurs, oceanic lithosphere is pushed under
another oceanic or continental plate.
The result is volcanism, which is triggered by fluids in the
subducting plate.
In the second case, a continental plate collides with another
continental plate.
This results in the formation of a mountain range.
A prime example of this are the Himalayas, which continue to increase
in height due to the movement of the Indian subcontinent.
Volcanism does not occur with this type of plate boundary.
Divergence zones may occur in oceans or on continents.
One example of the latter is the Great Rift Valley in eastern Africa.
In this case, the plates are drifting apart, that means Africa is
literally breaking apart, albeit extremely slowly.
Here you see a photo of the edge of a rift that is moving away from
the center of the rift.
However, divergence can also occur in the oceans at what are called
mid -ocean ridges.
For example, in the Pacific at the boundary between the Pacific plate
and the Nazca plate.
Here, material from the earth's interior is brought to the surface and
new seafloor is formed, which is then pushed away to both sides.
Indicators of such zones are black smokers, hydrothermal vents from
which hot water flows.
They are a weak form of volcanism.
Up to this point, we have spoken about seismicity and volcanism at
plate boundaries.
However, earthquakes and volcanism can also occur within lithospheric
plates.
The volcanic islands of Hawaii, for example, are fed by such a
hotspot, which more or less burns a hole in the overlying plate.
Today, GPS technology confirms the concept of plate tectonics.
GPS measurements serve not only to locate a receiver, but can also be
used to determine the distance between two measurement stations.
Earthquakes can provide information about the direction of underground
movements.
With the help of GPS, it is also possible to accurately measure the
speed of the movements.
These measurements can detect plate movements on a scale of several
centimetres per year, which is approximately the rate at which
fingernails grow.
Let me summarise once again.
Earthquakes occur at all plate boundaries.
Volcanism, on the other hand, typically occurs at convergent
lithospheres or as a hotspot in the interior of the plates.
Volcanism also occurs in a weaker form during the creation of new
seafloor at divergent mid-ocean ridges.
In addition, earthquakes and volcanism may also occur together at
other plate boundaries.
For example, when plate boundaries change their type over the course
of millions of years or when hotspots occur at plate boundaries.
Finally, let's come back to Alfred Wegener.
You have seen that combining various geoscientific observations can
lead to a deeper understanding of the processes going on in the earth.
Just like how Wegener used such observations to formulate a new
hypothesis over a century ago, interdisciplinary work being done today
can also bring about scientific progress.