When Plates Collide

Transcript

1. Welcome back to our Science Class! Today we will be discussing what happens when WHEN
PLATES COLLIDE.

2. You have learned that convergent plate boundary is a type of boundary wherein plates move
toward each other. This happens because these plates are pushed by other tectonic plates from
the other parts of world, which are diverging or moving away from each other.

Converging plates of different crusts results to different processes and events.

This video will teach you the processes involved, landforms to emerge and geologic events to
occur when different types of crustal plates move toward each other.

3. Before we proceed to our discussion, let us recall first some terminologies that will be
mentioned in this video.
Oceanic Crust – thinner but denser crust found under the ocean.
Continental Crust – thick but less dense in comparison with the oceanic crust.
Lithosphere – the outer solid part of the Earth which includes the crust and the upper mantle.
This is said to be divided into several pieces we refer to as “plates” in plate tectonics. This vary in
thickness and its thickness is relative to its age. Older lithosphere is thicker.
Asthenosphere – soft, weak layer of the upper mantle that lies beneath the lithosphere. It’s high
viscosity and malleability facilitates the floating and movement of lithospheric plates.
Mantle – Earth’s thickest layer that lies between the solid crust and the liquid, metallic outer
core.

Subduction – a process in which an oceanic crustal plate descends beneath another plate after
collision due to the difference in their densities.
Magma – molten rocks formed when oceanic crustal plate thrusts beneath another plate. This
plate, which carries water with it, will melt due to high temperature of the mantle.

Seismicity – the occurrence or frequency and distribution of earthquakes in a particular

geographical location
Tectonism – the process of deformation in the earth’s crust that produces different geologic
features such as ocean basins, mountains, plateau, rift and ridges
4. First type of convergence that we will be discussing is the Oceanic- Oceanic Convergence
wherein two oceanic crustal plates collide. Both crusts are dense yet only one of these will
thrust beneath the other because of gravity. This process is called subduction. The region on
which the slab of lithosphere descends back to the mantle is called subduction zone. The leading
portion of cold oceanic crustal plate will eventually melt as it approaches the hot mantle. Basalt-
rich magma produced, is less dense causing it to rise and find its way to the surface of the
overriding plate. Volcanic eruption occurs. After millions of years of continuous volcanic
activities, volcanic island arc is formed. Volcanic island arc is a chain of active volcanoes. Parallel
to this arc is a very long depression on the ocean floor called trench. Trenches, which are
considered to be the deepest part of the Earth, are proofs that lithospheric plates plunges
underneath. Another proof for this process is the daily seismic activities recorded by the
seismographs. This very slow movement of lithospheric plates along the subduction zone
creates active fault planes. Epicenters, magnitudes and depths of Earthquakes occuring on these
zones are recorded.

5. This figure shows the record of seismicity in Japan from 1990 to 2006. The colored dots refer to
the location of the epicenters of the past earthquakes. The different sizes of these dots pertain
to the magnitude. Meanwhile the different colors represent its depth. It is important to note
that in the east portion of Japan, dots are colored orange showing that the earthquakes here
were shallow with up to 33km depth. But as you move to the west, epicenter dots changed in
color to green, blue, violet and red, showing that earthquakes were constantly getting deeper
and deeper up to the depth of 800km.

6. We can therefore conclude that the Philippine Sea Plate which lies in the south of Japan and the
Pacific Plate in the east are both subducting beneath the Eurasian Plate. This subduction led to
the formation of deep trenches marking the boundaries of these plates. Parallel to these
trenches is a volcanic island arc named Japan. One of it well-known and active volcanoes is the
Mount Fuji.

7. Another best example of a volcanic island arc formed by an oceanic-oceanic convergence is the
Philippine Archipelago. Although Philippines is not entirely a product of subduction process,
large portion of it is formed due to the subduction of Philippine Sea Plate to Sundan Plate and
the oceanic crust of Eurasian Plate to the Philippine Sea Plate. This tectonism was the cause of
the formation of our active volcanoes, presence of the left and right trenches and occurrence of
earthquakes.

8. The next type of converging plate boundary is characterized by oceanic-continental

convergence. In here, an oceanic crustal plate collides to a continental plate. Recall that an
oceanic crust is thinner yet denser than a continental crust. Because of this, gravitational force
will cause the oceanic plate to thrust beneath the continental plate in the process called
subduction. As the oceanic plate plunges back to the mantle, it carries water with it. This
amplifies the melting of the subducting plate forming magma. These silica-rich molten rocks will
rise and will be expelled through vents. Continuous eruption will lead to the formation of
continental arc. Continental volcanic arc is a chain of active volcanoes formed in the continents.
 This formation could also rise to great heights and becomes mountain ranges. Parallel to this arc
is a marginal trench. Trenches mark the subduction zone that is associated with large
earthquakes.

9. This figure depicts the seismicity in the western side of South America from 1975 to 1995. Note
that along the continental margin, the record of earthquake epicenters are marked by orange
color. This indicates that earthquakes here were shallow with the depth of 33km. As we move
eastward, records of epicenters changed colors signifying that earthquakes get deeper from 150
km to 300km deep.

10. These seismic data verify that the oceanic Nazca Plate is subducting underneath the continental
South American Plate forming the longest mountain ranges in the world—the Andes Mountains.
Mountain ranges here are characterized by continental volcanic arc.

11. The last type of converging plates is the continental-continental convergence. This happens
when two continental crusts collide toward each other. Since both of these lithospheric plates
are thick and less dense, none of these subducts. Compressive forces caused these two plates to
buckle and be pushed upward or sideways. Buckling of a rock layer is called folding. This causes
the formation of great mountain ranges and mountain belts. As the continental plates continue
to fold, crusts are destroyed and faulting takes place. This is proven by the occurrence of the
earthquakes along this convergent boundary.

12. This figure displays the seismicity of Central Asia from 1977-1997. It is observable that most of
the recorded earthquake epicenters are colored red which indicates that these earthquakes’ foci
lie within the depth of 0 to 33km. This could only mean that converging continental plates
causes the crusts to be folded upward.

13. Indian Plate and Eurasian plate are both continental crustal plates. As Indian Plate collides to
Eurasian plate, the crusts were folded and crushed to form the Himalayas. It is the tallest
mountain ranges in the world and the Mount Everest has the highest peak.

14. In Summary, Convergent Plate Boundary manifests different geologic processes and formations
depending on the type of crustal plates involved. This boundary is said to be a Destructive
boundary because crust is destroyed when plates collide.