Learn More - plATE TECTONICS"THE EARTHS PUZZLE"
Features found at plate boundaries
Convergent Plate Boundaries.
In plate tectonics, a convergent boundary, also known as adestructive plate
boundary (because of subduction), is an actively deforming region where two (or more) tectonic
plates or fragments of the lithosphere move toward one another and collide. As a result of
pressure, friction, and plate material melting in the mantle, earthquakesand volcanoes are
common near convergent boundaries. When two plates move towards one another, they form
either a subduction zone or a continental collision. This depends on the nature of the plates
involved. In a subduction zone, the subducting plate, which is normally a plate with oceanic
crust, moves beneath the other plate, which can be made of either oceanic or continental crust.
During collisions between two continental plates, large mountain ranges, such as
the Himalayasare formed. It must be noted that there are three types of convergent plate
These are long, curved chain of oceanic islands associated with intense volcanic and
seismic activity and orogenic (mountain-building) processes. Prime examples of this form of
geologic feature include theAleutian-Alaska Arc and the Kuril-Kamchatka Arc.
Most island arcs consist of two parallel, arcuate rows of islands. The inner row of such a double
arc is composed of a string of explosive volcanoes, while the outer row is made up of nonvolcanic islands. In the case of single arcs, many of the constituent islands are volcanically
Island arcs are formed where two lithospheric plates (enormous rigid slabs that constitute
segments of the Earth’s surface) converge. Upon colliding, one of the plates—that bearing
heavy, oceanic crust—buckles downward and is forced into the partially molten lower mantle
beneath the second plate with lighter, continental crust. An island arc is built up from the surface
of the overriding plate by the extrusion of basalts and andesites. The basalts are thought to be
derived from the semimolten mantle, whereas the andesites are probably generated by the partial
melting of the descending plate and the sediments that have accumulated on its surface.
An ocean trench is a long, deep depression in the ocean floor, similar to deep chasms on the
Earth’s dry land. Some trenches are near continental shelves. Others are found near chains
of volcanic islands, often calledvolcanic arcs. Trenches are formed as a result of plate tectonics,
or the movement of the Earth’s crust. Tectonic plates slip underneath each other in a process
known assubduction. When the leading edge of a heavy plate meets the edge of a lighter plate,
the heavier plate bends downward. This place where the heavier plate melts (subducts) beneath
the lighter one is called the subduction zone. In the ocean, subduction zones can create huge,
Ocean trenches can be formed by subduction betweencontinental crust and oceanic crust.
Continental crust is always lighter. The long series of Peru-Chile Trenches off the west coast of
South America is formed by the oceanic crust of the Nazca plate subducting beneath the
continental crust of the South American plate.
Ocean trenches can also be formed when two plates carrying oceanic crust meet. These are more
rare. The Mariana Trench, in the South Pacific Ocean, is formed as the massive Pacific plate
subducts beneath the Philippine plate.
Fold mountains are mountain ranges that are formed when two of the tectonic plates that make
up the Earth'scrust push together at their border. The extreme pressure forces the edges of the
plates upwards into a series of folds.
Fold mountains are created through a process called "orogeny." An orogenic event takes millions
of years to create a mountain range because tectonic plates move only centimeters every year.
The Earth has two different types of crust: continental crust and oceanic crust. Orogenic events
can occur on both types of crust. The Himalaya Mountains are still growing as the Indian
continental plate folds over the Eurasian continental plate. Orogeny can also involve oceanic
plates. Beneath the microcontinent of Zealandia, the Pacific plate is being folded over the
Australian plate. The result is New Zealand's Southern Alps.
Most people think of the rugged, soaring heights of the Himalayas, Andes, and Alps when they
think of fold mountains. But some of the Earth's smaller mountain ranges were once soaring
peaks, too. The Appalachian Mountains started forming 480 million years ago when the North
American and Eurasian continental plates collided. The Appalachians were once taller than the
Himalayas. The range stretches from Newfoundland in southeastern Canada down through the
eastern United States to central Alabama. However, erosion has taken its toll on the
Appalachians. Today, some of its higher peaks are less than a third of the height of Everest.
Fold mountains are the most common type of mountain on Earth. Other types of mountains
are volcanic mountains, erosional mountains, and fault-block mountains. Volcanoes create
volcanic mountains. Erosional mountains are created as wind and water wear away soft portions
of land and leave rocky hills. Fault-block mountains are created where parts of continental crust
Divergent Plates- Oceanic
When a divergent boundary occurs beneath oceanic lithosphere, the rising convection current
below lifts the lithosphere producing a mid-ocean ridge. Extensional forces stretch the
lithosphere and produce a deep fissure. When the fissure opens, pressure is reduced on the superheated mantle material below. It responds by melting and the new magma flows into the fissure.
The magma then solidifies and the process repeats itself.
The Mid-Atlantic Ridge is a classic example of this type of plate boundary. The Ridge is a high
area compared to the surrounding seafloor because of the lift from the convection current below.
(A frequent misconception is that the Ridge is a build-up of volcanic materials, however, the
magma that fills the fissure does not flood extensively over the ocean floor and stack up to form
a topographic high. Instead, it fills the fissure and solidifies. When the next eruption occurs, the
fissure most likely develops down the center of the cooling magma plug with half of the newly
solidified material being attached to the end of each plate.
Divergent Plate Boundary – Continental
When a divergent boundary occurs beneath a thick continental plate, the pull-apart is not
vigorous enough to create a clean, single break through the thick plate material. Here the thick
continental plate is arched upwards from the convection current's lift, pulled thin by extensional
forces,and fractured into a rift-shaped structure. As the two plates pull apart, normal faults
develop on both sides of the rift and the central blocks slide downwards. Earthquakes occur as a
result of this fracturing and movement. Early in the rift-forming process, streams and rivers will
flow into the sinking rift valley to form a long linear lake. As the rift grows deeper it might drop
below sea level allowing ocean waters to flow in. This will produce a narrow, shallow sea within
the rift. This rift can then grow deeper and wider. If rifting continues a new ocean basin could be