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plates (and therefore most deformation) occur along their boundaries. Plates are bounded by
three distinct types of boundaries, which are differentiated by the type of relative movement they
exhibit.
Divergent boundaries—where plates move apart, resulting in upwelling of material from
the mantle to create new seafloor. Plate spreading (divergence) occurs mainly along the oceanic
ridge. As plates pull apart, the fractures created are immediately filled with molten rock that
wells up from the asthenosphere below. This hot material slowly cools to become solid rock,
producing new slivers of seafloor. This happens again and again over millions of years, adding
thousands of square kilometres of new seafloor. This mechanism has created the floor of the
Atlantic Ocean during the past 160 million years and is appropriately called seafloor spreading.
Convergent boundaries—where plates move together, resulting in the subduction
(consumption) of oceanic lithosphere into the mantle. Convergence can also result in the
collision of two continental margins to create a major mountain system. As two plates slowly
converge, the leading edge of one slab is bent downward, allowing it to slide beneath the other.
The surface expression produced by the descending plate is a deep-ocean trench. Plate margins
where oceanic crust is being consumed are called subduction zones. Here, as the subducted plate
moves downward, it enters a high-temperature, high-pressure environment. Some subducted
materials, as well as more voluminous amounts of the asthenosphere located above the
subducting slab, melt and migrate upward into the overriding plate. Occasionally this molten
rock may reach the surface, where it gives rise to explosive volcanic eruptions. However, much
of this molten rock never reaches the surface; rather, it solidifies at depth and acts to thicken the
crust.
Transform fault boundaries—where plates grind past each other without the production
or destruction of lithosphere. These faults form in the direction of plate movement and were first
discovered in association with offsets in oceanic ridges.

Task 3. Look at Figure 5.1. If you examine it, you can see that each large plate is
bounded by a combination of these boundaries. Movement along one boundary requires
that adjustments be made at the others. Tell which boundaries is the Eurasian plate
bounded by? The South American plate?

Task 4. Look at Figure 5.2. Convergent boundaries occur where two plates move
together, as along the western margin of South America. Divergent boundaries are located
where adjacent plates move away from one another. Look at the figure and try to name the
example of such a boundary.

Task 5. Fill in the gaps with one of the suitable words below. Pronounce the
following words correctly: the Aleutian, Mariana, and Tonga islands. Translate the text into
Ukrainian.

Volcanoes, plate, volcanic island arc, convergence, subduction.
The simplest type of … occurs where one oceanic … is thrust beneath another. At such
sites … results in the production of magma in a manner similar to that in the Andes, except …
grow from the floor of the ocean rather than on a continent. If this activity is sustained, it will
eventually build a chain of volcanic structures that emerge from the sea as a …. Most volcanic
island arcs are found in the Pacific Ocean, as exemplified by the Aleutian, Mariana, and Tonga
islands.

Task 6. Name the type of plate boundary with which each of the following is
associated: deep-ocean trench, seafloor spreading, subduction, oceanic ridge.

Task 7. Discuss the following:
1. Divergent boundaries.