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is protuberances resembling large bed pillows stacked one atop the
other, hence the name pillow basalts.
The lowest unit of the ocean crust develops from
crystallization within the central magma chamber itself. The first
minerals to crystallize are olivine, pyroxene, and occasionally
chromite (chromium oxide), which settle through the magma to
form a layered zone near the floor of the reservoir. The remaining
melt tends to cool along the walls of the chamber to form massive
amounts of coarse-grained gabbro. This portion accounts for up to
5 of the 7 km of ocean crust thickness.
Although molten rock rises continuously from the
asthenosphere toward the surface, seafloor spreading occurs in
pulselike bursts. As the melt begins to accumulate in the lens
shaped reservoirs, it is blocked from continuing upward by the stiff
overlying rocks. As the amount of melt entering the magma
reservoirs increases, pressure rises. Periodically, the pressure
exceeds the strength of the overlying rocks, which fracture and
initiate a short episode of seafloor spreading.

Task 3. Answer the following questions, using the
vocabulary from Task 1.
0. How do geologists know that thickness of ocean basin
averages only about 7 km?
0. What kind of rocks is the entire ocean basin composed of?
0. Briefly describe the four layers of the ocean crust.
0. How do the rocks above lens-shaped magma chambers
ascend into the young oceanic crust above?
0. How does the sheeted dike complex form?
0. What does the lowest unit of the ocean crust develop from?

Task 4. Find English equivalents for the following (see
the text).
Увесь басейн океану; сейсмічні зондування; складатись
(утворюватись) з; формувати літосферну мантію; зразки

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