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moves upward along the newly formed openings, often generating outpourings of lava
for weeks, months, or even years.
All magmas contain some water and other volatiles that are held in solution by
the immense pressure of the overlying rock. Volatiles tend to be most abundant near the
tops of magma reservoirs containing silica rich melts. When magma rises (or the rocks
confining the magma fail) a reduction in pressure occurs and the dissolved gases begin
to separate from the melt, forming tiny bubbles.
When fluid basaltic magmas erupt, the pressurized gases escape with relative
ease. At temperatures of 1000°C and low near-surface pressures, these gases can
quickly expand to occupy hundreds of times their original volumes. On some occasions,
these expanding gases propel incandescent lava hundreds of meters into the air,
producing lava fountains. Although spectacular, these fountains are mostly harmless
and not generally associated with major explosive events that cause great loss of life and
property. At the other extreme, highly viscous, rhyolitic magmas may produce
explosive clouds of hot ash and gases that evolve into buoyant plumes called eruption
columns that extend thousands of meters into the atmosphere. Because of the high
viscosity of silica-rich magma, a significant portion of the volatiles remain dissolved
until the magma reaches a shallow depth, where tiny bubbles begin to form and grow.
Bubbles grow by two processes, continued separation of gases from the melt and
expansion of bubbles as the confining pressure drops. Should the pressure of the
expanding magma body exceed the strength of the overlying rock, fracturing occurs. As
magma moves up the fractures, a further drop in confining pressure causes more gas
bubbles to form and grow.
When magma in the uppermost portion of the magma chamber is forcefully
ejected by the escaping gases, the confining pressure on the molten rock directly below
drops suddenly. Thus, rather than a single “bang,” volcanic eruptions are really a series
of explosions. This process might logically continue until the entire magma chamber is
emptied. However, this is generally not the case. It is typically only the magma in the
upper part of a magma chamber that has a sufficiently high gas content to trigger a
steam-and-ash explosion.

Task 3. Discuss the following question:
Why is a volcano fed by highly viscous magma likely to be a greater threat to
life and property than a volcano supplied with very fluid magma?

Task 4. Look at Figure 6.2. It is a schematic drawing showing the movement
of magma from its source in the upper asthenosphere through the continental
crust. During its ascent, mantle-derived basaltic magmas evolve through the
process of magmatic differentiation and by melting and incorporating continental
crust. Magmas that feed volcanoes in a continental setting tend to be silica-rich
(viscous) and have a high gas content. Look at the scheme and try to describe the
way volcanoes can erupt.

Individual work

Task 1. Read the text and translate it into Ukrainian (in written form).
Build up a list of key terms to the text.

Materials Extruded during an Eruption: lava
Volcanoes extrude lava, large volumes of gas, and pyroclastic materials (broken
rock, lava “bombs,” fine ash, and dust).

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