Lecture : The Rock cycle (2)
Andesitic and Rhyolitic Lava Flows Because of its greater viscosity, andesitic lava cannot flow
as easily as basaltic lava. When erupted, andesitic lava forms a mound above the vent. This
mound advances slowly down the volcano’s flank at only 1–5 m/d, becoming a lumpy flow with a
bulbous snout. Typically, andesitic flows are less than a few kilometers long.
Because the lava moves so slowly, the outside of the flow has time to solidify, so as it moves,
the surface breaks up into angular blocks, and the whole flow looks like a jumble of rubble
known as blocky lava. On steep slopes, the blocks may tumble downhill, so the flow may evolve
into a landslide of blocks.
Rhyolitic lava is the most viscous of all lavas because it has the highest silica concentration and
the coolest temperature. Therefore, it tends to accumulate above the vent in a bulbous mass
called a lava dome (see Fig. 9.3b–c). Sometimes rhyolitic lava freezes while still in the vent and
then pushes upward to form a column-like lava spire or lava spine, which may rise up to 100 m
above the vent. Rhyolitic flows, where they do form, rarely extend for more than 1–2 km from
the vent and have broken and blocky surfaces.
Volcaniclastic Deposits
On a mild day in February 1943, as Dionisio Pulido prepared to sow the fertile soil of his field
330 km west of Mexico City, an earthquake jolted the ground, as it had dozens of times in the
previous days. But this time, to Dionisio’s amazement, the surface of his field visibly bulged
upward by a few meters and then cracked.
Ash and sulfurous fumes filled the air, and the farmer fled. When he returned the following
morning, his rich land lay buried beneath a 40-m-high mound of gray cinders. Dionisio had
witnessed the birth of a new volcano named Paricutín. During the next several months,
Paricutín erupted continuously, at times blasting clots of lava into the sky like fireworks. By the
following year, it had become a steep-sided cone over 300 m high. Nine years later, when the
volcano stopped erupting, its lava and debris covered 25 km2, and Dionisio’s farm and those of
his neighbors were gone.
This description of Paricutín’s eruption, and that of Vesuvius at the beginning of this chapter,
emphasize that volcanoes produce large quantities of fragmental material.
Geologists use the term pyroclastic debris (from the Greek pyro, meaning fire) for fragmented
igneous material forcefully ejected from a volcano. Pyroclastic debris includes both material
solidified from clots or droplets of lava that freeze, either in the air or after they fall, and clasts
formed by the breakup of already solid rock during an eruption.
It accumulates on or around the volcano to form a layer of tephra, which, if it becomes solid and
coherent, becomes a layer of pyroclastic rock. In some cases, pyroclastic debris mixes with
water (from rain, snow, or ice) and flows in a muddy slurry down the side of the volcano. Where
there’s enough water, streams may transport pyroclastic debris away from the volcano,
eventually depositing it elsewhere in sorted sedimentary layers.
We refer to any accumulation of fragmental volcanic material—pyroclastic debris, or the
deposits of slurries and streams—as a volcaniclastic deposit. Let’s now look more closely at the
various components of volcaniclastic deposits. You’ll see that different types form in association
with different kinds of eruptions.
Pyroclastic Debris from Basaltic Eruptions Basaltic magma rising in a volcano may contain
dissolved volatiles, such as water. As such magma approaches the surface, the volatiles form
gas bubbles. In basaltic magma, the bubbles can rise faster than the magma itself, and when
the bubbles reach the surface, they burst and eject clots and drops of molten lava upward to
form dramatic fountains. To picture this process, think of the droplets that spray from a
just-opened bottle of soda.
Geologists recognize several different types of fragments formed from frozen clots or drops of
lava. Pea- to golf ball–sized fragments of glassy lava and scoria are known as cinders.
Geologists consider cinders to be a type of lapilli (singular lapillus, from the Latin word for little
stones). Lapilli accumulate in layers.
Occasionally, flying droplets of lava will develop long, thin tails as they move through the air.
These tails freeze into hair-like filaments of glass known as Pelé’s hair, named after the
Hawaiian goddess of volcanoes. The droplets themselves freeze into tiny, streamlined, glassy
beads known as Pelé’s tears.
Basaltic eruptions also produce apple- to refrigerator-sized fragments, called blocks, which
consist of already-solid volcanic rock broken up during the eruption. Blocks tend to be angular
and chunky. Lumps of erupted igneous rock that have streamlined, polished surfaces are known
as bombs. Bombs form when lava has already solidified, but remains hot and soft as it squirts
out of the vent and then flies through the air.
Pyroclastic Debris from Andesitic and Rhyolitic Eruptions Andesitic and rhyolitic lavas are more
viscous than basalt and tend to be more gas-rich. Eruptions of these lavas also tend to be
explosive. Volcanic explosions can produce immense quantities of pyroclastic debris, much
more than can come from a basaltic volcano.
Debris ejected from explosive eruptions includes ash, which consists of glassy particles less
than 2 mm in diameter formed when frothy lava or recently formed pumice breaks up explosively
during an eruption, or when pre-existing volcanic rock gets pulverized by the force of an
explosion; pumice lapilli, which consists of angular pumice fragments; and accretionary lapilli,
which consists of snowball-like lumps of ash formed when ash mixes with water in the air and
then sticks together to form small balls.
Ash, or ash mixed with lapilli, becomes a type of pyroclastic rock called tuff when buried and
lithified. In some cases, the grains bond during deposition because they are so hot that they
weld together. But more commonly, the coherence of tuffs occurs when the grains are cemented
together either by minerals precipitated from groundwater or by minerals that grow in the ash as
it reacts with groundwater.
Other Volcaniclastic Deposits
- The nature and origin of fragmental material produced during and after eruptions
continue to be the subject of active research because deposits of this material prove to
be difficult to interpret. As we’ve noted, geologists use the term volcaniclastic deposit for
any material that consists of volcanic igneous fragments, and they recognize three
categories:
Pyroclastic deposits, as we have just seen, consist of fragments ejected during an eruption, and
they accumulate directly from the clouds of debris ejected into the sky or sent in avalanches
down the flank of the volcano. The fragments in such deposits have not moved subsequent to
their original deposition.
Volcanic-sedimentary deposits consist of volcanic material (lava and pyroclastic debris) that
later moved downslope and was redeposited elsewhere subsequent to accumulating after an
eruption. Some of this material tumbles in landslides, breaking up to varying degrees as it
moves. If volcanoes are covered with snow and ice or are drenched with rain, water mixes with
debris to form a volcanic debris flow, which moves downslope like wet concrete.
Very wet, ash-rich debris flows downslope in a fast-moving slurry called a lahar, which can reach
speeds of 50 km per hour. Lahars tend to follow river channels and may travel for tens of
kilometers away from the volcano. When debris flows and lahars stop moving, they yield a layer
consisting of volcanic blocks suspended in ashy mud. Rivers may eventually sort and transport
some of this volcanic sediment. Where this material accumulates, perhaps far downstream, it
forms deposits of volcanic sandstone and/or volcanic conglomerate.
Fragmental lava deposits consist of debris produced when lava breaks up into angular clasts
while flowing, without ever being ejected into the air. As we’ve seen, fragmentation (also called
brecciation) happens when the inside of a lava flow continues to move after its surface has
frozen and the crust breaks up due to the movement. But fragmentation may also happen when
lava freezes very quickly and shatters upon erupting into water or ice. The resulting material,
hyaloclastite, consists of glassy fragments embedded in ash that has reacted with hot water.
Volcanic Gases
Most magma contains dissolved volatiles, including water (H2O), carbon dioxide (CO2), sulfur
dioxide (SO2), and hydrogen sulfide (H2S). Generally, felsic lavas can contain more volatiles
than can mafic lavas—in fact, up to 9% by weight of a felsic magma consists of volatiles. As
we’ve seen, these dissolved gases come out of solution when the magma approaches the
Earth’s surface. This process happens for two reasons. First, the ability of a liquid to hold
dissolved gas decreases as the pressure acting on the liquid decreases.
You see this phenomenon when you pop off the top of a carbonated beverage—the beverage
was injected with CO2 under pressure when it was bottled, and because popping the top
