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Lava Flows. The vast majority of lava on Earth, more than 90 % of the total
volume, is basaltic in composition. Andesites and other lavas of intermediate
composition account for most of the rest, while rhyolitic (felsic) flows make up as little
as 1% of the total.
AA AND PAHOEHOE FLOWS. Two types of lava flows are known by their
Hawaiian names. The most common of these, aa (pronounced ah-ah) flows, have
surfaces of rough jagged blocks with dangerously sharp edges and spiny projections.
Crossing an aa flow can be a trying and miserable experience. By contrast, pahoehoe
(pronounced pah-hoy-hoy) flows exhibit smooth surfaces that often resemble the
twisted braids of ropes. Pahoehoe means “on which one can walk.” Aa and pahoehoe
lavas can erupt from the same vent. However, pahoehoe lavas form at higher
temperatures and are more fluid than aa flows. In addition, pahoehoe lavas can change
into aa lavas flow, although the reverse (aa to pahoehoe) does not occur.
LAVA TUBES. Hardened basaltic flows commonly contain cave-like tunnels
called lava tubes that were once conduits carrying lava from the volcanic vent to the
flow’s leading edge. These conduits develop in the interior of a flow where
temperatures remain high long after the surface hardens. Lava tubes are important
features because they serve as insulated pathways that facilitate the advance of lava
great distances from its source. Lava tubes are associated with volcanoes that emit fluid
basaltic lava and are found in most parts of the world.
BLOCK LAVAS. In contrast to fluid basaltic magmas, which can travel many
kilometers, andesitic and rhyolitic magmas tend to generate relatively short prominent
flows, a few hundred meters to a few kilometres long. Their upper surface consists
largely of vesicle-free, detached blocks, hence the name block lava. Although similar to
aa flows, these lavas consist of blocks with slightly curved, smooth surfaces, rather than
the rough, clinkery surfaces.
PILLOW LAVAS. Much of Earth’s volcanic output occurs along oceanic
ridges (divergent plate boundaries). When outpourings of lava occur on the ocean floor,
the flow’s outer skin quickly congeals. However, the lava is usually able to move
forward by breaking through the hardened surface. This process occurs over and over,
as molten basalt is extruded—like toothpaste from a tightly squeezed tube. The result is
a lava flow composed of numerous tube-like structures called pillow lavas, stacked one
atop the other. Pillow lavas are useful in the reconstruction of geologic history because
whenever they are observed, they indicate that the lava flow formed in an underwater
environment.

Task 2. Look at Figures 6.3; 6.4; 6.5. They show lava flows. Continue the
ideas:
1. A typical slow-moving, basaltic, … flow.
2. A typical fluid … lava.
3. Lava flows often develop a solid crust while the molten lava below continues to
advance in conduits called … .

Task 3. Describe pahoehoe and aa lava flows.

Test yourself

Task 1. Tell whether the sentences are true or false. Correct the false ones.
1. All eruptions are very explosive.
2. There are four primary factors that determine whether a volcano extrudes magma
violently or “gently”. They are: the magma’s composition, its temperature, the
amount of dissolved gases it contains and lava flows.

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