Lava Flows
Basaltic magma produces two characteristic types
of lava flows. Since most of the research on island vulcanism
has been performed in Hawaii, the names of these flows are hawaiian
- pahoehoe ("ropy") and aa ("hurt").
These two types of flows have distinctly different appearances,
although their chemical make-up may be quite similar.
Pahoehoe is smooth,
with a twisted, or ropy texture while aa is sharp and blocky
(it hurts to walk on it). During a pahoehoe flow the outer
skin of lava cools and becomes viscous. The underying lava, however,
is insulated and remains quite liquid. As it flows, it carries
the cooler skin along with it, causing it to crumple and fold
into twisted shapes. Early in the flow, when the lava is hotter,
these twists have the appearance of ropes, but later, as it cools
and becomes more viscous, the twists are shaped more like entrails.
Underneath the solidifying surface, the liquid lava continues
to flow, often draining out and leaving hollow cavities which
later collapse.
Aa lava in general
is more viscous. The outer skin tends to form a rubbly, sharp-edged
surface. As the flow moves along, the surface rubble at the edge
of the flow falls off of the front edge and is over-ridden. Thus,
a cross-section of an aa flow reveals a rubbly surface,
a solid inner layer of slowly-cooled magma, and finally a narrow
zone of over-ridden rubble. The differences between pahoehoe and
aa lavas are subtle, but one important aspect is the speed of
flow. Flows over steeper terrain move more rapidly and tend to
be of the aa type while slower flows tend to form pahoehoe.
 .
In Galapagos aa flows tend to be more abundant
than pahoehoe flows, but they are rare at visitor sites
- for obvious reasons. One well known aa flow can be seen
on Fernandina. The best place to see a
pahoehoe flow is on Santiago. Just
before the turn of the century, massive lava flows covered the
southern part of the island, extending from Sullivan Bay on the
east side, around to James Bay on the west. Visitors who walk
on the Sullivan Bay flow are often profoundly affected by the
seemingly endless barrenness of the lava. At Punta Moreno, on
the west side of Isabela, visitors have
the opportunity to walk out on a lava flow that is about 1,000
years old. It is interesting to compare the two lava flows. At
Pta Moreno, the lava has been oxidized from shiny black to dull
brown, and it much more broken. Stepping over the lava fragments
results in loud clinking sounds (these rocks are often called
"clinkers") and there is a much greater variety of plants
growing there than can be found at Sullivan bay.
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