Geology Reference
In-Depth Information
The evolution of magmas occurs deep in the earth's crust, under conditions
of high temperatures. After a magma has erupted (becoming lava), its composi-
tion doesn't change, but the size of the crystals may be different depending on
how quickly it solidifies into rock.
Which minerals are melted into a magma also determines how fluidly the magma flows.
This characteristic of magma is called its
viscosity.
Viscosity is how strongly a fluid res-
ists flowing. Magma with high amounts of silica is more viscous; it flows more slowly
than magma with low amounts of silica. For this reason, extrusive rocks (formed by
mafic magmas that have flowed up to the surface) are more often basalt, while intrusive
rocks (created by felsic magmas that resist flow and stay trapped underground) have a
granite composition.
Magmatic differentiation, mixing, and partial melting explain how so many dif-
ferent types of igneous rocks exist and may help you understand that igneous
rocks cannot always be neatly categorized as one type or another; sometimes
they fall somewhere in between two categories.
A basalt by any other name: Pahoehoe and A'a
The content of a lava — which elements are in the liquid and whether there are dissolved gases —
gives the lava certain characteristics, such as how fluidly it flows. Hawaii's volcanoes called Mauna
Loa and Mauna Kea provide a safe location to observe basalt magma flows up close. From observa-
tions of Mauna Loa and Mauna Kea,
volcanologists
(scientists who study volcanoes) distinguish two
very different types of basalt magma flow.
Pahoehoe
(pah-hoi-hoi) is a wrinkled, ropy-looking lava (and eventually basalt rock) with a smooth sur-
face; it's shown in the picture below. As the lava with low amounts of silica and very little dissolved
gasses flows, the outer surface, exposed to the air, cools and solidifies. However, the inner lava is still
flowing, which creates the wrinkles and twists in the cooled crust of the flow.
Another type of lava produced from Hawaiian volcanoes is a'a (ah-ah) lava.
A'a
lava is also composed
of basalt but is very different from pahoehoe. Instead of fluidly flowing, a'a is blocky, rough, and jagged.
As it flows forward it looks, and sounds, like an advancing pile of basalt rubble. A'a lava usually has a
high amount of gas dissolved in it, and as it cools the gas escapes, leaving sharp spiny projections of
basalt.
