Igneous Rocks and Intrusive Igneous Activity - The Changing Earth: Exploring Geology and Evolution - page 86

locations, although when it reaches the surface, it tends to lose
some of its gas content because of decreased pressure. Any
magma is less dense than the rock from which it formed, so it
tends to rise toward the surface, but much of it solidifi es deep
underground, thereby accounting for the origin of plutons.
The magma that does reach the surface issues forth as streams
of lava, or lava fl ows , and some of it is forcefully ejected into
the atmosphere as particles known as pyroclastic materials
(from the Greek pyro , “fi re,” and klastos , “broken”).
All igneous rocks derive ultimately from magma; how-
ever, two separate processes account for them. They form
when (1) magma or lava cools and crystallizes to form
aggregates of minerals, or (2) pyroclastic materials are con-
solidated. Those igneous rocks derived from lava fl ows and
pyroclastic materials, both of which are extruded onto the
surface, are known as volcanic rocks or extrusive igneous
rocks . Plutonic rocks or intrusive igneous rocks , on the
other hand, are the ones that formed when magma cooled
below the surface, that is, from magma intruded into the
crust.

The Most Common Types of Magmas
and Their Characteristics

TABLE 4.1

Sodium, Calcium,

Silica Potassium, Iron, and

Type of Magma Content (%) and Aluminum Magnesium

Ultramafi c <45 Increase

Mafi c 45-52

Intermediate 53-65

Felsic >65 Increase

Most lava temperatures are taken at volcanoes that show
little or no explosive activity, so our best information comes
from mafi c lava fl ows such as those in Hawaii ( Figure 4.2).

In contrast, eruptions of felsic lava are not as common, and
the volcanoes that these fl ows issue from tend to be explosive
and thus cannot be approached safely. Nevertheless, the tem-
peratures of some bulbous masses of felsic lava in lava domes
have been measured at a distance with an optical pyrometer.
The surfaces of these lava domes are as hot as 900°C, but
their interiors must surely be even hotter.
When Mount St. Helens erupted in 1980 in Washing-
ton State, it ejected felsic magma as particulate matter in
pyroclastic flows. Two weeks later, these flows had temper-
atures between 300° and 420°C, and a steam explosion took

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Composition of Magma
By far the most abundant minerals in Earth's crust are sili-
cates such as quartz, feldspars, and several ferromagnesian
silicates, all made up of silicon and oxygen, and other ele-
ments shown in Figure 3.10. As a result, melting of the crust
yields mostly silica-rich magmas that also contain consid-
erable aluminum, calcium, sodium, iron, magnesium, and
potassium, and several other elements in lesser quantities.
Another source of magma is Earth's upper mantle, which
is composed of rocks that contain mostly ferromagnesian
silicates. Thus, magma from this source contains compara-
tively less silicon and oxygen (silica) and more iron and
magnesium.
Although there are a few exceptions, the primary con-
stituent of magma is silica, which varies enough to distin-
guish magmas classifi ed as felsic, intermediate, and mafi c.*
Felsic magma , with more than 65% silica, is silica rich and
contains considerable sodium, potassium, and aluminum,
but little calcium, iron, and magnesium. In contrast, mafi c
magma , with less than 52% silica, is silica poor and contains
proportionately more calcium, iron, and magnesium. And as
you would expect, intermediate magma has a composition
between felsic and mafi c magma (Table 4.1).

How Hot Are Magma and Lava?
Everyone knows that lava is very hot, but how hot is hot?
Erupting lava generally has a temperature in the range
of 700° to 1200°C, although a temperature of 1350°C was
recorded above a lava lake in Hawaii where volcanic gases
reacted with the atmosphere. Magma must be even hot-
ter than lava; however, no direct measurements of magma
temperatures have ever been made.

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Figure 4.2 How Hot Is Lava? A geologist uses a remotely-operated,
hand-held device for measuring temperature of a lava fl ow visible through
an opening in the rocks in Hawaiian Volcanoes National Park, Hawaii.

*Lava from some volcanoes in Africa cools to form carbonitite, an igneous
rock with at least 50% carbonate minerals, mostly calcite and dolomite.