Pumice. This is a light to medium gray, frothy, extrusive igneous rock. The large amount of
void space in the rock causes it to have a very low density. (Pumice is the only rock that will float on water).
Flow marks or blisters present in these types of flows may sometimes be distinguished among the light photo
tones on aerial photographs.
Scoria. This is a fine-grained, frothy, extrusive igneous rock with an appearance similar to
that of pumice. However, scoria is dark-colored, ranging from shades of brown to black or brick-red, and it is
more dense than pumice. (Scoria will sink in water.) Scoria appears dark on aerial photographs; however, it may
still be possible to distinguish flow marks and vesicles within the flow region.
(d) Pyroclastic Materials. Fragments of solid rock material aerially ejected from volcanic vents are
collectively referred to as pyroclastic material. The size of the rock fragments ejected from a volcano range from
fine dust or ash through volcanic sand, lapilli, cinders, and bombs to large blocks. These rock fragments may, at
some later time, become consolidated to form a pyroclastic rock. Such rocks, composed of volcanic dust and ash,
are called tuffs, whereas the term volcanic breccia is generally reserved for pyroclastic rocks made up of
fragments greater than four millimeters in diameter. Consolidated pyroclastic rocks are photographically similar
to sedimentary rocks, which are discussed in Lesson 1.B.2, page 1-18.
(3) Relationship of Topography to Extrusive Igneous Rocks.
(a) Landforms Developed in Areas of Extrusive Rocks. The extrusive igneous rock bodies
previously discussed (volcanoes and lava flows) may be readily identified by their characteristic landforms. In
fact, three different types of volcanic cones may be distinguished based on their topographic expressions.
Cinder cones are steep-sided, symmetrical cones composed of angular fragments of rock
ejected during volcanic eruptions. If the fragments are large, the side slopes of the cone will be inclined at angles
of 30 to 40 degrees from the horizontal. If the pyroclastic material is fine-grained, the side slopes are more gentle.
Figure 1-7, page 1-14, illustrates the topography of two weathered cinder cones, covering approximately eight
square miles.
Shield volcanoes are very broad, slightly arched "cones" resulting from outpourings of fairly
fluid lavas. The most notable shield volcanoes are those of the Hawaiian Islands. Two such volcanoes are
depicted in figure 1-8, page 1-15.
Composite cones are composed of alternating layers of lava flows and rock fragments. The
side slopes of these volcanic cones are intermediate between those of cinder cones and those of shield volcanoes.
An example of a large composite volcano can be seen in figure 1-9, page 1-16.
Lava flows, in contrast to the marked relief of volcanoes, form extensive plains and plateaus.
Young flows have a level to gently sloping surface, and lobate edges marking the terminus of the flow may yet be
visible. Mature flows, on the other hand, are highly dissected, with a surface of rolling to rugged hills. The
distinctive topography associated with young extrusive flows will rapidly disappear in humid climates, and a thick
soil profile will develop. The region shown in figure 1-10, page 1-17, is composed of several isolated basaltic
mesas, a common feature of mature lava flows.
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