FIGURE 3.16 Outcrop
of welded tuff from
Valles Caldera near Los
Alamos, New Mexico. Tuff
is composed mainly of
ash-sized particles and may
contain larger fragments of
pumice or other volcanic rocks.
(Photo by Marli Miller)
Today, we know that silica-rich lava is too
viscous (thick) to flow more than a few
miles from a vent.
Pyroclastic rocks composed mainly of
particles larger than ash are called volcanic
breccia. The particles in volcanic breccia
can consist of streamlined fragments that
solidified in air, blocks broken from the
walls of the vent, crystals, and glass
fragments.
Unlike most igneous rock names, such
as granite and basalt, the terms tuff and
volcanic breccia do not imply mineral
composition. Thus, they are frequently
used with a modifier, as, for example,
rhyolite tuff.
C O N C E P T C H E C K 3 . 5
The classification of igneous rocks is based
largely on two criteria. Name these criteria.
How are granite and rhyolite different?
In what way are they similar?
Compare and contrast each of the following pairs of rocks:
a. granite and diorite
b. basalt and gabbro
c. andesite and rhyolite
How do tuff and volcanic breccia differ
from other igneous rocks such as granite
and basalt?
4
3
2
1
feldspar phenocrysts or green, glassy-appearing olivine phenocrysts embedded in a dark
groundmass.
Basalt is the most common extrusive igneous rock. Many volcanic islands, such as the
Hawaiian Islands and Iceland, are composed mainly of basalt. Further, the upper layers of
the oceanic crust consist of basalt. In the United States, large portions of central Oregon
and Washington were the sites of extensive basaltic outpourings (FIGURE 3.15). At some
locations, these once-fluid basaltic flows have accumulated to thicknesses approaching
3 kilometers.
GABBRO. Gabbro is the intrusive equivalent of basalt (see Figure 3.11). Like basalt,
it tends to be dark green to black in color and composed primarily of pyroxene
and calcium-rich plagioclase feldspar. Although gabbro is uncommon in the
continental crust, it makes up a significant percentage of oceanic crust.
Pyroclastic Rocks
Pyroclastic rocks are composed of fragments ejected during a volcanic eruption. One of the most common pyroclastic rocks, called
tuff, is composed mainly of tiny, ash-size fragments that were later
cemented together (see Figure 3.4B). In situations where the ash
particles remained hot enough to fuse, the rock is called welded
tuff. Although welded tuff consists mostly of tiny glass shards, it
may contain walnut-size pieces of pumice and other rock
fragments.
Welded tuffs blanket vast portions of formerly active volcanic
areas in the western United States (FIGURE 3.16). Some of these tuff
deposits are hundreds of feet thick and extend for tens of miles
from their source. Most formed millions of years ago as volcanic ash
spewed from large volcanic structures (calderas) in an avalanche style,
spreading laterally at speeds approaching 100 kilometers per hour. Early
investigators of these deposits incorrectly classified them as rhyolite lava flows.
FIGURE 3.15 The Columbia River basalts. At some locations these
once-fluid lava flows are more than 3 kilometers thick. Although
basalt is usually black, it often weathers to a reddish-brown color.
(Photo by Willianborg)
75
Naming Igneous Rocks
of welded tuff from
Valles Caldera near Los
Alamos, New Mexico. Tuff
is composed mainly of
ash-sized particles and may
contain larger fragments of
pumice or other volcanic rocks.
(Photo by Marli Miller)
Today, we know that silica-rich lava is too
viscous (thick) to flow more than a few
miles from a vent.
Pyroclastic rocks composed mainly of
particles larger than ash are called volcanic
breccia. The particles in volcanic breccia
can consist of streamlined fragments that
solidified in air, blocks broken from the
walls of the vent, crystals, and glass
fragments.
Unlike most igneous rock names, such
as granite and basalt, the terms tuff and
volcanic breccia do not imply mineral
composition. Thus, they are frequently
used with a modifier, as, for example,
rhyolite tuff.
C O N C E P T C H E C K 3 . 5
The classification of igneous rocks is based
largely on two criteria. Name these criteria.
How are granite and rhyolite different?
In what way are they similar?
Compare and contrast each of the following pairs of rocks:
a. granite and diorite
b. basalt and gabbro
c. andesite and rhyolite
How do tuff and volcanic breccia differ
from other igneous rocks such as granite
and basalt?
4
3
2
1
feldspar phenocrysts or green, glassy-appearing olivine phenocrysts embedded in a dark
groundmass.
Basalt is the most common extrusive igneous rock. Many volcanic islands, such as the
Hawaiian Islands and Iceland, are composed mainly of basalt. Further, the upper layers of
the oceanic crust consist of basalt. In the United States, large portions of central Oregon
and Washington were the sites of extensive basaltic outpourings (FIGURE 3.15). At some
locations, these once-fluid basaltic flows have accumulated to thicknesses approaching
3 kilometers.
GABBRO. Gabbro is the intrusive equivalent of basalt (see Figure 3.11). Like basalt,
it tends to be dark green to black in color and composed primarily of pyroxene
and calcium-rich plagioclase feldspar. Although gabbro is uncommon in the
continental crust, it makes up a significant percentage of oceanic crust.
Pyroclastic Rocks
Pyroclastic rocks are composed of fragments ejected during a volcanic eruption. One of the most common pyroclastic rocks, called
tuff, is composed mainly of tiny, ash-size fragments that were later
cemented together (see Figure 3.4B). In situations where the ash
particles remained hot enough to fuse, the rock is called welded
tuff. Although welded tuff consists mostly of tiny glass shards, it
may contain walnut-size pieces of pumice and other rock
fragments.
Welded tuffs blanket vast portions of formerly active volcanic
areas in the western United States (FIGURE 3.16). Some of these tuff
deposits are hundreds of feet thick and extend for tens of miles
from their source. Most formed millions of years ago as volcanic ash
spewed from large volcanic structures (calderas) in an avalanche style,
spreading laterally at speeds approaching 100 kilometers per hour. Early
investigators of these deposits incorrectly classified them as rhyolite lava flows.
FIGURE 3.15 The Columbia River basalts. At some locations these
once-fluid lava flows are more than 3 kilometers thick. Although
basalt is usually black, it often weathers to a reddish-brown color.
(Photo by Willianborg)
75
Naming Igneous Rocks