Intermediate (Andesitic) Igneous Rocks

ANDESITE. Andesite is a medium-gray, fine-grained rock of volcanic origin. Its name comes from South America’s Andes Mountains, where numerous volcanoes are composed of this rock type. Andesite commonly exhibits a porphyritic texture. Andesite often resembles rhyolite, so their identification usually requires microscopic examination to verify mineral make-up.

DIORITE. Diorite is the plutonic equivalent of andesite. It is a phaneritic rock that looks somewhat similar to gray granite. However, it can be distinguished from granite by the absence of visible quartz crystals and because it contains a higher percentage of dark silicate minerals. Diorite has a salt-and-pepper appearance.

Task 3. Look at Figure 15.2 and Figure 15.3. They provide the classification of major igneous rocks based on mineral composition and texture. Adding the information from the figures try to characterize:

• granite;

• rhyolite;

• obsidian;

• pumice;

• andesite;

• diorite.

Individual work

Task 1. Read the text and translate it into Ukrainian (in written form). Build up a list of key terms to the text.

Mafic (Basaltic) Igneous Rocks

BASALT. Basalt is a very dark green to black, aphanitic rock composed primarily of pyroxene and calcium-rich plagioclase feldspar, with lesser amounts of olivine and amphibole. 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.

GABBRO. Gabbro is the intrusive equivalent of basalt. 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. 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. 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.

Task 2. Compare and contrast each of the following pairs of rocks:

a. granite and diorite

b. basalt and gabbro

c. andesite and rhyolite

Task 3. Answer the question:

How do tuff and volcanic breccia differ from other igneous rocks such as granite and basalt?