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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?

Test yourself

Task: Tell whether the sentences are true or false. Correct the false ones.
1. Texture is an important property because it reveals a great deal about the
environment in which the rock formed.
2. Two factors influence the textures of igneous rocks.
3. The rock which has large crystals embedded in a matrix of smaller crystals, is
said to have phaneritic texture.
4. Igneous rocks composed of the ejected particles which might be very fine ash,
molten blobs, or large angular blocks torn from the walls of the vent during the
eruption, are said to have a pyroclastic texture.
5. Rhyolite is a coarse-grained rock composed of about 25 percent quartz and
roughly 65 percent feldspar, mostly potassium-and sodium-rich varieties.
6. Granite may be reddish or light gray in color.
7. Many samples of pumice will not float when placed in water.

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