Page 149 - Laboratory Manual in Physical Geology

Introduction ACTIVITY
Right now, there are more than a hundred volcanoes
erupting or threatening to erupt on continents and islands 5.2 Minerals That Form Igneous
around the world. Some pose direct threats to humans. Rocks
Others pose indirect threats, such as earthquakes and
episodic melting of glaciers. In the oceans, deep under THINK What are igneous rocks composed of?
water and far from direct influence on humans, there are About It | How is composition used to classify
likely hundreds more volcanoes. The exact number is and interpret igneous rocks?
unknown, because they are erupting at places on the sea
floor that humans rarely see. OBJECTIVE Identify samples of eight minerals that

Most of the world’s volcanoes occur along its form most igneous rocks and categorize them as mafi c
260,000 kilometers of linear boundaries between or felsic.
lithospheric plates. The rest are largely associated with hot
spots. All of the volcanoes overlie bodies of molten (hot, PROCEDURES

partly or completely melted) rock called magma , which 1. Before you begin , read Mafic and Felsic Rock-
is referred to as lava when it reaches Earth’s surface at the Forming Minerals below. Also, this is what you
volcanoes. In addition to their liquid rock portion, or melt , will need :
magma and lava contain dissolved gases (e.g., water, carbon ____ Activity 5.2 Worksheet (p. 144 ) and pencil
dioxide, sulfur dioxide) and solid particles. The solid ____ optional: a set of mineral samples (obtained
particles may be pieces of rock that have not yet melted as directed by your instructor)
and/or mineral crystals that may grow in size or abundance ____ optional: a set of mineral analysis tools
as the magma cools. Igneous rocks form when magma (obtained as directed by your instructor)
or lava cool to a solid state. The bodies of igneous rock 2. Then follow your instructor’s directions for
may be as large as those in Yosemite Park, where bodies completing the worksheet.
of magma cooled underground to form batholiths of
igneous rock, tens of kilometers in diameter. They may be
as small as centimeter-thick layers of volcanic ash, which
is composed of microscopic fragments of igneous rock
(mostly volcanic glass pulverized by an explosive volcanic
eruption).
Mafic and Felsic Rock-Forming
Minerals

ACTIVITY There are eight silicate minerals that form most igneous
rocks. This is because silicon and oxygen are the most
5.1 Igneous Rock Inquiry common elements in magma and lava. The silicon and
oxygen naturally forms silicon-oxygen tetrahedra, in which

THINK | What are igneous rocks composed of, and one silicon atom shares electrons with four oxygen atoms

( FIGURE 5.1 ). This creates a silicon-oxygen tetrahedron
About It how can they be classified into groups? (four-pointed pyramid) with four electrons too many, so
each oxygen atom also shares an electron with another

OBJECTIVE Analyze and describe samples of igneous adjacent silicon atom. The simplest ratio of silicon to
rock, then infer how they can be classified into groups. oxygen is 1:2, written SiO and called silica .The mineral

2
PROCEDURES quartz is a crystalline form of pure silica. However, with
1. Before you begin , do not look up defi nitions and the abundance of other chemicals in magma and lava,
information. Use your current knowledge, and silicon-oxygen tetrahedra often bond with other kinds of
complete the worksheet with your current level of metal atoms to make the other silicate minerals commonly
ability. Also, this is what you will need to do the found in igneous rocks. Although each one has its own
activity: unique properties that can be used to identify it, the
minerals are also categorized into two chemical groups.
____ Activity 5.1 Worksheet (p. 143 ) and pencil
____ optional: a set of igneous rock samples Felsic Minerals
(obtained as directed by your instructor)
The name felsic refers to feldspars ( fel -) and other silica-rich
2. Analyze the rocks, and complete the worksheet
(- sic ) minerals. The common felsic minerals in igneous
in a way that makes sense to you.
rocks are gray translucent quartz , light gray opaque
3. After you complete the worksheet , be prepared plagioclase feldspar , pale-orange to pink opaque potassium
to discuss your observations, interpretations, and feldspar , and glossy pale-brown to silvery-white muscovite.
inferences with others. They are all light colored because their chemical composi-
tion lacks iron and magnesium.

130 ■ L ABOR ATORY 5