Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Become familiar with
the textures and mineral
assemblages of igneous rocks
■■
Use texture and mineral
content to interpret the
history of igneous rocks
■■
Understand the classification
of igneous rocks
■■
Associate igneous rocks
with different plate-tectonic
settings
■■
Recognize the kinds of
volcanoes and understand why
they have different shapes
MATERIALS
NEEDED
■■
Set of igneous rocks
■■
Magnifying glass or hand lens
and, ideally, a microscope and
thin sections of igneous rocks
■■
Standard supplies for
identifying minerals (streak
plate, glass plate, etc.)
5.1 Introduction
Every rock has a story to tell. The story of an igneous rock begins when rock in the
lower crust or upper mantle melts to form molten material called magma, which
rises up through the crust. Some magma reaches the surface, where it flows as what
we call lava. Other magma explodes upward through cracks or vents in the Earth,
spewing tiny particles of volcanic ash into the air. Igneous rock that forms from
solidified lava or ash is called extrusive igneous rock because it comes out (extrudes)
onto the Earth’s surface.
Some magma never reaches the surface and solidifies underground to form
intrusive igneous rock, so called because it squeezes into (intrudes) the surrounding
rocks. Bodies of intrusive igneous rock come in many shapes. Massive blobs are
called plutons, and the largest of these—called batholiths—are generally composed
of several smaller plutons. Other intrusions form thin sheets that cut across layering
in the host rock (the rock into which the magma intruded). These intrusions are
called dikes, and those that form thin sheets parallel to the layers of host rock are
called sills (FIG. 5.1).
When looking at an igneous rock, geologists ask several questions: Where did the
rock cool (was it intrusive or extrusive)? Where in the Earth did the rock’s parent
magma form? In what plate-tectonic setting—ocean ridge, mid-continent, subduction
zone, hot spot—did it form? Most of these questions can be answered with
three simple observations: the rock’s grain size, color, and specific gravity. In this
chapter you will learn how to answer these questions and how to identify common
types of igneous rock. Few new skills are needed—just observe carefully and apply
geologic reasoning.
Exercise 5.1 shows how easy the process is. The following sections provide the
additional information you need to interpret the history of an igneous rock.
FIGURE 5.1 Intrusive and extrusive igneous rock bodies.
Ash fall
Volcanoes
Lava flow
Dike
Sill
Magma chamber
Pluton
112 CHAPTER 5 USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY