Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Understand the differences
between minerals and rocks
■■
Become familiar with the three
main classes of rock: igneous,
sedimentary, metamorphic
■■
Understand the different
processes that form rocks
■■
Learn how the grains in a rock
act as records of geologic
processes
■■
Use textures and mineral
content to determine
whether rocks are igneous,
sedimentary, or metamorphic
■■
Understand the rock cycle
and how rocks are recycled to
make new ones
MATERIALS
NEEDED
■■
An assortment of igneous,
sedimentary, and metamorphic
rocks
■■
Supplies to make artificial
igneous, sedimentary, and
metamorphic rocks: tongs for
moving hot petri dishes, hot
plate, sugar, thymol, Sodium
acetate in a dropper bottle,
sand grains, glass petri dishes,
Calcium hydroxide solution,
straws, Play-Doh, plastic coffee
stirring rods, plastic chips
■■
Magnifying glass or hand lens
■ ■ Mineral-testing supplies:
streak plate, steel nail, and
so forth
4.1 Introduction
A rock is an aggregate of mineral grains, fragments of previously existing rock, or a
mass of natural glass. Rocks form in several ways—by cooling and solidification of
a melt, cementation of loose grains, precipitation from water solutions, or changes
that happen underground in response to increasing temperature and pressure.
Geologists use two principal characteristics of rocks to evaluate how they formed
and to interpret aspects of the Earth’s history: their composition (the identity of minerals
or glass that make up a rock) and their texture (the sizes and shapes of grains,
and the ways that grains are arranged, oriented, and held together).
In this chapter you will first be introduced to the three basic classes of rock,
then learn how each of them forms by making your own “rocks.” These simulations
show that rock-forming processes leave textural evidence that enables us to identify
which class a rock belongs to and, in some instances, the conditions under which it
formed. The ultimate goal is to combine your skills in mineral identification with
this new textural knowledge to correctly group rock samples into the three classes.
On our dynamic planet, rocks don’t survive forever. The Earth is the original
recycler, reusing materials from one rock to form new ones. This chapter will also
introduce you to the rock cycle, the complex interaction among rock-forming processes
by which this recycling takes place.
4.2 The Three Classes of Rocks
Geologists struggled for centuries with the question of how to classify rocks. They
finally concluded that rocks can best be classified on the basis of how they formed,
and using this standard, we now group all rocks into three categories: igneous, sedimentary,
and metamorphic.
■ Igneous rocks form through the cooling and solidification of molten rock,
which is created by melting of pre-existing rock in the mantle or lower crust. We refer
to molten rock below the Earth’s surface as magma and to molten rock that has
erupted onto the surface as lava. Some volcanoes erupt explosively, blasting rock
fragments into the air, and when these fragments fall back to the Earth, coalesce,
and solidify, the resulting rock is also considered to be igneous.
■ Sedimentary rocks form at or near the surface of the Earth in two basic ways:
(1) when grains of pre-existing rock accumulate, are buried, and then are cemented
together by minerals that precipitate (grow and settle out) from groundwater; and
(2) when minerals precipitate out of water near the Earth’s surface, either directly or
through the life function of an organism, and either form a solid mass or are cemented
together later. The grains that become incorporated in sedimentary rocks form
when pre-existing rocks are broken down into pieces by processes involving interactions
with air, water, and living organisms. These interactions are called weathering.
Some weathering simply involves the physical fragmentation of rock. Other kinds of
weathering involve chemical reactions that produce new minerals, most notably clay.
The products of weathering can be transported (eroded) by water, ice, or wind to the
site where they are deposited, buried, and transformed into new rock.
■ Metamorphic rocks form when pre-existing rock is subjected to physical and
chemical conditions within the Earth different from those under which the original
rock formed: increased pressure and temperature and/or shearing at elevated
temperatures. For example, when buried very deeply, rock is warmed to high temperatures
and squeezed by high pressure. The texture and/or mineral content of
the pre-existing rock changes in response to the new conditions, but its solid state
does not change. Metamorphic rocks thus form without the melting or weathering
that creates igneous and sedimentary rocks.
90 CHAPTER 4 MINERALS, ROCKS, AND THE ROCK CYCLE