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Overburden
Feldspar
Cement
Quartz

Pore space

A After deposition B Compaction C Cementation

FIGURE 17.18 Lithification of sand grains to become sandstone. (A) Loose sand grains are deposited with open pore space between the
grains. (B) The weight of overburden compacts the sand into a tighter arrangement, reducing pore space. (C) Precipitation of cement in the
pores by groundwater binds the sand into the rock sandstone, which has a clastic texture.

The weight of an increasing depth of overlying sediments hot solutions from the intrusion of a magma. Pressures from
causes an increasing pressure on the sediments below. This movement of the crust can change the rock texture by fl attening,
pressure squeezes the deeper sediments together, gradually deforming, or realigning mineral grains. Temperatures from an
reducing the pore space between the individual grains. This intruded magma must be just right to produce a metamorphic
compaction of the grains reduces the thickness of a sediment rock. They must be high enough to disrupt the crystal structures

deposit, squeezing out water as the grains are packed more to cause them to recrystallize but not high enough to melt the
tightly together. Compaction alone is usually not enough to rocks and form igneous rocks (Figure 17.19).
make loose sediment into solid rock. Cementation is needed to The exact changes caused by heat and pressure depend on
hold the compacted grains together. the mineral composition of the parent rock and the extent of
In cementation, the spaces between the sediment parti- the pressure, temperature, and hot solutions that may or may
cles are filled with a chemical deposit. As underground water not be present to induce chemical changes. Pressure on parent

moves through the remaining spaces, solid chemical deposits rocks with flat crystal flakes (such as clays and mica) tends to
can precipitate and bind the loose grains together. Th e chemi- align the flakes in parallel sheets. This new crystal alignment
cal deposit binds the particles together into the rigid, cohesive is called foliation after the Latin for “leaf” (as in the leaves, or
mass of a sedi mentary rock. Compaction and cementation may pages, of a closed book). Foliation gives a metamorphic rock the
occur at the same time, but the cementing agent must have been property of breaking along the planes between the aligned min-
introduced before compaction restricts the movement of the eral grains, a characteristic known as rock cleavage. The extent
fluid through the open spaces. Many soluble materials can serve

as cementing agents, and calcite (calcium carbonate) and silica
(silicon dioxide) are common.

CONCEPTS Applied

Minerals in Sand
Collect dry sand from several different locations. Use a
magnifying glass to determine the minerals found in each
sample.

METAMORPHIC ROCKS
The third group of rocks is called metamorphic. Metamorphic
rocks are previously existing rocks that have been changed by
heat, pressure, or hot solutions into a distinctly diff erent rock.
The heat, pressure, or hot solutions that produced the changes FIGURE 17.19 Increasing metamorphic change occurs with

are associated with geologic events of (1) movement of the increasing temperatures and pressures. If the melting point is reached,
crust, which will be discussed in chapter 18, and (2) heating and the change is no longer metamorphic, and igneous rocks are formed.

446 CHAPTER 17 Rocks and Minerals 17-14