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Rocks
How Geologists Classify Rocks
When studying a rock sample, geologists observe three things:
1) Origin
2) texture
3) mineral composition.
Rocks
Texture
•Texture is the look and feel of the rock’s surface.
•Texture can be smooth, glassy, rough or chalky.
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stigation
Rocks
Grains
•Particles of minerals or other rocks that make up a rock
•Grains are large and easy to see = coarse-grained
•Grains so small they can only be seen with microscope
= fine-grained
Rocks
Grain Shape
•Grain shapes can be round or jagged
•Also result from crystal shape
Rocks
Grain Pattern
•Can be dotted throughout (non-banded)
•Can be organized in horizontal rows (banded)
Rocks
No Visible Grain
•No grain visible even under microscope
•This occurs when lava cools very quickly
•Extremely small pieces of silica make up the rock
Rocks
Mineral Composition
Using a microscope to identify minerals by cutting a thin slice
Use acid to detect carbon (Calcium Carbonate)
Also test with magnet to detect iron or nickel (magnetic properties
Rocks - Classifying Rocks
How Rocks Form
Geologists classify rocks into three major groups: igneous
rock, sedimentary rock, and metamorphic rock.
Rocks
Graphic Organizer
Rocks
can be
Sedimentary
Igneous
include
Intrusive
Metamorphic
include
Extrusive
Clastic
Organic
include
Chemical
Foliated
Nonfoliated
Rocks
Igneous Rock
Forms when molten rock (magma/lava) cools and
hardens
Cools slowly beneath Earth’s surface = intrusive
Cools rapidly on the surface = extrusive
Intrusive rocks usually have large, visible grains
Extrusive rocks usually have small to no visible
grains
Color is based on the amount of silica in the
magma
Rocks
Sedimentary Rocks
Forms from the compaction and cementation of rock pieces,
mineral grains, or organic matter called sediments
Can contain fossils
Sediments are formed through the process of weathering and
erosion of exposed rocks.
Rocks
3 kinds:
Clastic = made of mineral or rock pieces
(shale; conglomerate; sandstone; Breccia)
Organic = made from plant or animal
remains (coal; coquina; some limestones)
Chemical = made from dissolved minerals
in water (halite; other limestones)
Rocks
Metamorphic Rock
Forms when rocks are changed into different kinds of rocks
by great heat and pressure
They are heated, squeezed, folded, or changed by contact
with hot magma.
2 kinds:
foliated = grains are in bands
nonfoliated = no banding
Rocks - Igneous Rocks
Classifying Igneous Rocks
Igneous rocks are classified according to their origin, texture,
and mineral composition.
Rocks - Igneous Rocks
Identifying Main Ideas
As you read the section “Classifying Igneous Rocks,” write
the main idea in a graphic organizer like the one below. Then
write three supporting details. The supporting details further
explain the main idea.
Main Idea
Igneous rocks are classified by origin, texture, and composition.
Detail
Extrusive rock forms
from lava on the
surface; intrusive
rock forms from
magma from beneath
the surface.
Detail
Intrusive rocks have
larger crystals than
extrusive rocks
because they cool
more slowly.
Detail
High-silica rocks are
light colored; lowsilica rocks are dark
colored.
Rocks - Sedimentary Rocks
From Sediment to Rock
Most sedimentary rocks are formed through a series of
processes: erosion, deposition, compaction, and
cementation.
Rocks - Sedimentary Rocks
Outlining
As you read, make an outline
about sedimentary rocks. Use
the red headings for the main
topics and the blue headings
for the subtopics.
Sedimentary Rocks
I. From Sediment to Rock
A. Erosion
B. Deposition
C. Compaction
D. Cementation
II. Types of Sedimentary Rock
A. Clastic Rocks
B. Organic Rocks
C. Chemical Rocks
III. Uses of Sedimentary Rocks
A. Building Materials
B. Tools
Rocks - Rocks From Reefs
Using Prior Knowledge
Before you read, look at the section headings and visuals to
see what this section is about. Then write what you know
about coral reefs in a graphic organizer like the one below.
As you read, write what you learn.
What You Know
1.
2.
3.
Coral reefs grow in the oceans.
Florida has coral reefs.
Oceans used to be where there is dry land now.
What You Learned
1.
2.
3.
Coral animals cannot live below 40 meters.
In the United States, only the coasts of Florida and Hawaii have coral
reefs.
Some limestone deposits on land formed from ancient reefs.
Rocks - Metamorphic Rocks
Previewing Visuals
Before you read, preview Figure 17. Then write two
questions that you have about metamorphic rocks in a
graphic organizer like the one below. As you read, answer
your questions.
Previewing
Q. Why do the crystals in gneiss line up in bands?
A. Gneiss is a type of metamorphic rock that is foliated—the
crystals are flattened to form parallel lines.
Q. How does quartzite form from sandstone?
A. High temperature and pressure on the minerals in sandstone
cause them to be changed into minerals that make up
quartzite.
Rocks - The Rock Cycle
A Cycle of Many Pathways
Forces deep inside Earth and at the surface produce a slow
cycle that builds, destroys, and changes the rocks in the
crust.