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Rocks
Activator:
Look at your sample of marble and conglomerate using the
hand lens. Describe each rock. What is the color and texture of each? Try scratching the
surface of each rock with the edge of a penny. Which rock seems harder? Hold each rock in
your hand. Allowing for the fact that the samples aren’t exactly the same size, which rock
seems denser?
• Marble
• Conglomerate
Classification
 Rocks are classified by their texture, mineral
content and how they form (origin).
 Geologists group rocks into three different categories.
 Those categories are:
– Igneous
– Metamorphic
– Sedimentary
When studying a rock sample, geologists observe the
rock’s color, texture and mineral composition to
classify it.
• Texture (look and feel) – Color alone does not
provide enough information to identify a rock but
texture is an important characteristic.
Examples - chalky, smooth, glassy, rough
• Most rocks are made up of minerals and other
rocks called grains.
To describe a rock’s texture, geologists use a number
of terms based on the size, shape, and pattern of the
rock’s grains.
Texture (cont.)
• Grain size: fine-grained (hard to see) like slate and coarsegrained (easy to see) like diorite
• Grain Shape: smooth-and-round or jagged
• Grain Pattern: banded or non-banded, wavy, stacked layers
• No Visible Grain: no crystal grains when they form because
may have cooled very quickly, very smooth and shiny or particles are
extremely small
Why does grain shape matter?
• Rocks with round grains are more likely to
absorb water (porous) than those rocks with
interlocking grains. This allows porous
rocks to become more crumbly and softer
than rocks with interlocking grains.
granite
sandstone
Mineral Composition
• Geologists must look very closely to
determine the minerals that make up a rock.
• They can do this by cutting a small sliver of
the rock and look at it under a microscope.
• They can also perform test on the properties
of the minerals making up the rock such as
how it reacts to an acid test or does it
possess any magnetic properties.
Igneous Rocks Text pgs. 150-153
 Igneous rocks are classified by their origin, texture,
and mineral composition.
 Scientists classify igneous rock according to how they are
formed.
 If igneous rocks form from lava on the
surface of the Earth they are known as
extrusive igneous rocks.
 If igneous rocks form from magma beneath the Earth’s surface
they are known as intrusive igneous rocks.
Granite is the most abundant intrusive rock in the Earth’s crust.
Igneous Rocks Classification
Igneous rocks are classified by texture. Textures depend on
how fast the magma or lava cools. The four texture categories
include:
 Fine Grained – Crystals not visible. Cools
quickly. Example: Rhyolite
 Coarse Grained – Large crystals. Cools
slowly. Example: Granite
 Glassy – No crystals. Cools quickly
Example: Obsidian
 Porphyritic – Two different-sized crystals.
Large crystals scattered on the background of
smaller ones. Cools slowly then quickly.
• Granite is an Igneous Rock.
– Quartz and feldspar minerals, and mica.
Copyright © 2010 Ryan P. Murphy
Mineral Composition
• Remember that the silica
content of magma and lava
affect how easily the magma or
lava will flow. Lava low in
silica will usually form darkcolored rocks such as basalt.
Basalt contains feldspar as well
as certain dark colored
minerals, but does not contain
quartz
• Magma that is high in silica
usually forms light-colored
rocks such as granite. However,
granite can be many colors,
Granite’s color changes with its
mineral composition. Granite
rich in feldspar will be pinkish,
while granite rich in hornblende
will be grayish.
• Pumice
Copyright © 2010 Ryan P. Murphy
• Obsidian
– Glassy
Copyright © 2010 Ryan P. Murphy
• Rhyolite
• Which is Rhyolite and which is Granite?
• Which is Rhyolite and which is Granite?
• Which is Rhyolite and which is Granite?
• Which rock is Obsidian and which is Pumice?
Copyright © 2010 Ryan P. Murphy
• Which rock is Obsidian and which is Pumice?
Copyright © 2010 Ryan P. Murphy
• Which rock is Rhyolite, and which is Pumice?
Copyright © 2010 Ryan P. Murphy
• Which rock is Rhyolite, and which is Pumice?
Copyright © 2010 Ryan P. Murphy
• Gabbro
– Same composition as basalt but cooled slower
(Intrusive)
Copyright © 2010 Ryan P. Murphy
• Gabbro
– Same composition as basalt but cooled slower
(Intrusive)
Copyright © 2010 Ryan P. Murphy
• Gabbro
– Same composition as basalt but cooled slower
(Intrusive)
Copyright © 2010 Ryan P. Murphy
• Which of the rocks below cooled quickly
(Extrusive) and which cooled slowly (Intrusive)?
A
B
• Which of the rocks below cooled quickly
(Extrusive) and which cooled slowly (Intrusive)?
• Can rocks float?
• Picture of floating Pumice Island.
Copyright © 2010 Ryan P. Murphy
Many igneous rocks are hard,
dense, and durable
Sedimentary Rocks Text pgs. 154-158
Most Sedimentary rocks are made from particles
deposited by water and wind and ice over time. These
particles are known as sediments. Sediment is small,
solid pieces of material that come from rocks and living
things. Destructive forces are constantly breaking up
and wearing away the rocks on the Earth’s surface.
Erosion and deposition are two of the processes that
help build sedimentary rocks.
After sediment has been deposited, the processes of
Compaction and Cementation
Cementation
While compaction is taking place,
the minerals in the rock slowly
dissolve in the water. The
dissolved minerals seep into the
spaces between particles of
sediment. Cementation is the
process in which dissolved
minerals crystallize and glue
sediments together.
Example: sandstone
Compaction
• At first sediments fit together
loosely. But gradually over
millions of years, thick layers of
sediment build up. These layers
become heavy and press down on
the layers beneath them,
compacting them together.
• Compaction is the process that
presses sediments together. These
layers often remain visible.
• Example: shale
Sedimentary Rocks
Most Sedimentary rocks are made from bits and pieces
of other rocks that cement together over time. There
are three classifications of Sedimentary rocks. They are
Clastic, Chemical, and Organic. Sedimentary rocks
are usually found in layers. Most fossils are found in
Sedimentary rocks.
Sedimentary Rocks
Clastic Sedimentary Rocks – Clastic rocks are made of different
rock particles that are squeezed together. Clastic Sedimentary
rocks are classified by particle size.
Examples:
• Conglomerate
• Sandstone
• Shale
Sedimentary Rocks
Organic Sedimentary Rocks – Not all sedimentary
rocks are made from particles of other rocks. Organic
rocks form from the remains of dead plants and
animals.
Examples:
Limestone
Coal
Sedimentary Rocks
Chemical Sedimentary Rocks – Chemical rocks form
when oceans, lakes, and rivers evaporate and leave
behind sediment particles. They are sometimes called
evaporites. Minerals dissolve and crystallize.
Examples:
Gypsum
Calcite
Halite
Can you determine the order of
these rocks?
Metamorphic Rock
is a rock that has changed its form
Text pgs. 162-165
Metamorphic rocks form from heat and/ or pressure deep beneath the
Earth’s surface. Any rock can be changed into a metamorphic rock.
 Changes this rock goes through
appearance  texture  crystal structure mineral content
Metamorphic rocks can be formed by Contact Metamorphism or Regional
Metamorphism.
 Contact Metamorphic Rocks – Form when existing rock comes in
contact with magma. The changes that happen are because of great
amounts of heat.
 Regional Metamorphic Rocks – Form when pressure from magmas
change rocks over a large area.
Metamorphic Rocks
Minerals in the original rock determine the mineral
composition of the metamorphosed rock.
Metamorphic rocks are classified by arrangement of
the grains that makeup the rock.
The two metamorphic textures are Foliated and
Nonfoliated.
• Foliated Texture – Minerals line up in
bands or parallel layers.
Example – Gneiss
• Nonfoliated Texture – Minerals do not
line up in bands.
Example - Marble
Examples of Metamorphic Rocks
• The sedimentary rock
shale can change into
the metamorphic rock
slate.
• The sedimentary rock
sandstone can change
into the metamorphic
rock quartzite.
The igneous rock granite can be
melted and changed into the
metamorphic rock, gneiss
Changing Rocks
Limestone changes to marble
Sandstone changes to quartzite
Shale changes to slate
The Rock Cycle Text pgs. 166-169
• Which came first?
granite
sandstone
quartzite
A Cycle of Many Pathways
• Earth’s rocks are not as unchanging as they seem. Forces
inside and at the surface produce a rock cycle that builds,
destroys, and changes the rocks in the crust. The rock
cycle is a series of processes on Earth’s surface and inside
the planet that slowly change rocks from one kind to
another. What drives the rock cycle? Earth’s constructive
and destructive forces – including plate tectonics- move
the rocks through the rock cycle.
• New Area of Focus. The Rock Cycle
Copyright © 2010 Ryan P. Murphy
Quiz
Answer the following questions.
1. What are the three major classes of rocks?
2. How are intrusive rocks different from
extrusive rocks?
3. What are the four igneous rock textures?
4. What are two conditions that must be
present in order to form metamorphic
rocks?
5. How is Contact Metamorphism different from
Regional Metamorphism?
6. What are three classifications of Sedimentary
Rocks?
7. How are Clastic Sedimentary rocks classified?
8. What is another name for Chemical Rocks?
9. List examples of Igneous Rocks.
10. List examples of Sedimentary Rocks.