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12/8/05
4:29 PM
MAZER
Page 172
SOLIDS, LIQUIDS, AND GASES
Resurfacing the
Landscape
8:3.d Students know the
states of matter (solid,
liquid, gas) depend upon
molecular motion.
The landscape of northern California shows evidence of much
volcanic activity. It is a place of many different types of rocks
formed during various eruptions. Volcanic eruptions can be very
violent or very slow. The type of eruption determines the type of
rock formed.
Molten and Solid Rock
The lack of mineral crystals in
obsidian is evidence that the rock
cooled very quickly.
The history of volcanic eruptions can be told as a story of solids,
liquids, and gases. Molten rock below Earth’s surface is called
magma. Magma is a mixture of liquid rock, solid minerals, and
dissolved gases. Liquid rock is less dense than solid rock. Thus,
magma tends to rise slowly through the surrounding solid rock.
As magma gets closer to the surface, the pressure from the rocks
above it becomes lower. The trapped gases expand, increasing the
pressure inside the magma. A very rapid expansion of the trapped
gases may cause an explosive eruption. Slower expansion causes
less violent eruptions.
Magma that reaches the surface is called lava. Molten lava
cools to form solid rock again. The chemical composition of the
lava––that is, the elements and compounds it is made of––determines
the type of rock that is formed. But even lavas of similar composition
can result in very different types of rock. Obsidian and pumice, shown
on the left, are examples.
Smooth and Foamy Rocks
Pumice is full of holes. The holes
formed as expanding bubbles
of gas escaped from rapidly
cooling lava.
Obsidian is a glasslike rock that is high in silica (SiO2). It forms when
silica-rich lava cools rapidly. The fast rate of cooling does not give
crystals time to form. When obsidian breaks, it forms sharp edges.
Native Americans traditionally used obsidian to make tools.
Pumice has a chemical composition that is similar to obsidian,
but it looks very different. It is full of holes, like a pile of soap bubbles.
These holes make it less dense, so it may even float in water. The
holes formed when gases expanded quickly. Then the rock cooled
so rapidly that the holes did not have time to collapse. This is one
way in which different types of rock form.
WRITING ABOUT SCIENCE
This ground is quite young in
geologic terms. The pumice, with
its tiny holes, is evidence of violent volcanic eruptions. Notice Mt.
Shasta in the background.
172 Unit 2: The Structure of Matter
Pumice and obsidian have many uses today. For example,
obsidian is used in jewelry, and obsidian blades are even used
to make surgical tools. Research some of the uses of pumice
or obsidian and describe in a paragraph how the composition
and texture of the rocks suit their uses.
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