Chapter 5 Section 1
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Lava
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Igneous Rocks
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
Rocks that form when lava or magma cools and
minerals crystallize
Partial Melting
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
Magma that flows out onto Earth’s surface
Some minerals melt at low temperatures while
others remain solid
Fractional Crystallization

The first minerals that crystallize from magma are
the last minerals that melted during partial melting.

Heat that melts rocks

Theory
 The remaining energy from Earth’s molten formation
 Heat generated from decay of radioactive elements


Type of igneous rock that forms depends on
the type of magma
Magma

A slushy mix of molten rock, dissolved gasses and
mineral crystals.
Silica most abundant
 Others include:
 Oxygen, silicon, aluminum, iron, magnesium, calcium,
potassium, sodium

Silica


Greatest effect on magma characteristics
Magma classifications-based on silica amounts
Basaltic
 Andesitic
 Rhyolitic

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Silica content affects melting temperature and
how quickly it flows

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Silica content: 42-52%
Hawaiian Islands
Freely flowing liquid
Oozes out through
cracks in ocean floor
or spills out of
volcanoes




Silica content 52-66%
Cascade Mountains,
Andes Mountains
Composition between
basaltic and rhyolitic
Can produce violent
volcanic eruptions





Silica Content more
than 66%
Yellowstone National
Park
Less dense than basalt
Thick, stiff magma
Violent volcanic
eruptions

Magma forms
Melting Earth’s crust
 Melting within mantle


Four main factors

Temperature
 Geothermal gradient- Earth’s temp. increases

Pressure
 Increases with depth

Water content
 Amount of water present can change the melting point of the
rock.

Mineral content


Elements and compounds give magma its
overall character.
Melting points of minerals differ
Basalt melts at high temperature
 Granite melts at low temperature


Rocks rich in iron and magnesium melt at
higher temperatures than rocks containing
higher levels of silicon

Not all parts of a rock melt at the same temp.


Elements and compounds affect melting temp
Igneous rocks are a combination
 Some parts of rock melt at lower temp than others.
 If the temp is not high enough to melt the whole rock
 Resulting magma will have a different composition than
the original rock


N.L. Bowen showed that as magma cools and
crystallizes, minerals form in predictable
patterns.
Two main branches


Continuous gradual change (feldspar group)
Abrupt change (iron group)


Undergo abrupt changes as magma cools
As the temperature drops enough for a new
mineral to be formed

Previously formed minerals react with the magma
and new minerals are formed


Continuous change
As magma cools



Earliest formed minerals have high levels of calcium
As they cool farther and react with the magma, they
become sodium rich
If cools too quickly, calcium rich minerals do not
react completely leaving zoned crystals P. 115