Properties of Rocks
By Trista L. Pollard
Forces inside the Earth and on the surface are responsible for rock
formation on our planet. When you look at a rock on the Earth's surface, you
are getting a glimpse at its chemical composition and the composition of its
minerals. The chemical stability of rocks has a huge impact on the life of
rocks. A rock's chemical stability will determine the rate at which it
weathers and breaks apart. The stability of rocks also affects how rocks and
minerals form. Thanks to a scientist during the early 1900s, we now have an
understanding of the mineral crystallization process.
1
N.L. Bowen, a Canadian geologist, developed the Bowen's reaction
series in 1928. This series explains the order that minerals crystallize or
form as magma cools and solidifies. Bowen studied the crystallization
process and learned that some minerals crystallize first. During this process,
the forming minerals remove certain elements from the magma, changing its
composition. Once this occurs, other minerals with different elements begin
to form. From this observation, Bowen learned that different minerals form
at different times during the magma solidification process and form in the
same order. His simple flow chart, the reaction series, was born.
2
Bowen also developed a hypothesis that explained the two ways that
minerals form. He stated that minerals form continuously over time. The
second way is through sudden changes in the type of minerals formed. The
chemical composition of magma also affects the formation of these minerals.
In fact, it may determine the order in which the minerals form. Since we are
talking about composition, let's look at the chemical stability of minerals
and the physical stability of rocks.
3
A mineral's chemical stability is the measure of how well the mineral
maintains its original chemical composition, instead of breaking down to
form a different substance. It's the chemical stability that determines the rate
at which minerals break down. Scientists have found that the strength of the
mineral's chemical bonds between its atoms determines its chemical
stability. So what is the relationship between the chemical atomic bonds and
the stability of the mineral? Well, the minerals that have the highest number
of bonds between the silicon and oxygen atoms tend to be the most resistant
to weathering and breaking down.
4
5
The physical stability of rocks is different from the chemical stability of
minerals. Rocks have different points where stress has a greater effect on
their surface. Simply put, some rocks have weak spots or zones. Huge
igneous rocks have joints or evenly spaced areas of weaknesses which cause
them to break into columns. Since they form in layers, sedimentary rocks
may break between the layers. Metamorphic rocks also break into layers
during the process of mineral formation. Rocks that form inside the Earth
may also develop zones of weakness, especially under intense heat and
pressure. When these rocks are brought up to the surface, the sudden
decrease in pressure and temperature may cause joints and fractures to form.
Once this occurs, the rocks are more susceptible to chemical and physical
weathering. Where and how rocks form has a huge impact on their physical
stability. The work of Bowen and other scientists has provided a lot of
information about rock and mineral formation. This information provides
even greater understanding of out planet's crust formation.
Copyright © 2013 edHelper
Name
_____________________________
Date
___________________
Properties of Rocks
1. Bowen's reaction series explains
the ______.
Weathering process of
minerals
Order of mineral formation
Crystal structure of minerals
None of the above
3. True/False: As the minerals form
they remove certain elements
from the magma.
False
True
2. Explain chemical stability and
how it is related to minerals.
4. Which of the following
statements is not true?
Rocks develop different
zones of weaknesses as they
form.
Rocks that form inside the
Earth under intense pressure and
heat may form zones of weakness
when exposed to a sudden
decrease in pressure and
temperature.
As minerals are formed the
composition of the magma is
changed.
The strength of a mineral's
chemical bonds between its atoms
determines its physical stability.
5. What are the two ways that
minerals may form, based on
Bowen's hypothesis?
6. True/False: As magma cools and
solidifies, minerals all form at the
same rate.
Name
_____________________________
Date
___________________
Properties of Rocks
7. Based on context clues, what does 8. How would you know where the
weathering mean?
zone of weakness of sedimentary
rocks is located?