Science 3360
Lecture 14: The Rock Cycle
A look at rocks, minerals, and the
evidence of a dynamic Earth
Questions to Answer…
From our previous discussion of the hydrological cycle and
the ocean nutrient cycle, two important questions arise:
We know that rivers deliver materials from the eroding
continents to the oceans (remember weathering?). On
average, rivers deliver 2-4 x1012 kg of material to the
oceans every year. Why haven’t the oceans filled in?
We know from the ocean nutrient
cycle that some of the nutrients are
buried in the sediment. If this is
occurring over millions of years, why
haven’t the ocean’s biota run out of
nutrients?
Answers…
The answers to these questions are buried
in the processes related to the solid earth.
To fully understand these concepts, we must
first investigate the structure of the Earth,
rocks and minerals, and the rock cycle.
The Structure of the Earth
The Earth consists of 3
major zones:
The Crust
The Mantle
The Core
We’ll discuss these in
more detail next
lecture.
Aside: Volcanoes
1. Magma chamber
2. Bedrock
3. Conduit (pipe)
4. Base
5. Sill
6. Branch pipe
7. Layers of ash emitted by the volcano
8. Flank
9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
From
Rocks and Minerals
A mineral is a naturally occurring inorganic
element or compound with a definite
internal arrangement of ions and a
chemical composition that is fixed or only
varies slightly.
Rocks are aggregates of minerals.
A useful analogy* is
Letter
work
Element
mineral
sentence
rock
* Conte, D.J., D.J. Thompson, and L.L. Moses, Earth
Science: An Integrated Perspective, Wm.C. Brown Publishers,
Chicago, IL, 432pp, 1997.
Minerals
There are 92 naturally occurring elements but most minerals
contain 8 main elements. These are the elements that make
up must of the Earth’s crustal material.
Element
Oxygen
Silicon
Aluminum
Iron
Calcium
Sodium
Potassium
Magnesium
All others
Average Weight Percent in Crust
46.6
27.7
8.1
5.0
3.6
2.8
2.6
2.1
1.5
NB: While O is the most abundant element, it only appears in
combinations with other elements as oxides (i.e. silicates)
Minerals
Items of note:
1. Native elements = pure
elements
2. Other categories comprise
compounds where ions are
combined with a common
element or group (e.g.,
sulfides, sulfates, oxides,
halides, etc).
3. Silicates are most abundant
(> 90% of all minerals found
are silicates). They are
composed of varying
numbers of silicate
groups(SiO42- ).
4. Some minerals formed by
ionic substitution (e.g.
plagioclase, olivine).
You don’t need to memorize all these names.
Minerals
•Within each group or class there can be many individual
minerals
•Any given mineral will have a limited chemical composition. If
the elements in that crystal are changed, it will no longer be the
same mineral.
• sodium chloride is the mineral halite;
• potassium chloride is the mineral sylvite;
• calcium carbonate is the mineral calcite;
• calcium sulfate is the mineral gypsum.
•There is some minor flexibility in chemical make-up caused by
ionic substitution. Some of the ions are similar in size and ionic
charges (Ex.: Fe+2 and Mg+2). If the mineral is crystallizing out of
a liquid that is rich in both elements, the crystal may incorporate
some of the "substitute" ions instead of the more appropriate
ion.
• This substitution is common in the silicate families
Minerals: Ionic Substitution
For example: The
plagioclase series comprises
minerals that range in
chemical composition.
Na or Ca can substitute for
each other as can differing
amounts of Al and Si.
So plagioclase can range from
pure NaAlSi3O8 (Albite) to
pure CaAl2Si2O8 (Anorthite)
and includes combinations of
Albite and Anorthite.
Mineral Identification
How do we identify unknown minerals?
By comparing physical properties of the substance to
known values we can logically identify it.
Properties:
1. Hardness
2. Color
3. Streak
4. Luster
5. Density
6. Cleavage/Fracture
7. Crystal Form
8. Magnetic Properties
Hardness
Hardness scale developed by
Friedrich Mohs.
Runs from 1 to 10 with one being
the softest.
Take an item of known hardness
and try to scratch the unknown
item. If the known is harder, it will
scratch the unknown.
Common Objects and Their Hardness Values
2.5
3.5
Fingernail Penny
5.5
6.5
8.5
Glass
Steel
Knife
Emery
Cloth
So if a fingernail
can’t scratch it but a
penny can, its
hardness is between
2.5 and 3.5
Color
Color is as simple as it sounds. Some minerals
are easily identified by color because they always
have the same color characteristics. An example
of this is malachite which is always green.
However, other minerals can change color
drastically due to chemical impurities. An
example of this is quartz. In its purest for,
quartz is colorless. Quartz with traces of iron
becomes violet (amethyst). With traces of
manganese it turns pink (rose quartz).
Streak
When a mineral is rubbed firmly across an unglazed tile of white
porcelain (a streak plate), it leaves a line of powder called a streak.
The color of the streak is always the same, regardless of impurities that
may be contained in the mineral.
For example, quartz always leaves a white streak regardless of whether
it’s clear (pure quartz), violet (amethyst), or pink (rose quartz).
Luster
Luster is the way the surface of a mineral reflects light. Luster should
always be observed on a fresh cut of freshly broken surface.
There are two general types of luster: metallic and non-metallic.
The terms used to describe luster are:
Metallic -- example: gold
Vitreous (glassy) -- example: quartz
Adamantine (brilliant) -- example: diamond
Resinous (like resin or sap from a tree) -- example: sphalerite
Greasy or waxy -- example: turquoise
Pearly -- example: talc
Silky -- example: asbestos
Dull or earthy -- example: bauxite
Cleavage
When a mineral breaks it does so by fracturing or cleaving. Crystal cleavage is a
smooth break producing what appears to be a flat crystal face. How a mineral
cleaves is controlled by its crystalline structure.
Some minerals break only in one direction while others cleave in two or more
directions.
Examples: Basal Cleavage is cleavage exhibited on a horizontal plane of the
mineral. Minerals with basal cleavage can sometimes be “peeled”. E.g. Mica
Cubic Cleavage: Mineral cleaves perfectly in 3 planes. E.g. Calcite
Minerals
So how are minerals formed?
Minerals are formed by the
process of crystallization.
Within the Earth’s interior high
temperatures and pressures
cause rocks to melt, forming
magma.
As the magma cools, various
elements and ions come together
to form solids with a specific
structure (a crystal)
Different minerals form at
different temperatures. This
relationship is described by
Bowen’s Reaction Series.
Rocks
Rocks are aggregates of minerals.
The variety of rocks that are found is determined by the processes that
lead to the formation of rocks and their mineral constituents. As a general
rule, the minerals that form a given a rock will tend to crystallize at similar
temperatures and pressures.
Rocks
Rocks fall into 3 broad categories depending on how they
were formed
• Igneous rock is formed from the crystallization of magma
– Extrusive rocks means they formed on the surface;
resulted from faster cooling
– Intrusive rocks formed below the surface; resulted
from slow cooling
• Sedimentary rock is formed by the sedimentation and
compaction of material (called lithification)
• Metamorphic rock is rock that has been chemically
altered while in the solid state from exposure to high
temperature and pressure (called recrystallization)
Igneous Rock
Igneous Rocks can be classified by where they were formed
Extrusive Rocks. Cooled quickly on the surface. An example of an
extrusive igneous rock is Basalt. Basalt is comprised of olivine,
pyroxene, and plagiocloase
Intrusive Rocks. Cooled slower below the surface. An example is
granite. Granite is often comprised of feldspar, quartz, and mica
Sedimentary Rock
Formed by depositing layers of material down over time. Can be
classified by the type of material that was compacted.
Detrial: Shale (made from mud) and sandstone (made from sand)
Biological: Limestone (made from shells, coral, etc) and Chert
(made from diatoms)
Evaporites: Gypsum (form when chemical solutions i.e. seawater
evaporate)
Metamorphic Rocks
Formed when rocks of another type in solid form were chemically
altered by high temperature and pressures.
Examples: Slate. Dominant minerals are clay, chlorite, mica and
quartz. Slate is metamorphized shale. How many planes of cleavage
does slate have?
Marble. Dominant mineral is calcite. Marble is metamorphized
limestone.
Gneiss. Dominant minerals are mica quartz, and feldspar. Gneiss is
metamorphized granite or slate.
Metamorphic Rocks
Sedimentary Rocks start as a collection of lose
mineral grains.
Pressure pushes the grains closer together,
eliminating void spaces. There is no change to
the grains themselves yet.
Additional pressure changes the shape of the
grains to form an interlocking lattice.
Grain sizes change as many small grains reform
into large ones. There may or may not be a
change to the chemical composition. Note the
decrease in volume as the small grains
recrystallize into larger ones.
The Rock Cycle
All three rock types are linked by the rock cycle.
Teachnet-lab.org
Wordpress.com
But Wait!
• The schematics on the previous page
describe how the rock-types are linked by
processes, but it does not indicate:
– How does sedimentary material get transported to the
deep interior where it can be metamorphized?
– What causes uplift to move the igneous and
metamorphic rocks to the surface to be weathered?
– Where does the energy come from to drive the cycle?
Somehow there are dynamical processes
moving material through the Earth. We’ll
explore this next time with Plate Tectonics