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Rocks and Minerals and
the Geosphere
The Geosphere
The geosphere is the portion of the Earth
system that includes the Earth's interior,
rocks and minerals, landforms and the
processes that shape the Earth's surface.
Earth’s Molten Stage
– During the early formation of the Earth it was molten
(liquid)
– During this early stage, the heavier elements, such as iron
and nickel, sank to the deeper interior of the Earth.
– This left a thin layer of lighter materials on the surface
that is now called the crust.
Rocks vs. Minerals
– The solid materials of the Earth’s crust and are known as
rocks and minerals.
– A mineral is solid inorganic material of the Earth that
has both a known chemical composition and a crystalline
structure that is unique to that mineral
– A rock is a solid aggregate (combination) of one or more
minerals that have been cohesively brought together by a
rock-forming process.
98% of the Earth is
made of 8
elements…
(Graph A) The
percentage by weight of
the elements that make
up Earth's crust.
(Graph B) The
percentage by weight of
the elements that make
up the whole Earth.
Minerals (mineral crystals)
• A mineral crystal is composed of a structural
unit that is repeated in three dimensions. (This
is the basic structural unit of a crystal of sodium chloride,
the mineral halite.)
• The structural unit for a crystal of table salt, sodium
chloride, is cubic, as you can see in the individual grains.
• These quartz crystals are hexagonal prisms.
• Compare the dark colors of the ferromagnesian silicates
augite (right), hornblende (left), and biotite to the lightcolored nonferromagnesian silicates.
• Compare the light colors of the nonferromagnesian silicates
mica (front center), white and pink orthoclase (top and
center), and quartz, to the dark-colored ferromagnesian
silicates.
Physical Properties of Minerals
– Color
• A visual measure.
• Not very useful for identification as color of minerals varies
considerably.
– Streak
• This is the color of the mineral when it is finely powdered.
• Rubbed across a piece of tile, leaving a fine powder of the
mineral on the tile.
– Hardness
• Resistance of the material to being scratched.
• Measured using the Mohs hardness scale, which compares the
hardness of the mineral to 10 reference minerals.
• (A)Gypsum, with a hardness of 2, is easily scratched
by a fingernail. (B) Quartz, with a hardness of 7, is
so hard that even a metal file will not scratch it.
MORE Physical Properties of Minerals
– Crystal form
• Related to the internal geometric arrangement of the atoms
that make up the crystal structure.
– Cleavage
• the tendency of mineral to break along smooth planes.
• Depends upon zones of weakness in the crystal structure.
– Fracture
• The broken surface is irregular and not in a flat plane.
Even more Physical Properties of Minerals
Luster
• Metallic – shiny like metal
• Pearly – multicolored like pearl
• Vitreous/glassy – light can pass through
• Earthy – like dirt
Density – ratio of the mass of a mineral to its volume.
• Depends on:
– Kind of atoms which make up the mineral
– How the atoms are arranged in the crystal lattice.
Influences on the mineral forming process
• Temperature
• Pressure
• Time
• Availability and concentration of ions that are in
solution
Bowen’s Reaction Series
Minerals at the top of the series (olivine, augite, and calcium-rich plagioclase)
crystallize at higher temperatures.
Later, the left-over magma cools and lighter-colored, less
dense minerals (orthoclase feldspar, quartz, and white mica) crystallize.
Rocks
- Elements are chemically combined to form
minerals then these minerals are physically
combined to form rocks.
- Three types = Igneous, Sedimentary, and
Metamorphic
Igneous Rocks
– Form from molten rock material
– Intrusive igneous rock
» Formed when magma cools deep within the
Earth’s surface
» Cools very slowly as it is in contact with molten
rock.
» Produces course-grained igneous rock.
– Extrusive igneous rock
» Magma that cools above the Earth’s surface.
» Produces fine-grained igneous rocks.
» This rapid cooling does not allow time for
crystals to form.
• Intrusive Igneous = Granite is a coarse-grained igneous rock
composed mostly of light-colored, light-density minerals.
The earth's continental areas are mainly made of granite and
by rocks with the same mineral composition of granite.
• Extrusive Igneous rock: This is a piece of obsidian, which
has the same chemical composition as the granite. Obsidian
has a different texture because it does not have crystals and
is a volcanic glass.
Sedimentary Rocks
– Form from material from previously existing rock
• Material is provided by weathering of previously
existing rock
– Chemical Sediments
• Dissolved rock materials that can combine together
– Clastic Sediments
• Clumped pieces of rocks that can be cemented
together
• This is a sample of breccia, a coarse-grained
sedimentary rock with coarse, angular fragments.
Compare the grain sizes to the centimeter scale.
• This is a sample of sandstone, a sedimentary rock that
formed from sand grains in a matrix of very fine-grained
silt, clay, or other materials. The grains in this sample are
mostly the feldspar and quartz minerals, which probably
accumulated near the granite from which they were eroded.
• This is a sample of limestone, a sedimentary rock made of
calcium carbonate that formed under water directly or
indirectly from the actions of plants and animals. This finegrained limestone formed indirectly from the remains of
tiny marine organisms.
Compaction helps form
sedimentary rocks
• As sediments are laid down grain by
grain, the mass becomes greater.
• The increasing mass of the sediment layer
above creates pressure on the layers
below.
• Eventually this pressure becomes great
enough to compact the existing layers into
a cohesive rock layer.
Cementation also helps form
sedimentary rocks
After, or during, the compaction process, the
spaces between the sediment particles become
filled with a chemical deposit.
This deposit holds the compacted layers into a
cohesive mass of sedimentary rock.
Metamorphic Rocks
– Rocks changed by heat and pressure due to:
• Movement of the Earth’s crust
• Heat generated by intrusion of hot magma
• Pressure can change rock by flattening, deforming, or
realigning mineral grains.
– Foliation
• When the pressure on flat crystal flakes tends to align the flakes
into parallel sheets.
• This is a sample of marble, a coarse-grained metamorphic
rock with interlocking calcite crystals. The calcite crystals
were recrystallized from limestone during metamorphism.
• This banded
metamorphic rock is
very old; at an age of
3.8 billion years, it is
probably among the
oldest rocks on the
surface of the earth.
The Rock Cycle
• The Rock Cycle describes the continually changing
structure of rocks.
– Igneous, sedimentary, or metamorphic rock are just
temporary stages in the continuing changes that all rocks
undergo.
This is a diagram of the rock cycle, which shows that
geologic processes act continuously to produce new
rocks from old ones.