Chapter 3 – Igneous Rocks






What is Magma?
o Completely or partly molten material which on coolikng solidifies to form
igneous rock
How does lava differ from Magma?
o Lava is magma that has reached the Earth’s surface; is no longer under
the crust.
How does the rate of cooling influence the crystallization process?
o Slow cooling permits ions to migrate freely until they eventually join an
existing crystalline structure
 Slow cooling  fewer but larger crystals
o Rapid cooling – ions quickly lose mobility
 Rapid cooling  solid mass of small intergrown crystals
o Nearly instantaneous cooling – no time for ions to arrange in ordered
crystalline network
 Unordered ions  glass
In addition to the rate of cooling, what two other factors influence the
crystallization process?
o The amount of silica present
o The amount of dissolved gases in the magma
The classification of igneous rocks is based largely on two criteria; name the
criteria.
o Texture
o Mineral composition
The statements that follow relate to terms describing igneous rock textures.
For each statement, identify the appropriate term:
o Openings produced by escaping gas
 Aphanitic – crystals so small that they cannot be seen without a
microscope
 Vesicular texture – voids left by gas bubbles that escape as
lava solidifies
 Happens when lava cools very rapidly near the surface
 Obsidian exhibits this texture
 Glassy texture – when unordered ions are “frozen” before
they are able to unite into an orderly crystalline structure
o A matrix of fine crystals surrounding phenocrysts
 Porphyritic texture – large phenocrysts surrounded y a matrix of
smaller crystals called groundmass
 Form over tens of hundreds of thousands of years
 Termed porphyry
o Texture in which crystals are too small to be seen without a microscope
 Aphanitic






o A texture characterized by two distinctly different crystal sizes
o Coarse-grained texture, with crystals of roughly equal size.
 Phaneritic Texture – when large masses of magma slowly solidify
far below the surface
o Exceptionally large crystals exceeding 1 cm in diameter
 Pegmatitic texture  Pegmatites
 Form in the late stages of crystallization ; consequence of fluid
rich environment that enhances crystallization
What does a porphyritic texture indicate about an igneous rock?
o That it was formed over a very long period of time and possibly changed
environments.
How are granite and rhyolite different? In what way are they similar?
o Granite is Phaneritic or course grained whereas Rhyolite is Aphanitic or
fine grained ; Granite is plutonic or intrusive and rhyolite is extrusive or
volcanic
o The are both Felsic or granitic rocks
Compare and contrast each of the following pairs of rocks:
o Granite and diorite
 Granite and Diorite are both phaneritic or coarse-grained rocks
 Granite is a Felsic or Granitic rock whereas Diorite is an
Intermediate or Andesitic rock
o Basalt and gabbro
 Basalt and Gabbro are both Mafic or Basaltic rocks
 Gabbro is a phaneritic or coarse-grained rock whereas basalt is an
aphanitic or fine grained rock.
 Gabbro is the intrusive or plutonic form of Basalt
o Andesite and rhyolite
 Andesite and Rhyolite are both Aphantic or fine-grained rocks
 Rhyolite is a Felsic or granitic rock whereas Andesite is an
Intermediate or Andesitic rock
How does tuff differ from other igneous rocks such as granite and basalt?
o It is pyroclastic or fragmental – composed of multiple types of
rock/minerals.
What is the geothermal gradient?
o Increase of temperature with depth
Describe the three conditions that cause rock to melt.
o Heat – causes melting but at higher temperatures at greater depth
because of a greater confining pressure
o Pressure – decrease in pressure causes decompression melting
o Volatiles – Introduction of volatiles or water can lower a rocks melting
point sufficiently; melting is not complete so partial melting produces a
melt made of the lowest melting temperature minerals (higher in silica
than original rock)




What is magmatic differentiation? How might this process lead to the
formation of several different igneous rocks from a single magma?
o It is the process of developing more than one magma type from a
common magma
o Crystal settling causes previously formed denser to settle from the liquid
portion at the bottom of the magma chamber; remaining melt may
solidify to form a rock with a different chemical composition.
Relate the classification of igneous rocks to Bowen’s reaction series.
o The mineral makeup of an igneous rock is determined by the chemical
composition of the magma from which it crystallizes; minerals with
higher melting points crystallize before minerals with lower melting
points and the reaction series illustrates the sequence of mineral
formation – those minerals that form at similar melting points will
typically be found in together in the same igneous rocks
What is partial melting?
o Partial melting produces a melt made of the lowest-melting temperature
minerals
How does the composition of a melt produced by partial melting compare with
the composition of the original rock?
o The composition of the partial melting material is much higher in silica
than the original rock; magmas generated by partial melting are nearer to
the felsic end than the rocks from which they formed.
Chapter 5 – Weathering and Soils







If two identical rocks were weathered, one mechanically and one chemically,
how would their products differ?
o The rock that was mechanically weathered would form a product
identical to the original rock. Mechanical weathering merely breaks rock
into smaller fragments
o The rock that was chemically weathered would form a product that is
chemically different than the original rock
How does mechanical weathering add to the effectiveness of chemical
weathering?
o Mechanical weathering breaks the rock into smaller pieces thus creating
more available surface area for chemical attackers
Describe the formation of an exfoliation dome. Give an example of such a
feature.
o The processes of sheeting and unloading form exfoliation Domes.
Sheeting occurs when large masses of igneous rock (granite) are exposed
by erosion, slabs begin to break loose. This sheeting happens because of
unloading or a reduction in pressure when overlying rock is eroded away.
The outer layers expand more than the rock below and separate.
Weathering causes the slabs to fall of creating exfoliation domes like
Liberty Cap
Granite and basalt are exposed at the surface in a hot, wet region. Which type
of weathering will predominate? Which of the rocks will weather most
rapidly? Why?
o Chemical weathering will predominate
o Basalt will weather more rapidly because it is lower in the stability scale
Heat speeds up a chemical reaction. Why then does chemical weathering
proceed slowly in a hot desert?
o There is a lack of available moisture
How is carbonic acid formed in nature? What results when this acid reacts
with potassium feldspar?
o Carbon dioxide is dissolved in water; rain dissolves some carbon dioxide
as it falls through the atmosphere and additional amounts released by
decaying organic matter are acquired as the water percolates through
the soil
o It produces clay mineral, [potassium ions, bicarbonate ions, and silica
ions] in solution
Relate soil to the earth system
o It is an interface or a common boundary where different parts of a
system interact
o Soil forms where the geosphere, the atmosphere, the hydrosphere, and
the biosphere meet


o Soil is dynamic and sensitive to every aspect of its surroundings so any
change in environment results in a change in the soil characteristics until
equilibrium is reached again.
What factors might cause different soils to develop from the same parent
material or similar soils to form from different parent materials?
o Time – if weathering has been going on shortly, the soil will still greatly
resemble the parent material (and vice versa)
o Climate – variations in precipitation and temperature determine whether
chemical or mechanical weathering predominates as well as the rate and
depth of weathering. Controls leaching as well as plant and animal life
o Plants and Animals – the biosphere; types and abundance of organisms
affect physical and chemical processes.
Which control of soil formation is the most important? Explain.
o Climate is the most influential. Temperature and precipitation influence
the predominance of either chemical or mechanical weathering (thus the
chemical composition of the soil). Influences rate and depth of
weathering (thick or thin layer of soil) This also decides the degree to
which materials are removed from the soil affecting its fertility.
Determines plant and animal life.
Chapter 6: 13-18



What is Diagenesis? Give an example.
o Collective term for all of the chemical, physical, and biological changes
that take place after sediments are deposited and during and after
lithification.
 Example: recrystallization- the development of more stable
minerals from less stable ones (aragonite the less stable form of
calcium carbonate)
Compaction is most important as a lithificaton process with which sediment
size?
o Fine grained sedimentary rocks because sands and other coarse
sediments are less compressible.
List three common cements for sedimentary rocks. How might each be
identified?
o Calcite – effervesces with dilute hydrochloric acid
o Silica – forms the hardest sedimentary rocks
o Iron oxide – produces an orange or dark red color



Distinguish between clastic and nonclastic textures. What type of texture is
common to all detrital sedimentary rocks?
o Clastic: broken; rocks with a clastic texture consist of discrete fragments
and particles that are cemented and compacted together
 Some spaces between particles remain
 All detrital rocks are clastic
o Nonclastic/crystalline: minerals form a pattern of interlocking crystals
(may be microscopically small or visible) – ex: evaporates.
 Intergrown crystals
What is probably the single most characteristic feature of sedimentary rocks?
o Strata or beds since sedimentary rocks form as layer upon layer of
sediment accumulates.
Distinguish between cross-bedding and graded bedding.
o Cross bedding – sediments accumulate in layers that are inclined to the
horizontal
o Graded beds – particles within a single sedimentary layer gradually
change from coarse at the bottom to fine at the top.