Rocks and the Rock Cycle
What is the rock cycle ?
This lecture is designed as a brief introduction
to the concept of the rock cycle and the principal categories of rocks. The
material presented below is not meant as a comprehensive treatment of rocks
and the rock cycle, but is designed largely to provide necessary background
for other aspects of this course. If you wish to learn more about these
concepts and might enjoy learning more about minerals and rocks and how to
identify them, you should consider taking an introductory level geology class.
What is the rock cycle ?
taken from the Mineralogical Society of America Web site)
The rock cycle refers to the diverse set of natural processes that
lead to the formation and transformation of igneous, sedimentary, and metamorphic
rocks. A short list of such processes includes erosion and weathering,
sediment burial, seafloor spreading, volcanism, tectonism, sediment transportation
and cementation. Few or no rules apply to the rock cycle. For
example, sedimentary rocks can be formed from fragments of pre-existing igneous,
metamorphic, or sedimentary rocks, or any combination thereof, or can be
formed by organic or inorganic precipitation of common ions dissolved
in salt or fresh water. Igneous rocks can be formed via melting and
extrusion or intrusion of pre-existing sedimentary, metamorphic, and/or
igneous rocks. Metamorphic rocks can be formed by alteration of pre-existing
rock types through hydrothermalism, fault slip, or exposure to high temperatures
and pressures due to deep burial. The rock cycle operates in all geologic
settings, ranging from the deepest part of the oceans (where oceanic seafloor
reacts with seawater to form new "hydrated" minerals) to the highest peaks,
where glaciation erodes and transports pre-existing rocks to lower elevations.
Convection of the earth's mantle and its surface expression, plate tectonics,
give rise to a dynamic planetary surface, characterized by large scale
horizontal transport and vertical uplift and subsidence
of rocks. Exposure of rocks to the
atmosphere and hence to weathering and erosion result directly from plate
tectonic processes. These are the driving forces of the rock cycle.
The graphic below shows one example of how plate tectonics impacts
the rock cycle. As new oceanic crust is created at a seafloor spreading
center, the seafloor on either side of the spreading axis moves symmetrically
away from it. Black "smokers", which carry hot water away from nearby
sub-surface magma bodies near the seafloor spreading axis, create metal-rich
deposits on the seafloor. Wind-blown dust, fine-grained sediments
transported by turbidite flows, and microscopic shells of organisms that dwell
in the oceans create an ever-thickening sedimentary "ooze" on the seafloor.
As this "ooze" gets thicker over millions of years, the pressure
and temperature on the lower sediment layers increases. Water is squeezed
out and the sediments eventually lithify (turn to solid rock), thereby creating
a column of flat-lying sedimentary rocks on top of the igneous oceanic crust.
Artistic view of processes that create deep-ocean sediments, by
C. DeMets. The original paper version of this stunning color graphic
is now found in the New York Metropolitan Museum of Art and is considered
a late-20th century masterpiece . The blue fish and red snail are strongly
influenced by the neo-primitive style of Gaugin, and the dots that represent
the wind-blown dust and foraminferal shells recall the pointillist technique
perfected by Post-Impressionist painter Georges Seurat.
If motion of the oceanic plate eventually carries the crust and
sediments to a subduction zone (see below), most
sediments on top of the basaltic seafloor will be scraped off and added (accreted)
to the edge of the overlying plate. These sediments may fracture, fold,
and possibly metamorphose. Some sediment may be carried into the mantle
with the subducting plate. Partial melting of the mantle occurs above
the subducting plate in response to water that is released by the dehydrating
minerals in the subducted oceanic crust (see previous lecture). The
resulting magmas rise into the overlying plate. Magma that is trapped
deep beneath the surface cools slowly, giving rise to plutonic igneous
rocks. Other magma finds pathways to the surface, where it cools more
rapidly, giving rise to a variety of volcanic igneous rocks. These
igneous rocks are, in turn, weathered and eroded, transported by water and
wind, and deposited as sediments.
Nearly all rocks are composed of minerals, which are naturally occurring,
inorganic, crystalline substances with definite chemical compositions.
Crystalline substances are identified by the regularly ordered arrangements
of the atoms that construct them. The types of minerals that result
from a particular magma depend on the chemical composition of the magma,
and the temperatures and pressures that the magma experiences as it cools.
The size of the crystals for a given mineral depends strongly on how
quickly or slowly a magma cools. Slow cooling allows time for atoms
to arrange themselves into larger, visible crystals. Cooling near or
on the surface allows less time for atoms to form larger crystals, hence
many volcanic igneous rocks have crystals so small that they can be seen
only with a magnifying glass or microscope. Magmas that are erupted
into the atmosphere cool so rapidly that there is no time for crystal formation
- glassy materials form instead. The volcanic igneous rock obsidian,
shown below, is such a material - it has a disordered internal atomic arrangement
and reveals no crystals at any level of magnification. Obsidian has
exactly the same chemical composition as some other kinds of volcanic rocks
with easily seen crystals. The difference in their "textures" results
entirely from the difference in the rates at which their parent magmas cooled.
For a given magma body, minerals typically crystallize out of the melt in
a well established sequence that is known as the Bowen's reaction series.
At a basic level, minerals with the highest melting temperatures crystallize
before those with lower melting temperatures, although the actual process
is somewhat more complicated and beyond the level of this class.
Obsidian - a glassy volcanic igneous rock
Just to show that not all igneous rocks are either plutonic or volcanic,
the photo at the right shows one that exhibits characteristics of both.
Large greenish crystals, called phenocrysts, are clearly visible. These
crystals are surrounded by much smaller crystals. The phenocrysts formed
by slow cooling within a deeply buried magma chamber. Rapid eruption
of the magma brought these crystals to the surface where they were surrounded
by magma that cooled rapidly during the eruption. (Photo taken on beach
at north edge of Lake Huron, Ontario, Canada)
are rocks that have solidified from magma
. Magmas that cool
beneath the surface result in plutonic
igneous rocks (Pluto was the
god of the underworld in Roman mythology). Plutonic igneous rocks are also
referred to as intrusive
. Magmas that reach the earth's surface
are referred to as lava
Cooling of lavas results in the formation of volcanic igneous rocks
also referred to as extrusive igneous rocks. Seafloor spreading is
the process responsible for creating the largest volume of extrusive igneous
rocks. Unlike sedimentary and metamophic rocks, igneous rocks are not
directly created from the remnants of pre-existing rocks. Even if some
of the magma that forms a particular igneous rock might have come from a
pre-existing rock, no identifiable characteristics of the pre-existing rocks
are preserved in a newly cooled igneous rock.
The photo to the right
shows granite, a plutonic igneous rock. Notice that the rock contains
visible minerals, reflecting the slow cooling history of its parent magma.(Granite
boulder on beach at north edge of Lake Huron, Ontario, Canada)
The photo to the right shows a volcanic (extrusive) igneous rock. Minerals
that compose the rock are clearly much smaller and many could be seen with
a magnifying lens or microscope. (Photo taken on beach at north edge of
Lake Huron, Ontario, Canada).
Metamorphic rocks result from metamorphism (a changing of form) of pre-existing
rocks. Numerous factors determine the type of metamorphic rock that will
result from a pre-existing rock. These include the chemical composition
of the original rock, the temperature and pressure at which metamorphosis
occurs, the types of stresses that are imposed on the rock, the original
fabric of the rock, the presence or absence of water, and the amount of time
over which metamorphosis occurs. Several common types of changes experienced
by rocks that metamorphose are recrystallization of the original minerals,
changes in mineral types, changes in the rock fabric and rock hardness, and
even partial melting of the existing minerals. Metamorphism frequently
occurs deep within the crust or beneath mountain ranges, where high temperatures
and pressures provide favorable conditions for metamorphism.
One common type of metamorphic rock, gneiss, exhibits distinctive dark and
light flow bands that are consistent with plastic flow of the rock at deep
levels in the crust. This particular gneiss contains dark fragments
of a relatively unaltered igneous rock through which it has flowed. (Photo
taken on beach at north edge of Lake Huron, Ontario, Canada)
Sedimentary rocks are formed from cemented fragments of pre-existing rocks
or from particles that are precipitated organically or inorganically from
ions dissolved in water. Entire textbooks are devoted to describing this
economically important rock type. A short summary is presented below
and is, by necessity, incomplete.
Sedimentary rocks can be divided into two simple categories: clastic and
chemical. Clastic rocks are built from fragments (clasts)
of pre-existing rocks. For example, one of the most common sedimentary
rocks, a sandstone, consists of cemented quartz grains (and possible impurities
such as clay particles).
The picture at the left shows a quartz-rich sandstone from a beach on the
north edge of Lake Huron in Ontario, Canada. The image on the left
shows a microscopic image of a sandstone at varying magnification levels.
At the left, the upper one-half of this boulder consists of well-sorted,
small sand grains. The lower half consists of more poorly sorted pebbles
and grains. The poorly sorted mixture in the lower half qualifies this
part of the rock as a conglomerate, whereas the well-sorted character
of the upper half qualifies it as a sandstone.
As the most common mineral in continental rocks, quartz sand grains are being
transported and deposited in numerous geologic settings. Two are shown below.
Great Sand Dunes National Monument, Colorado. Geology by Light
Plane Web site, courtesy of Dr. Lou Maher.
Lower Wisconsin River. Geology by Light Plane Web site, courtesy
of Dr. Lou Maher.
Although sandstones frequently accumulate in horizontal layers, wind- and
water-deposited sandstones often exhibit cross-bedding. Cross
bedding preserves the tilted sides of dunes and sand ripples. Due
to the migration of sand dunes and ripples, cross-bedded sandstones are distinctive
and easily recognized. They are also excellent indicators of the local
environment in ancient times (flowing water, beach, desert).
Cliffs, Arizona, Photo credit– Thomas F. Osborne.
Cross bedding of these sandstones is exhibited
dramatically in this landscape, which has been sculpted by flowing water.
Clastic sedimentary rocks are typically named in reference to the size of
the rock fragments that compose them. For example, rocks with fragments
that are larger than 2 millimeters are typically named breccias or conglomerates
depending on whether the majority of the large clasts are angular or well
rounded. Rocks with average grain sizes between one-sixteenth of a
millimeter and 2 millimeters are called sandstones, and rocks with grain
sizes smaller than one-sixteenth of a millimeter are called siltstones or
Chemical sedimentary rocks form by precipitation of dissolved
ions in water-rich settings, either inorganically or through the actions
of organisms. For example, clams, oysters, and numerous other marine
organisms are capable of extracting carbonate and calcium ions that are dissolved
in water and constructing their shells from them. Examples of the shells
of microscopic marine organisms are shown below.
White Cliffs of Dover, England. English Channel. The cliffs consist
of thick deposits of the limestone shells of microscopic ocean-dwelling organisms
(see above). A close-up view of chalk shown below shows that individual
shells cannot be distinguished.
Rocks composed of calcium carbonate, whether organic or inorganic in origin,
are generically referred to as limestones. Limestones are also an extremely
common sedimentary rock. Limestones are generally classified as biochemical
or inorganic depending on whether they were precipitated by an organism or
How does rock type contribute to geological research ?
Although understanding and learning to recognize minerals and rock
types can be a satisfying experience, geoscientists are typically more interested
in the information that can be gleaned from rocks about the conditions under
which a particular rock formed. For example, sedimentary rocks often
reveal an enormous amount of information about ancient climate or the environment
of deposition. Similarly, the fabrics and minerals found in metamorphic
rocks can be used to characterize past periods of deformation, possibly related
to plate motions. Igneous rocks and their chemical compositions and crystal
sizes help to determine the locations of past volcanic or intrusive igneous
activity and whether the volcanism occurred along a convergent or divergent