Rocks and the Rock Cycle

What is the rock cycle ?
Igneous rocks
Metamorphic rocks
Sedimentary rocks
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 ?

(Graphic 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.






Minerals

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











Igneous Rocks

Igneous rocks 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).



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)


Metamorphic rocks

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
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).

Vermillion 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 shales.  

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 inorganically.

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  plate boundary.