The Rock Cycle

NOTE: This is sort of a review of basic geology, but it will reinforce some of the concepts as you think in terms of planetary processes. Look up words in boldface type, and visit all designated websites:

Three general types of rocks occur on the surfaces and within the crusts of terrestrial planets:

igneous, sedimentary, and metamorphic

The relative proportions of these rock types depend on internal and external processes for any given planetary body. Another rock type, breccia, is a major constituent of the lunar surface and other planetary surfaces that have experienced intense bombardment by bolides; however this specific type is often considered as either metamorphic or sedimentary due to its formation by reworking of pre-existing rock bodies (see below).

• Vist the U.S. Geological Survey website on Rocks and Minerals.

IGNEOUS ROCKS

Igneous rocks are those that are formed by the cooling of molten rock called magma. Magma is a 3-phase system of silicate liquid, solid minerals, and perhaps a vapor phase. It takes on various forms including pods, lenses, diapirs, veins, or dikes depending on viscosity, density, and the forces applied to it by surrounding rock. Magma genesis, i.e. partial melting and mobilization of rock, is a planetary internal process, one that depends on heat generation, pressure differentials or, in some cases, presence of water and other volatile compounds.Sometimes magma genesis involves complete segregation of liquid from the residual unmelted rock, whereupon the liquid begins to crystallize new minerals. Other times magma genesis involves mobilization of the liquid and some or all of the residual solid mass.

Beneath the surface magma tends to cool slowly which allows for the growth of large crystals within the rock. Rocks formed in this way are intrusive or plutonic rocks. Mineral growth is often enhanced by aqueous fluids that can help carry the chemical components necessary for rapid crystal growth. Common examples of plutonic rock are granite, diorite, gabbro, anorthosite, and syenite; the rock names depend on the relative proportions of constituent minerals and the bulk chemistry.

Magma that erupts onto the surface is called lava that cools very quickly because the temperature at the surface is relatively low, and crystals will probably cease growth because the liquid is quenched. Thus, an extrusive or volcanic rock is formed. Volcanic glass is a type of quenched lava, and typical volcanic rocks are rhyolite, basalt, and andesite.

Volcanic (extrusive) Rocks

Basalt: the most abundant extrusive rock type. It is found in all regions of the Earth and is known to exist on the Moon, Mars, Venus, and Mercury. A fine-grained dark rock made of plagioclase and pyroxene (often with olivine and Fe-oxides), it usually occurs as lava flows or tephra (fragmented lava that makes up cinder cones, spatter cones, etc). Basaltic magma is generated by partial melting of mantle rock, and rises through the crust to the surface.
	Basalt (with ultramafic nodules): has pieces of the mantle or lower crust from which it was derived or from the last place where the magma resided before erupting on the surface. The nodules are made of olivine and pyroxenes that are dominant minerals in the mantle and lower crust.
Obsidian (silicic black volcanic glass): This obsidian came from a composite volcano in the Cascade Range of Oregon. Obsidian is typically homogeneous, but it can have flow lineations and compositional banding. This specimen contains several spherulites, round marble-sized zones where the glass devitrified outward from a central nucleus of crystallization that contained a small amount of water.
	Volcanic glass with pumiceous fragments: The lava in this sample was broken into fragments as it expanded with volatiles (H2O vapor mostly) and chilled while the flow was moving. Interstitial material is continuous between the fragments and shows evidence of oxidation which causes a change in color to more red and orange hues.
Andesite: Volcanic rocks that have phenocrysts (crystals embedded in a fine matrix) are termed "porphyritic" and are common in magmatic arc terranes and illustrate multiple stages of crystallization.. Magma begins to crystallize soon after it is generated by partial melting. Crystals that form in the magma chamber are surrounded by finer groundmass that cooled rapidly. This sample is porphyritic andesite, and came from a lava flow that erupted in central Idaho about 50 million years ago. Andesite is named for the Andes Mountains in South America, where numerous volcanoes occur above a subduction zone. NOTE: Not all andesites look like this; many are dark, almost basalt-like, with fewer phenocrysts.

Plutonic (intrusive) Rocks

Diorite: The plutonic equivalent to andesite. Diorite and andesite have similar chemical components, but much different texture. Intrusive rocks are coarsely crystalline because they take a long time to cool within the crust relative to volcanic rocks that are erupted on the surface and cool quickly. This rock has plagioclase, quartz, pyroxene, Fe-oxide (magnetite), and minor K-feldspar).

Granodiorite: is similar to granite except that granite has more K-feldspar (orthoclase or microcline) than plagioclase, whereas granodiorite has more plagioclase than K-feldspar. Granodiorite is distinguished from diorite by having a substantial amount of quartz.

SEDIMENTARY ROCKS

Any type of rock that is exposed to the surface environment begins to weather, i.e. breakdown chemically or physically. Physical weathering breaks the rock into pieces that get smaller and smaller until they may eventually become sand, silt, or dust-sized particles. Chemical weathering is the process whereby minerals in rocks are dissolved or changed to other minerals, such as feldspar weathers to a clay mineral (both are alumino-silicates). Hydration, oxidation, and ion exchange cause chemical weathering. Weathering is a planetary surficial process that depends on the presence of an atmosphere or a hydrosphere.

For an excellent ADVANCED discussion of chemical weathering, read the following PDF file:
W. M. White's online geochemistry textbook, chapter 13.

(NOTE: When downloading PDF files, you may want to open in a new window to make sure that your Acrobat Reader is installed properly and linked to your browser.)

Sediment that weathers from exposed rocks is transported, either in solution or as suspended particles, to places where they become deposited. These particles of rock, and the chemical precipitates from concentrated solutions, are called sediment and when sediment becomes lithified (solidified), it forms sedimentary rock. Often, sedimentary grains are lithified by cementation, but they can become lithified by dewatering and compaction as they become buried deep beneath additional layers of sediment.

A special type of "weathering" involves mechanical processes along fault zones, whereby mineral grains are pulverized into smaller grains and dust. This is actually a type of metamorphism called cataclasis, and depends on forces related to tectonics (faulting). Another form of cataclasis is impacting, which involves planetesimal bodies such as asteroids striking the planet with enough energy to pulverize, or even melt, rock. Both faulting and impacting produce breccia, rock that is made up of smaller rock fragments and interstitial material. On planets such as the Moon and Mercury, the layer of "soil" on the surface is actually pulverized rock called regolith. Impacting is so dynamic that it can combine the processes of physical weathering, metamorphism and magmatism all in one event!

The grain size and form of the sedimentary deposit depends on the type of material supplied by the parent rock, the transport medium, and the environment of deposition. For example, compared to water, wind is less capable of carrying large sand grains, but the dispersal pattern of wind-carried material can be much broader than that of fluvial systems. Examples of sedimentary rocks include sandstone, siltstone, limestone, breccia, conglomerate, and shale.

			Limestone: is a rock formed by chemical precipitation of CaCO3 (calcite) often in the presence of marine organisms that supply calcite. This particular example has rather large fossils of organisms buried in limy muck that later compacted under the weight of overlying sediment.
Conglomerate: Much like a piece of road material or concrete, conglomerate (as the name implies) is a cemented mixture of rounded pebbles transported from various places. Rounding is evidence of stream transport that causes the sharp edges and corners to become worn down as the particle is bounced along with other particles in a stream.
			Sandstone: A common detrital rock, sandstone represents water-deposited beaches or wind deposited dunes and is made up of fine particles that are easily transported in a fluid medium. Siltstone and claystone are other examples of detrital rocks, but with smaller grainsize than sandstone. Detrital sediment is classified according to the size of the constituent particles in decreasing order: boulder, cobble, pebble, sand, silt, and clay. The large sized particles are transported and deposited during intense flooding or in steep, fast moving streams. The smallest sized particles are deposited only in nearly calm conditions, such as lakes and deep marine environments.
Ripples: are formations in fine-grained sediment caused by the movement of water or wind over a depositional surface. They can be asymmetrical indicating flow direction, or they can be fairly symmetrical indicating an oscillating current. These ripples occur in a slab of sandstone, which was probably deposited in a shallow area of a stream or along the margin of a large lake or sea.

METAMORPHIC ROCKS

If any type of rock is subjected to a change in temperature and/or pressure so that minerals in the rock are transformed to new minerals in a new P-T regime, it becomes a metamorphic rock. Elevated heat and pressure, perhaps by being near a body of hot magma or burial deep within an orogenic belt causes changes in the stability of minerals in rocks. Metamorphic rocks can also form by the addition or subtraction of chemical components, a process called metasomatism. Indeed, hot aqueous fluids can be generated by magma that heats groundwater or gives off primary water. Such hydrothermal fluids can alter rocks that surround intrusive bodies. This is how some economic mineral deposits are formed.

Examples of metamorphic rocks are gneiss, schist, slate, marble, granulite, amphibolite, and quartzite. Parent rocks (protoliths) for metamorphic rocks can be igneous, metamorphic, or sedimentary. The mineralogy of a metamorphic rock depends on temperature and pressure maintained in the metamorphic conditions, as well as the chemical composition of the protolith.

	Gneiss: is a high-grade metamorphic rock that has experienced intense recrystallization and segregation of compositional layers. This example is part of a formation made up mostly of schist, a foliated rock that usually contains micas oriented subparallel to one another. Foliation is an important texture in metamorphic rocks, caused by alignment of platy minerals such as mica, or compositional layering shown in this picture. It is the manifestation of directed stresses that occur within the crust.
NOTE: Tabular minerals will align perpendicular to directed pressure or recrystallize into new minerals aligned perpendicular to directed pressure. Imagine standing on a deck of cards which is initially standing on edge. The cards collapse and become aligned with the floor.
Metamorphic Rocks: occur in various textural forms, commonly with foliation or other evidence of intense directed pressures.
	Phyllite: is a weakly metamorphosed rock that was initially a shale or mudstone. This example contains mechanically aligned minerals that form crenulations due to variation in the direction of applied stress. Although this rock is low-grade, a similar rock could be metamorphosed to slate, schist, gneiss, or even granulite depending on the pressure and temperature (and time) of metamorphism.

The Rock Cycle

The three principal rock types and the ways they can be formed are shown in the following diagram commonly called The Rock Cycle. The cycle can take many shortcuts, depending on the local conditions, and not all processes will occur on other planets. It is interesting to note that classical depictions of the rock cycle have always left out impacting which is the most widespread process in the Solar System!

Important rock-forming minerals are silicates such as quartz, plagioclase feldspar, alkali feldspars, amphiboles, micas, pyroxenes, and olivine; plus lesser amounts of Fe -Ti oxides, clay and calcite. Clay is normally derived by weathering of feldspars and other alumino-silicates; sand and silt are relicts of pre-existing mineral grains that become broken down by mechanical processes (crushing and abrading); and calcite and other carbonates (as well as gypsum and salt) are minerals precipitated from solution by either chemical saturation or biological activity.

FAULTING and PLATE TECTONICS

Review the patterns of fault motion and the forces that are responsible for each one:

See Module 4 – Earth for details on faulting and plate tectonics. Learn to associate geologic processes observed on Earth with those on other planetary bodies.  Also visit Module 5 of ISU's Environmental Geology course on the web to see more information about earthquakes and plate tectonics, and a lot of informative images and diagrams.

Vocabulary Words to Know:

Glossary

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