2006 Historical Geology - Part III
Outline of Historical Geology by Ellin Beltz
Part I
Introduction, Environment, Stratigraphy
Part II
Taxonomy and Taphonomy
Part III
You are Here
Rock Cycle
Part IV
Plate Tectonics
Part V
A brief history of Earth
2006 by Ellin Beltz

Historical Geology - Part III - Rock Cycle

Review of the Rock Cycle

Rocks record their environment of formation and reveal the Earth's history. All the rocks of Earth's crust can form or be formed from other rocks.

If you're rusty on this topic, click here to review the Basic Rock Cycle before advancing to this slightly more advanced material.

Composition of crustal rocks is dependent on the abundance of elements in the earth's crust. By descending weight percent these elements are:

ElementPercent
Oxygen 46.2
Silicon 27.7
Aluminum 8.1
Iron 5.0
Calcium 3.6
Sodium 2.8
Potassium 2.5
Magnesium 2.1
Other elements 1.7
Total 100%

The uppermost mantle is considered to be mafic and ultramafic, except where blobs of subducted plates may be changing the mineralogy in localized areas. Since the early Earth is considered to have been hotter and the crust to have formed as a froth on the surface of the hot liquid, igneous rocks are discussed first.

Igneous rocks
form from hot magma. The source of the magma determines the chemical composition of the rock (mafic, intermediate, or felsic). Mafic rocks form from mafic lavas associated with a deep mantle source. Felsic rocks form from reworking and recycling of primarily continental sedimentary material. Intermediate rocks often form by a mafic lava travelling upward through a felsic continent, melting and diluting the magma as it travels upward. Igneous rock texture (big crystals, little crystals, no crystals and/or glassy) is a function of cooling time and the amount of water present in the magma. For example, granite is classified as a felsic rock with an intrusive texture (crystals >~5mm) while andesite is an intermediate extrusive rock with tiny crystals formed during rapid cooling following volcanic eruption.

The intrusive igneous rocks are gabbro (mafic), diorite/diabase (intermediate) and granite (felsic). All cooled slowly and have large crystals because their source magmas never reached the surface. The extrusive igneous rocks are basalt (mafic) including the Hawaiian lavas pahoehoe/aa and pillow lavas, andesite (intermediate) and rhyolite (felsic).

Mafic rocks are high in iron and magnesium minerals, primarily Olivine, Pyroxene, Amphibole/Hornblende and aluminosilicate minerals like Calcium Feldspar/Labradorite and Calcium/Sodium feldspar mixes. Felsic rocks contain fewer iron magnesium minerals but are high in aluminosilicate Sodium Feldspar/Albite and Potassium Feldspar/Orthoclase, as well as sheet silicates/Biotite and Muscovite Micas and pure silica tetrahedrons/Quartz. Intermediate rocks contain some mafic minerals and some felsic minerals.

The most notable igneous features are active volcanos and igneous structures exposed by erosion like necks, dikes, sills, batholiths and plutons associated with extinct volcanos. Volcanos erupt both on land and under the ocean. In 1996, a volcano erupted under a glacier in Iceland causing immense damage and drainage rates from subglacial rift zone in Antarctica are being monitored. Volcanos erupt magma and deposit ash, tuff, tufa, lava, water and gases. When the heat source for the magma is lost, fluid rock solidifies slowly, creating large crystal intrusive rocks. Radial stream drainage is a volcanic feature controlled by steep slope angle and soft rocks. Exposed batholiths and plutons are often called "elephant rocks" or "jumbo rocks" as they tend to joint and weather roundly before eroding away.

Sedimentary rocks
form from chemical and physical particles of other rocks and sediments. Wind and water weather and erode rocks and sediments at the Earth's surface. Water and wind may act as agents of transport, moving particles far from their source. Sediments laid down in a "depositional environment" may lithify to sedimentary rocks if they are subjected to the forces of compaction and cementation. One importance of sedimentary rocks to plate tectonics is that when deposited in an environment which eventually obducts or subducts, the metamorphosed sediments record the previous environment. "Suspect terranes," foreign rocks obducted onto continents are often composed of basalt platforms with associated sedimentary rocks. The major importance of sediments to plate tectonics, however, is that they record environments which - although not directly impacted (or squished) - were affected by weather changes and current changes associated with the changing face of the Earth.

Sediments and sedimentary rocks are classified primarily by particle size and/or chemical composition. For example, red sandstone is composed of sand sized particles of silicon dioxide (Quartz) with iron oxide in the cement; while limestone is a chemical sedimentary rock composed of calcium carbonate (Calcite).

There are two types of weathering:

  1. Physical weathering is the process by which large rocks are made into small rocks. It includes jointing, frost wedging, exfoliation weathering, and the actions of plants and animals.
  2. Chemical weathering results in ions and molecules from the parent rock being dissolved and carried away in solution.

The type of weathering influences the sediments formed:

Physical weathering MineralChemical weathering
sand-sized particlesQuartzsoluble silica
clay-sized particlesFeldspars/micassoluble salts/soluble silica
black sandsFe+/Mg+soluble Fe+ and Mg+ ions

Wentworth's classification of particle sizes is used for both sediments and clastic sedimentary rocks. Particles sized larger than 256 mm are considered boulders, from 256 to 64 mm are cobbles, from 64 to 2 mm are gravel, from 2 mm to 1/6 mm are sand, from 1/16 to 1/256 mm are silt, and less than 1/256 mm are clay-sized particles.

Rocks formed from large particles are called conglomerate if the rocks are rounded and breccia if the rocks are broken but not rounded. The rounding results from transport. Smaller particle sizes (sand, silt, and clay) result in fine-grained sedimentary rocks (sandstone, siltstone, and claystone). If there is a mix of fine particle sizes, adjectives can be used to describe the stone, thus a sandy siltstone could be called an "arenaceous siltstone," and a clayey sandstone can be called and "argillaceous sandstone."

Chemical sediments are usually smaller than clay-sized particles and are primarily carbonates (xCO3) where x is a cation like iron (Fe+), sodium (Na+), calcium (Ca+), or a combination like calcium/magnesium (Ca/Mg). Chemical sedimentary rocks may contain fossilized hard parts of plants and animals, their clasts, shells, tests and communal dwellings, reefs and burrows, or they may be formed entirely of chemically precipitated calcite if they formed at a depth below which calcite is insoluble in seawater.

The carbonates that form from iron, sodium, calcium and calcium/magnesium cations include:

Composition Formula Mineral name Rock name
iron carbonateFeCO3SideriteIronstone
sodium bicarbonateNaHCO3Baking sodaNahcohlite
calcium carbonateCaCO3CalciteLimestone
calcium, magnesium carbonate(Ca, Mg)CO3DolomiteDolostone

Rocks formed primarily of carbonates are sometimes called "calcareous rocks." Siderite concretions are found at Mazon Creek, IL and in exposures in Kansas which contain perfectly preserved plant and animal fossils. Limestone and dolostone exposures contain fossilized remains of hard-shelled animals from the last 600 million years of earth history.

Sulfates and evaporites including halites form when saline or briny waters evaporate and deposit their dissolved solids as minerals or rocks. The dry saline lakes of the American west are major sources of evaporite minerals and thick salt deposits of Silurian Age are below the state of Michigan.

Composition Formula Mineral name Rock name
hydrated calcium sulfateCaSO4 & 2H2OSeleniteGypsum
sodium chlorideNaClHaliteRock salt

Oxides form during weathering as corrosive Oxygen from the atmosphere reacts with ionic iron (Fe+) and water (H2O) reacts with ionic aluminum (Al+) and iron (Fe+). The "desert varnish" found on rocks is an Iron/Manganese dioxide formed by the reaction of mafic minerals in the rock with air and water. Other oxides that are currently forming include Limonite and Water. Some oxides cannot form under current conditions. Most notably, the banded iron formations in the upper peninsula of Michigan formed when the oxygen level in Earth's atmosphere was much lower. Photosynthesis of bacteria and plants changed the atmospheric oxygen levels sufficiently to change aqueous and rock geochemical reactions.

Composition Formula Mineral name Rock name
iron oxide Fe2O3 Hematite Iron ore
iron oxide Fe3O4 Magnetite/Lodestone Iron ore
hydrated iron oxide 2Fe2O3 & 3H2O Limonite Rust/iron ore
hydrated aluminum oxide Al2O3 & 2H2O Bauxite Aluminum ore
aluminum oxide Al2O3 Corundum Ruby/sapphire

The siliceous chemical sedimentary rocks include chert, flint, jasper, chalchedony, agate and opal. They are all composed of silica tetrahedra of quartz. The varying colors are due to impurities in the crystal matrix.

The final category of sedimentary rocks are organic sedimentary rocks which include the carbonaceous black shales and the carboniferous peats and coals.

Sedimentary environments include all depositional environments from aeolian sand dunes to the radiolarian chert of the oceanic abyss. Each is governed by a unique series of physical factors recorded in the rocks. In a confined aquatic environment (ponds or lakes, in certain glacial environments, and under abyssal pressures) large particles settle first and the lighter sediments settle on top of them. However, if sediments are unconfined as in rivers and oceans, the largest particles settle first and the lighter particles are carried away by the current and deposited far from their source.

Sedimentary structures include mudcracks, cross-bedding, ripple marks, evaporite deposits, reefs and atolls, point bars, long-shore current islands, beaches, dunes, loess bluffs, moraines, outwash fans, braided rivers, talus, alluvium, till, drift, varves, kames, drumlins, lakes, streams and rivers.

Follow this link to my Outline of Sedimentary Rocks for a more detailed discussion of sedimentary processes.

Metamorphic rocks
form when parent rocks (or sediments) are subjected to any or all of the three agents of metamorphism: heat, pressure and chemically active fluids. Metamorphism may be localized or affect whole regions. Local metamorphism is often contact metamorphism - the boundary zone where a hot rock touches a colder country rock can be metamorphosed while nearby rocks are unaffected (skarn-type ore deposits). Large meteor impacts result in unique local metamorphic products like tektites and shock quartz. Regional metamorphism is a large-scale process usually resulting from the collision of continents.

Different rocks and sediments respond differently to metamorphic forces. Metamorphic rocks are defined by their parent material and the amount of deformation to which they have been subject. Thus, sandstone becomes quartzite under pressure while limestone becomes marble. These two metamorphic products do not show layering or banding of mineral grains and are called "unfoliated" metamorphic rocks. Foliated metamorphic rocks often appear banded and include: slate, schist and gneiss. Large crystals can form in foliated metamorphic rocks; garnets as large as ping-pong balls can be seen in museums while smaller garnets (iron aluminosilicates) are so plentiful in some mica schists that they are mined and extracted for sandpaper. Banded gneisses are commonly seen as building and tombstones. Their swirled appearance results from a near-melting of the parent material which then begins to separate out into mafic and felsic minerals.

Mafic rocks have a unique metamorphic sequence which begins with serpentine. More pressure turns serpentine to talc or jade depending on the amount of water trapped with the parent rock. Felsic rocks most often metamorphose from schist to gneiss. Sedimentary rocks may have overlapping metamorphic products:

Sedimentary rock Metamorphic product (low to high grade)
Conglomerate Meta-conglomerate, schist, gneiss
Breccia Meta-breccia, schist, gneiss
Sandstone Quartzite, mica schist
Shale/siltstone/claystone Phyllite, slate, schist, gneiss, "fubarite"
Limestone/dolostone Marble (color shows "argillaceous" or "carboniferous")
Black shale Black slate
Peat/coal Anthracite through graphite with or without associated natural gas, petroleum and asphalt; Mississippi-type lead and zinc deposits

Chemical metamorphic rocks and minerals include hydrothermal quartz veining and native metal/ore mineral deposition as well as regional lead/zinc and fluorite deposits.

CompositionMineral NameRock Name/Ore
Au Gold Gold Ore
Ag Silver Silver Ore
Cu Copper Copper Ore
Pt PlatinumPlatinum Ore
S Sulfur Native Sulfur
CaF2FluoriteFluorospar

Deposition of native metals, sulfur and fluorite appears to occur at relatively high temperatures associated with regional metamorphism. Native metals may also deposit in sulfide solutions (AgS silver sulfide and CuS copper sulfide being the more common). Native metals precipitate out of sea water around "black smokers," undersea volcanos associated with ridge spreading, and at the ocean water/hot rock boundary along island arcs. Many sulfides are transported in warm water (hydrothermal) solutions:

CompositionMineral NameRock Name/Ore
PbSGalenaLead
ZnSSphaleriteZinc
FeS2Pyrite/MarcasiteIron
(Cu,Fe)S2ChalcopyriteCopper

Pyrite and marcasite crystals and masses may occur in limestones and cherts. They may be iron and sulfur ions concentrated during anaerobic decomposition of plants and animals. Some fossils are "pyritized."

Carbonates and silicates can be reworked by warm water solutions:

CompositionMineral NameRock Name
SiO2 transparent...Quartz, rock crystal
SiO2 translucent...Jasper, flint, chert, quartz, chalchedony, agate, silicified fossils, petrified wood
CaCO3CalciteTravertine marble, geodes, stalactites/stalagmites

Regional metamorphic structures, the result of large-scale folding and faulting, exert regional influence on the landscape. Radial drainage is found on volcanic slopes. Rectangular and trellis drainage patterns result from direct bedrock control of topography. Canyon drainage results from regional uplift while dendritic drainage occurs in areas with a fairly uniform bedrock or sediment cover.

Major regional metamorphic structures include

Outline of Historical Geology by Ellin Beltz
Part I
Introduction, Environment, Stratigraphy
Part II
Taxonomy and Taphonomy
Part III
You are Here
Rock Cycle
Part IV
Plate Tectonics
Part V
A brief history of Earth
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2008 by Ellin Beltz -- January 10, 2008