Rocks Identified on the basis of composition and texture (arrangement of features). Classification depends on description and interpretation of these features. Three major categories: 1) igneous = fiery 2) sedimentary = settled 3) metamorphic = changed form
The Rock Cycle
Crustal Abundances of Rock Types
Igneous Rocks Form by the cooling and hardening (crystallization/glassification) of magma. Most magma crystallizes before it can reach the surface, producing bodies called plutons made of intrusive (plutonic) igneous rock. Some magma (known as lava) reaches the surface while still at least partially molten, producing volcanic eruptions and extrusive (volcanic) igneous rocks.
Plutons Classified by shape and relationship with surrounding (country) rock into which the intrusion occurred. Two Main Categories: Concordant (parallel with surrounding layers) such as sills, laccoliths, and lopoliths Discordant (cuts across layers) such as dikes, batholiths and stocks
Common Types of Plutons
Stocks are irregular-shaped plutons that have areas <100 km 2. The stock shown above has deformed the layered country rock.
Dikes are tabular intrusive bodies that are oriented perpendicularly or obliquely to layers of country rock.
Volcanic necks are the central portion of the frozen plumbing system of a volcano. Over time, the less resistant extrusive rock of the volcano s flank is eroded away leaving the more resistant neck exposed in relief.
Shiprock, New Mexico is a classic example of a volcanic neck. Note radial dikes extending from the neck. Ring dikes (concentric circles) are also possible.
Igneous Composition and Texture A magma is a multi-component material with a bulk composition which almost always changes as it moves and cools. The resulting rocks provide clues to the processes which shaped them: Composition: types and abundances of different minerals and nonminerals Texture: sizes, shapes, and boundary relationships of the mineral grains and other components
Igneous Textures Slow cooling produces large grains, rapid cooling produces small (or no) grains. Practical Differences: Phaneritic ( obvious ): visible to unaided eye, also called coarse-grained. Usually intrusive. Aphanitic ( not obvious ): crystalline, but not visible, also called fine-grained. Usually extrusive. Glassy: not crystalline. Extrusive. Porphyritic ( purple ): coarse grains (phenocrysts) surrounded by fine grains (groundmass). Began crystallizing underground, then erupted and finished solidifying on surface. Extrusive.
Gabbro Diorite Granite Phaneritic igneous rocks crystallize slowly (usually underground). Chemical composition also plays a role in determining the specific rock type.
Phaneritic grains are distinguishable to the unaided eye. This rock contains quartz (light gray), plagioclase feldspar (white) and biotite (black) crystals.
Aphanitic rocks contain mineral grains which are too small to distinguish clearly with the unaided eye. Same magnification as the previous image.
Obsidian has a glassy texture. It may contain a few isolated mineral grains or even an abundance of submicroscopic crystal seeds (crystallites), but it is mostly amorphous, lacking the long-range order of crystal structure.
Porphyritic rock is partially coarse and partially fine. The large phenocrysts formed first, slowly, in the subsurface, whereas the groundmass crystallized quickly after eruption onto the surface.
Another Igneous Texture Violent volcanic eruptions produce an explosive spray of lava which hardens (at least partially) while in flight. The resulting fragments may or may not weld to one another upon landing, but usually retain the outlines of their initial crusts. This texture is called pyroclastic ( broken by fire ). Individual particles range from dust-sized, called ash, to building-sized, called bombs, and are typically a mixture of minerals and glass.
A large pyroclastic eruption of Mount Pinatubo in the Philippines (1992). The ash and other volcanic derived clasts can become welded together to form finegrained tuff or coarse-grained volcanic breccia.
Volcanic ash (tephra) derived from the Mount Mazama (Crater Lake, Oregon) eruption 6800 years ago.
One Last Aspect of Texture As a magma approaches the surface, it undergoes decompression and cooling. This decreases its ability to hold various gases (H 2 O, CO, CO 2, etc.) in solution. These gases will separate as bubbles which will either escape or remain trapped as the magma hardens around them. Trapped bubbles are called vesicles and the rock texture is said to be vesicular.
Pumice (shown) or scoria (darker) form when gas bubbles are trapped in rapidly cooling pyroclastic materials. The rocks are glassy and frothy.
Igneous Composition Various igneous environments will produce magmas which differ in silica content and the abundances of metals such as Fe, Mg, Ca, Na, and K. Mafic: poor in silica (~50%), rich in Fe, Mg, Ca, poor in Na and K, typically dark or green minerals. Felsic: rich in silica (~70%), poor in Fe, Mg, Ca, rich in Na and K, typically light or pink minerals. Intermediate: between mafic and felsic. Ultramafic: beyond mafic, even more mafic than mafic
Magmas are subdivided largely by silica (SiO 2 )content. As silica content increases, iron (Fe), magnesium (Mg), and calcium (Ca) content decreases. Note that lighter elements, such as sodium (Na) and potassium (K) content follow the silica trends. Chemical compositions are often described in terms of oxide composition as a way of keeping track of the abundant oxygen in these rocks.
Gabbro Diorite Granite Examples of mafic (gabbro), intermediate (diorite), and felsic (granite) compositions.
Typical Magmatic Sources The mantle is ultramafic. Unusually extensive melting will produce ultramafic magmas, but routine partial melting produces mafic magmas. Partial melting of subducting oceanic crust (mafic) and its associated sediments produces mafic and intermediate magmas. Interaction with continental material is required for the production of felsic magmas.
Trends with Composition Mafic (Basalt/Gabbro) Density about 3.3 g/cm 3 Crystallization ~1200 C Low viscosity Typically mild eruptions Felsic (Rhyolite/Granite) Density about 2.7 g/cm 3 Crystallization ~700 C High viscosity Typically violent eruptions
Mafic lavas often erupt in a gentle fashion and flow great distances across the landscape. Their low viscosities make it less likely that gas pressure will build to the point of explosiveness.
Intermediate and felsic lavas often erupt with great violence in large part because gases cannot easily escape them. When they do not explode, they instead ooze slowly and do not travel far.
Even when molten, the silicate tetrahedra will polymerize into chains. These will become entangled and thereby inhibit flow. Over the seemingly modest range of 50% to 70% silica content, this extent of tangling results in a change of about seven orders of magnitude in viscosity, or a factor of around 10,000,000. Mafic (basaltic) magmas can flow almost like water. Felsic (rhyolitic) magmas are far more sluggish than toothpaste.
Changes in Bulk Chemistry Further complications arise if materials are removed during solidification. Several fractionation processes: 1) Gravitational settling of initial solids 2) Flow segregation as the magma moves 3) Filter pressing of residual fluid 4) Loss of volatiles (water, gases) along with readily-dissolved elements which don t fit well in the crystallizing silicate minerals
Differentiation of magma can occur from fractional crystallization involving the removal of crystals as they accumulate. The solid phase will have a composition that is relatively more mafic than the remaining melt phase.
Felsic Volcanism Most felsic magma does not reach the surface, but the localized heating of continental crust can produce large outpourings of material, such as at continental hotspots. Some felsic melt is produced at continentalcontinental collision zones, but the overthickened crust impedes its ascent.
Granitic composition magma reaches to the surface in Yellowstone Park because the continental crust is being heated by upwelling magma generated from an asthenospheric hotspot.
The Yellowstone Caldera (Wyoming) formed following a very large eruption ~600,000 years ago. The rhyolite flows are very viscous and internal gas pressures can be very high.
As the North American plate passes over the Yellowstone hotspot, the area affected appears to move across the continent. Throughout the last 17 Ma (million years), a major eruption has occurred every few hundred thousand years.
Area of significant ash deposits from the last major Yellowstone event, about 630,000 years ago.
Intermediate Volcanism Ocean-ocean subduction zones will produce magmas ranging from mafic to intermediate. At ocean-continent subduction zones, the opportunity for interaction with the felsic continental material leads to magmas spanning the entire range from mafic to felsic, but intermediate (andesitic) magmas are the most abundant.
Intermediate (andesitic) magma is produced from a partial melt of oceanic crust along subduction zones. The presence of water from the subducting plate lowers the melting point of the upper mantle, as well. Mixing with the overlying continental crust (if present) also alters the composition of the final product.
Stratovolcanoes (composite cones) form from interlayered pyroclastic and intermediate (andesite or dacite) lava flows. Stratovolcanoes are poorly consolidated and therefore erode rapidly.
Volcanic breccia forms from a mixture of welded volcanic clasts, ash, and mud.
Eruptions along the flank of a stratovolcano can occur where extensional cracks develop due to magma upwelling within the magma chamber. Over time, these magmas typically become more silica-rich through fractionation and the volcano s life cycle can end with a cataclysmic eruption and emptying of the magma chamber.
The steepness of stratovolcanoes is controlled by the stability of their unconsolidated pyroclastic layers and the viscosity of their silica-rich andesite and dacite flows. Shown above are part of the Aleutian chain in Alaska.
The High Cascades are typical stratovolcanoes with slope angles between 25-35. Rainier is shown in the foreground with St. Helens to the upper right. Adams and Hood are in the background. Note the deep erosion of Rainier.
Mafic Volcanism Mantle plumes (hot spots) produce large quantities of mafic magma. Divergent boundaries (mid-ocean ridges) also give rise to mafic activity. Continental rifting may produce flood basalts.
OCEANIC CRUST MANTLE Shield volcanoes are constructed by successive basalt flows. In the case of the Hawaiian hotspot, this has produced the largest volcano on Earth.
The morphology of a volcano is strongly controlled by the viscosity of the eruptive product. The Hawaiian shield volcano shown above is composed of interlayered basalt flows. Slope angles typically range between 7-10 for shield volcanoes.
At mid-ocean ridges, the rising mafic magma will intrude as a series of parallel dikes, resulting in a sheeted dike complex. Above this, the erupted lava will develop a distinctive pillow texture from rapid cooling as it reaches the sea water. A cross-section through oceanic crust will show a uniform sequence of layers known as an ophiolite.
The Columbia River Basalts (CRBs) erupted over an interval ranging from approximately 17 to 6 million years ago. They cover 200,000 square kilometers and are in places up to 4 km thick. Thousands of individual flows poured from a series of fissures (cracks) which themselves are buried under the basalt.