IGNEOUS ROCKS

GEOL 1121 (Written by T. Weiland)

*All rocks were originally derived from igneous rocks. Elevated planetary temperatures during earth formation produced widespread melting. The study of igneous rocks not only provides important information about earth’s origin and evolution, but also records important data on more recent global volcanism and crustal instability.

I. Igneous Rock Definition - rocks formed by the cooling and solidification of magma. (Magma - naturally-occurring molten-rock material generated within the earth. Magma is made primarily of the elements found in the silicate minerals plus some gases.)

  1. Volcanic Rocks (Extrusive Rocks) - igneous rocks that form by the extrusion and cooling of magma on the earth's surface. They can be recognized by the fine-grained or glassy nature of the portion of the rock that cooled rapidly as it was extruded. Lava - another name for fluid magma which reaches the surface before it is totally solidified.
  2. Plutonic Rocks (Intrusive Rocks) - igneous rocks which form by the solidification of magma beneath the surface (or within the earth). Named after Pluton, god of the underworld. These are only seen at the surface today (like the granite at Stone Mountain) because of extensive erosion and/or uplift. Plutonic rocks are recognized by the presence of visible crystal throughout the rock.

II. Origin of Magma - Scientists originally believed that the interior of the earth was totally molten; however, an abundance of evidence now indicates that earth is almost totally solid down to the outer core. It appears that magma occurs as liquid segregations in localized areas (vein-like) throughout the upper part of the mantle and parts of the lower crust.

  1. Major Factors Which Cause Melting (Melting - the change from solid to liquid). Melting is most commonly caused by heating which increases ion vibration until the chemical bonds are broken.
    1. Temperature Changes - higher temperatures cause melting. Rocks are poor conductors of thermal energy so much of that original heat remains. Temperature could be increased by moving material deeper within the earth or by increasing the temperature of existing levels. The geothermal gradient is the increase in temperature with increasing depth. The geothermal gradient in the upper crust is approximately 25° C/Km.

    Sources of Heat

    1. Residual heat remaining from earth’s formation
    2. Heat given off from naturally-occurring radioactive elements
    3. Frictional heat from colliding plates (minimal)
    4. Solar heat(minimal)
    1. Pressure Changes - lower pressures generally decrease melting temperatures, such that rocks which are stable deep within the earth at high pressures become unstable and melt when moved upward in areas of lower pressure. Source of Pressure – weight of the overlying rocks
    2. Changes in Water Pressure - the addition of water generally lowers the melting temperature of rocks. The release of water from hydrated oceanic crust which is subducted along active margins is believed to cause melting of overlying rocks which were originally stable.Source of Water – subducted oceanic crust
  1. Major Areas of Magma Generation
    1. Mid-oceanic Ridges - hot mantle rock material is upwelled along elongate ridges forming new oceanic crust. Melting is largely due to decreased pressure in these areas of divergent (pull-apart) oceanic crust.
    2. Subduction Zones - oceanic crust is pushed beneath other oceanic or continental crust. Melting is largely due to water that is derived
    3. from subducted oceanic crust.
    4. Rifted areas - where continental crust is being pulled apart, molten mantle rock is often erupted. Melting is a result of elevated crustal temperatures and decreased pressures associated with the rifting.

III. Crystallization - change from liquid to a solid. Generally long-term (lava flows take years to hundreds of years, while intrusions take thousands of years). Different minerals form and react during cooling because they have different crystallization temperatures and stability ranges.

  1. Factors that Control Crystallization - Crystallization involves the arrangement of ions into an orderly atomic pattern. Requires slow cooling and low to moderate viscosity (magma thickness). Vibrational energy of the atoms is much higher at higher temperatures, therefore crystallization can't occur until some of this energy is released. If the energy is released too fast, the magma is solidified into a glass, a material without an ordered atomic arrangement. Melting is the opposite of crystallization - energy is added until the atomic bonds are broken.
    1. Cooling Rate - most important - rapid cooling leads to fine-grained or glass-rich rocks (glass - amorphous solid w/out an ordered atomic arrangement). Slow rates of cooling are required for coarse-grained crystalline rocks to form (ex. Stone Mt. probably took thousands of years to crystallize beneath the surface). Crystallization requires a slow decrease in thermal energy for extensive ion migration (movement of charged atoms) and nucleation (initial growth of seed crystals).
    2. Viscosity of the Magma - Viscosity is the resistance to flow. If the magma is too viscous, internal friction prevents ion migration.
  1. Mineral Reactions During Magma Cooling
    1. Continuous Reaction Series – Several minerals will display solid solution, a process where solid crystalline material will change chemical composition during cooling or heating like a liquid solution can change by adding or removing elements. Solid solution mineral series are define by end members that crystallize over a range of temperatures and which continuously change in composition as the temperature decreases (continuous reaction with the melt). Example - Anorthite (1553° C.) and Albite (1118° C.) Na exchanges for Ca in the crystalline structure. This involves a continuous reaction with the melt.
    2. Discontinuous Reaction Series – Several early-formed minerals will become unstable and react with the melt to form more stable lower-temperature minerals as the magma cools. Example - Olivine reacting to form pyroxene: Mg2SiO4(olivine) + SiO2(in melt) ----® 2MgSiO3 (pyroxene)
    3. No Reaction – Some minerals are stable over a large temperature range and will not chemically react with the magma unless they melt. An example is quartz.
    4. Bowen's Reaction Series - simplified sequence in which minerals crystallize in a magma as temperature drops. See your notes and textbook for a better description and diagram. *The series is not true in all magmas because temperature might not reach the lower range and compositional, pressure and viscosity differences also control mineral stability.
  1. Magmatic Differentiation - Basalt magma appears to be the parental composition of most igneous rocks. There are several important processes which subsequently change the magma composition and result in the great variation seen in the composition of igneous rocks.
    1. Assimilation - melting of the wall rock which surrounds the magma (wall rock must be a different composition).
    2. Crystal Settling - gravitational settling of the early-formed denser crystals that changes the composition of the residual magma.
    3. Magma Mixing - magma is mixed with other magma that enters the magma chamber.

IV. Classification of Igneous Rocks -based on texture, composition and stratigraphic (field) relationships.

  1. Texture - the general appearance or character of the rock. This includes the grain size, shapes, and the arrangement in the rock. **Records the cooling and crystallization history of the rock.
    1. Aphanitic - crystals are too small to be seen with the unaided eye. These rocks must be studied with the microscope or by geochemical methods. Volcanic rocks (extrusive) are at least partially aphanitic due to rapid cooling. Glassy - lava which cools so rapidly that the crystals don't have time to grow. (unordered atomic arrangement) Example: obsidian
    2. Phaneritic - mineral grains can be seen with the unaided eye. Plutonic (intrusive) rocks are totally phaneritic due to slow rates of cooling. Pegmatite - very coarse-grained rocks which crystallize from very water-rich magmas. The water in these magmas lowers the viscosity allow excessive ion migration and crystallization.
    3. Porphyritic - two distinct crystal sizes (can be either porphyritic or aphanitic). Larger crystals (phenocrysts)are surrounded by finer-grained crystals or glass (groundmass). This texture records two distinct cooling histories.
    4. Other Volcanic Textures - important only in volcanic rocks - Most magmas contain a few percent of dissolved gases (mostly water and carbon dioxide). Before eruption, this gas is in solution; however as the magma nears the surface, the gas is released from solution (as pressure is lowered) - resulting in violent eruptions. Example - Mt. St. Helen
      1. Pyroclastic Rocks - result from subaerial volcanic eruptions which expel magma so quickly that it cools forming dust-size glass fragments that is shattered and highly broken. Tuff - pyroclastic rock composed of compacted ash and rock fragments.
      2. Vesicular Rocks - rocks formed from the cooling of a froth of magma and gas. Gas is released more passively, especially in less viscous magmas. Pumice - light-colored, low density vesicular rock. Scoria - dark-colored, more-dense vesicular rock.
  2. Composition - mineralogy and chemical composition. Most magma consists of predominantly 8 elements (Si, O, Al, Mg, Na, K, Ca, Fe). Composition provides insight into the nature and origin of the magma. Quartz, feldspar and the ferromagnesian silicates are the most common minerals in igneous rocks; therefore, these minerals form the basis for the classification of igneous rocks.

General Mineralogic, Chemical and Textural Subdivisions

  1. Felsic - (Fel - feldspar, si - silica) applied to igneous rocks which have abundant light-colored minerals such as quartz and feldspar (non-ferromagnesian silicates).
  1. Granite - granular, coarse-grained, silica-rich igneous rock which is predominantly composed of K-feldspar, Na-plagioclase and quartz with minor biotite, iron oxides and/or amphibole. It is an abundant rock type on continents (example – Stone Mt. And Elberton granites).
  2. Rhyolite - aphanitic equivalent of granite. Commonly occurs as volcanic flows and tuffs along active continental margins.
  1. Mafic Rocks - (Ma - magnesium, fic - iron) - igneous rocks composed predominantly of dark ferromagnesium minerals such as amphiboles, olivine and pyroxene and occasionally Ca-rich plagioclase. *Most abundant rock types - 75% of all igneous rocks.
  1. Gabbro - granular, intrusive, mafic igneous rock composed of Ca-plagioclase, pyroxene and some olivine and iron oxides. Commonly found at lower crustal and/or mantle levels of both oceanic and continental crust.
  2. Basalt - aphanitic equivalent of gabbro. It is the dominant component of oceanic crust and oceanic islands. It is commonly erupted along fissures as extensive thick flows. Basalt rarely occurs as a tuff due to the low viscosity of basaltic magma.
  1. Intermediate Rocks - Because there is a complete gradation between the mafic and silicic igneous rocks, this group includes many of the compositions between the two extremes. These rocks generally contain a substantial amount of both ferromagnesian and nonferromagnesian silicates.
  1. Diorite - coarse-grained, granular rock which is generally darker than a granite but lighter than a gabbro (salt and pepper appearance). It usually contains abundant intermediate (Na-Ca) plagioclase, amphibole and/or pyroxene and iron oxides. It is differentiated from granite by the general absence of quartz.
  2. Andesite - extrusive equivalent of diorite. It is commonly found along subduction zones(active continental margins as both flows and tuffs.
  1. Ultramafic Rocks - composed almost entirely of ferromagnesian minerals with only minor plagioclase and iron oxides. These include dunite (mostly olivine) and peridotite (olivine and pyroxene). These rock types are the dominant components of the mantle, but are much less common than the other three types on the surface.
  1. Stratigraphic (Field) Relationships - the nature of the contacts between the magmatic body and the surrounding rocks.
  1. Intrusives
  1. Plutons - any large intrusive body
  2. Sills - tabular plutons that have formed by the injection of magma between beds of layered rocks.
  3. Dike - tabular bodies that cut across the layering of the country rock.
  4. Batholith - the largest of the plutons (>100 kms.2). Ex. Sierra Nevada Batholith
  1. Extrusives
  1. Lava - magma that is extruded passively onto the earth's surface (mostly basalt or andesite). Lavas usually follow the pre-existing topography.
  2. Tuffs and Pyroclastic Rocks - more silicic in composition. Tend to mantle the countryside.