Geology 306: Earth Science Laboratory: Rock Identification Supplement Hydrological cycle and the Rock Cycle The Rock Cycle 1. The rock cycle and hydrological cyle both involve the recycling of materials, such as water between the surface and the atmosphere or the recycling of the various types of rocks that make up the surface of the Earth. 2. All rocks are classified based upon their mineral content (mineral composition) and visual texture. a. Texture: The size, shape and arrangement of mineral (crystal) grains or fragments of material. 3. Types of Rocks: a. Igneous Rocks: are associated with the crystallization and/or solidification of Magma. i. Intrusive igneous rocks cool slowly underground and form larger crystals, such as granite. ii. Extrusive igneous rocks cool rapidly on or near the surface and form much smaller crystals (such as basalt) and in some cases no crystals at all, such as obsidian which is an amorphous or glassy material. Extrusive igneous activity can also produce violent volcanic eruptions which can produce more fragmental material. b. Sedimentary Rocks: are associated with the accumulation of weathering by-products produced by weathering and erosion on the surface, such as sandstone. c. Metamorphic Rocks: are associated with the alteration and deformation of pre-existing rocks known as parent rocks. These parent rocks are deformed by: Heat, Pressure, and Chemically- active fluids to produce new metamorphic rocks, such as Marble or Gneiss. 4. Igneous Rocks: are classified based on their overall mineral content and visual texture. a. Mineral composition: is based upon the abundance of Ferromagnesian (minerals rich in iron and/or magnesium) and Non- ferromagnesian minerals (minerals that lack iron and/or magnesium), which is in turn influenced by Bowen’s Reaction Series. Bowens Reaction Series i. Bowen’s Reaction Series: describes the sequence of mineral formation in a cooling magma. • Discontinuous Series or Branch: consists of the ferromagnesian minerals: Olivine, Pyroxene, Amphibole, and Biotite, which are all dark, ferromagnesian minerals. • Continuous Series or Branch: consists of the non- ferromagnesian minerals, Calcium-rich Plagioclase feldspar and Sodium-rich Plagioclase feldspar, which are lighter colored than the discontinuous branch. • Muscovite, Potassium Feldspar (Orthoclase), and Quartz: are all light-colored non-ferromagnesian minerals. ii. Mineral content: will reflect the minerals that are present at the temperature in which the rock solidified. For example, rocks that solidify at higher temperatures may contain more olivine, pyroxene, and calcium-rich plagioclase feldspar. Rocks that solidify at very low temperatures will contain more quartz and potassium feldspar. b. Compositional classes: can be determined by the amount of light (non-ferromagnesian minerals) and dark (ferromagnesian minerals). i. Ultramafic: the composition of the mantle (peridotite) and rarely found on the surface. Contains mostly olivine, pyroxene, and a small amount of calcium-rich plagioclase feldspar. ii. Mafic: Contains mostly dark, ferromagnesian minerals such as: olivine, pyroxene, as well as, calcium-rich plagioclase feldspar, and small amounts of amphibole minerals. These rocks tend to be dark and denser, such as the rock, Basalt. iii. Intermediate: Contains mostly amphibole, biotite mica, sodium-rich plagioclase feldspar, and small amounts of pyroxene and calcium-rich plagioclase feldspar. Typically seen as half ferromagnesian and half non-ferromagnesian minerals, but can have a range of compositions. Intermediate colors, such as the rock diorite and/or andesite. iv. Felsic: Contains mostly non-ferromagnesian minerals, such as: quartz, potassium feldspar, and muscovite mica, with small amounts of biotite, amphibole, and sodium-rich plagioclase feldspar. Typically light in color, such as granite Mineral content of felsic, Intermediate, mafic, and ultra-mafic rock compositions: Igneous Rock Textural Terms INTRUSIVE TEXTURES associated with Plutonic environments and slow cooling: Phaneritic: Uniform, slow-cooling, and coarse-grained, easily seen, e.g. granite (typical intrusive texture) Pegmatitic: very coarse-grained, larger than your thumb (1-inch) Often forms due to slow cooling of a fluid-rich magma and can produce semi-precious gemstones such as aquamarine, tourmaline, etc. (if this texture is present, the term Pegmatitic is included in the rock name) (E.g. Pegmatitic granite or granite pegmatite) Porphyritic: Two or more grain sizes are present due to multiple stages or rates of cooling. Phenocrysts (larger crystals) are set in a finer grained matrix due to two or more stages of cooling underground. (If this texture is present, the term Porphyritic is included in the rock name, e.g. porphyritic granite.) EXTRUSIVE TEXTURES associated with volcanic environments and rapid cooling or violent volcanic eruptions: Aphanitic: Uniform, rapid cooling, fine-grained, not visible except under a microscope, e.g. basalt (typical extrusive texture). Porphyritic: phenocrysts in a finer grained matrix due to two or more stages of cooling with the last stage of cooling occurring on the surface. (If this texture is present, the term Porphyritic is included in the rock name, e.g. porphyritic basalt.) Glassy: similar to glass, due to very rapid cooling and the lack of crystal formation, e.g. obsidian. Vesicular or Cellular: many visible vesicles formed as magma cooled around gas bubbles that were escaping, thus preserving the spherical shape of the gas bubbles. (When large very large vesicles are present the term vesicular is added to the rock name, e.g. vesicular basalt, otherwise specific rock names such as scoria may be used.) Amygdaloidal: Vesicles are filled in with secondary minerals, e.g. amygdaloidal basalt. (If this texture is present, the term Amygdaloidal is included in the rock name, e.g. Amygdaloidal Basalt.) Frothy: glassy & vesicular, many small vesicles, foamy-looking, e.g. pumice. Fragmental/Pyroclastic: particles, ash, fragments (which are typically angular) of rock, etc. fused together, e.g. welded tuff, volcanic breccia. Some Igneous Rock Classifications Emplacement of Plutonic Structures / Formation of Plutons Sedimentary Rocks The following diagram depicts the chemical equations important to chemical weathering and the formation of chemical cements. 1. Sedimentary rocks: form through the accumulation of weathering by- products, of which there are many types. These by-products will produce different types of sediments based upon their origin. Diagenesis describes the sequence of events or processes that transform a sediment into a sedimentary rock and often includes: deposition, burial, and Lithification (compaction and cementation) of sediments. Diagenesis and the transformation of Sediment into Sedimentary Rock 2. Types of Sediments and sedimentary rocks and their textures: a. Clastic Detrital Sediments: are particles or fragments of other materials, such as sand grains. These particles are broken bits of other rocks and/or minerals and can be transported by wind, ice, and water which can influence the shape and characteristics of the particles. These particles get deposited and undergo Diagenesis, which includes the cementing of the particles together to form a rock, such as sandstone. b. Non-Clastic Sediments: i. Chemical Sediments: form through the precipitation of formerly dissolved materials, such as the formation of salt crystals from an evaporating salty lake, to produce rock salt. These rocks often appear crystalline. ii. Biological, Organic Sediments: sediments associated with the accumulation of biological material, such as leaf litter, shells, fossils, coral reefs, etc. These rocks can be fossiliferous and contain many fossils, such as fossiliferous limestone. c. You can sometimes find combinations sediments accumulating in some environments. For example, an accumulation of mud will often form mudstone and shale, but if it contains abundant fossils too, it would be a fossiliferous mudstone or fossiliferous shale. Such a rock would have both clastic and biological material and could have a Bio-clastic texture. 3. As particles are transported, they may be reduced in size and could also undergo rounding and sorting of the particles. Typically, when clastic sediments first form, they may have a jagged, and fragmental appearance, but as they travel, the sharp corners are often knocked off and the particle becomes smaller with smoother, rounded shapes. For example, in a stream, particles will tumble along and become more spherical and uniform in shape. Sorting describes the uniformity of grain sizes in a sedimentary rock. 4. Cementing agents: are typically materials that have formed through chemical weathering reactions to produce dissolved materials that can infiltrate the deposit of sediments and then precipitate in the pore spaces between the sediments and thus bind the particles together, such as dissolved calcite, quartz, or iron oxides. CLASSIFICATION OF SEDIMENTARY ROCKS Clastic / Detrital Sedimentary Rocks Texture / Composition Comments Rock Name particle size Poorly sorted rounded rock CONGLOMERATE Coarse grained Quartz, quartzite, fragments of any rock type. and chert are Poorly sorted angular rock ( >2 mm) dominant BRECCIA fragments of any rock type. Poorly sorted, nonstratified, and Fragments of any angular rock fragments. Coarse to fine rock type Sometimes the larger particles are TILLITE grained (associated with elongate with striations on the flat glaciers) surfaces. Primarily