Geology Exam 2
Organic rock
Coal: forms from compaction and heating of partially decayed plant material preserved in oxygen depleted environments such as peat bogs and swamps.
Crater Lake caldera collapse
Crater Lake was formed about 7700 years ago when an enormous volcanic eruption of Mount Mazama emptied a large magma chamber below the mountain
The Standard Geological Time Scale
Eons, Eras, Periods, and Epochs (Precambrian, Panerozoic, Paleozoic, Mesozoic, Cenozoic, in that order and numerical ages for the boundaries between these eras)
Metamorphism, Metamorphic Rocks, and Hydrothermal Rocks
Metamorphic rocks form from preexisting rocks through the processes of metamorphism which result from heat and/or pressure conditions which cause changes to the texture and/or composition of minerals in the rock. a. Metamorphic changes occur to the rock in the solid-state, if temperatures get high enough to cause melting, then igneous processes occur rather than metamorphic processes. b. Metamorphic "fabrics" form as minerals recrystallize or new minerals form under high pressure conditions c. Minerals that are unstable at high temperatures and pressures will react to produce new minerals that are 4 stable at these conditions
vesicular texture
Rock that has a spongy appearance due to trapped gas bubbles in the lava.
What Are Sedimentary Rocks? What are the three types? Examples for each
Sedimentary rocks are formed by the accumulation of sediments. Clastic sedimentary rocks - form from the accumulation and lithification of mechanical weathering debris. Chemical sedimentary rocks - form when dissolved materials preciptate from solution. Organic sedimentary rocks form from the accumulation of plant or animal debris. Clastic- breccia, conglomerate, sandstone, siltstone, and shale. Chemical- chert, some dolomites, flint, iron ore, limestones, and rock salt. Organic- chalk, coal, diatomite, some dolomites, and some limestones.
Extrusive Rocks
Textures - generally fine-grained - some porphyritic or vesicular. Compositions-
Pyroclastic flow
The expulsion of ash, cinders, bombs, and gases during an explosive volcanic eruption. (Mt. St. Helens, Soufriere Hills, Montserrat)
Depositional Environments
There are many types of depositional environments. Whenever the energy of the transporting medium (streams, glaciers, wind, waves, etc.) drops too low to carry sediment of a particular size, that sediment will be deposited. Some depositional environments include: alluvial fans, lakes, streams, deltas, beaches, lagoons, reefs, abyssal fans, and the sea floor
The chain of volcanoes in the Cascades that includes Mt. St. Helens is the result of:
Transform Boundary
Volatiles and gases
Volatiles include nitrogen, carbon dioxide, ammonia, hydrogen, methane, sulfur dioxide and others
Mechanical weathering
a. Frost action - frost wedging b. Pressure release/exfoliation (sheeting)- cracks called sheet joints develop in igneous rocks that formed at depth under high pressure conditions when they are exposed at the Earth's surface. Exfoliation refers to the process in which sections of rock between sheet joints break loose in concentric layers to produce rounded domes. c. Plant growth -- plant roots break up bedrock d. Thermal cycling -- large temperature swings break up rocks through expansion and contraction
Numerical or Absolute Ages
a. Isotopic Principles and Radioactive Decay Mechanisms: decay of unstable parent isotope into stable daughter isotope through alpha decay (loss of alpha particle consisting of two protons and two neutrons), beta decay (loss of an electron from a neutron to produce a proton), and electron capture (capture of an electron by a proton to produce a neutron) b. Half-life: refers to amount of time required for half of existing parent isotope to decay to daughter product, is constant for a given radioactive isotope c. Common Isotopic dating systems: uranium-thorium-lead, potassium-argon, rubidium-strontium
Chemical rocks
a. Limestone: composed primarily of calcite. Can be from biochemical processes (from precipitation of calcite from seawater by organisms to produce their shells or skeletal structures), or through inorganic processes, such as precipitation of calcite to form ooids. Carbonate environments: reefs, lagoons, shallow warm marine environments. b. Marl/Chalk: very fine-grained bioclastic limestone that forms from accumulation of the remains of microscopic marine organisms. c. Dolomite: calcium magnesium carbonate that forms from replacement of part of the calcium in limestone with magnesium. d. Chert: formed from silica (microcrystalline quartz) either through accumulation of the remains of microscopic marine organisms that precipitate silica from sea water to produce their skeletal structures, or replacement of parts of limestone rocks with silica from ground water to produce chert nodules. e. Evaporites: form by precipitation of minerals during evaporation of water, such as gypsum & rock salt.
Differential stress vs. lithostatic stress
a. Lithostatic stress is a strong confining pressure (acting the same in all directions) due to the weight of the overlying rocks b. Differential stress is not the same in all directions, and leads to development of planar fabrics known as foliation. c. Compressive stress: a type of differential stress where rock is compressed greater in one direction, usually due to tectonic forces d. Shear stress: a type of differential stress where rock is sheared, commonly seen in fault zones, where rocks are sliding past each other
Main processes of weathering
a. Mechanical weathering - physically breaking down rocks into smaller fragments b. Chemical weathering - chemically alteration or dissolution of rocks from exposure to water and atmospheric gases
Classification of metamorphic rocks
a. Non-foliated types (e.g. marble—metamorphosed limestone, quartzite—metamorphosed sandstone, hornfels—metamorphosed shale or basalt) b. Foliated types (e.g slate, phyllite, schist, gneiss), increasing in metamorphic grade from slate to phyllite to schist to gneiss as temperatures and pressures increase.
Chemical weathering
a. Oxidation - when oxygen from the atmosphere reacts with minerals - particularly iron bearing minerals b. Dissolution by weak acids - some minerals such as calcite (found in limestone) dissolve in weakly acidic ground waters c. Feldspars are altered to form clays through hydrolysis- acidic rain water and ground water react with feldspar minerals to produce clay minerals (hydrous aluminum sheet silicates) d. Quartz is NOT susceptible to chemical weathering -- quartz remains quartz
Volcanic landforms and structures
a. Pillow basalts - form from sugmarine eruptions b. Columnar jointing - forms through contraction as lava flows cool c. Lava tubes
Effects of weathering
a. Ratio of surface area to volume is increased as rocks are broken into smaller pieces b. Spheroidal weathering -- the rounding of rock due to more rapid weathering of corners and edges c. Differential weathering - different rock types weather at different rates, leading to landforms such as cliffs and slopes
Development of Geological Reasoning
a. Religious perspectives on the age of the Earth b. Hutton & Lyell: "The Present is the Key to the Past" c. Uniformitarianism - relationship to process and rate
Soils
a. Soil Horizons - from top down: O - organic matter; A - organic matter mixed with minerals; E - zone of leaching; B - zone of accumulation of clays/Fe oxides; C - fragmented bedrock b. Climate - humid climates - more chemical weathering, thick soils; tropical climates - leached poor soils; arid climates - thin soils, evaporites, caliche c. Residual vs. transported soils - soils that form from bedrock below vs. those that form from transported sediments d. Paleosols -- old soils preserved as layers within the rock record
Effects of transportation
a. Sorting: well-sorted, smaller-sized sediments indicate long transport, poorly sorted indicate shorter transport from the sediment source b. Rounding: rounded sediments indicate long transport, angular grains indicate shorter transport from the sediment source
Role of climate
a. Temperature - higher temperatures generally lead to more rapid chemical weathering. b. Humidity - humid climates generally lead to more rapid chemical weathering
Combining Relative and Numerical Ages
a. The Age of the Earth i. Modern value of 4.6 Billion years: derived from dating meteorites ii. Early Methods and initial estimates b. Revised Geological Time Scale with Numerical Ages: assign numerical ages to boundaries of relative geologic time-scale
Effusive vs. Explosive eruptions
a. explosive eruptions caused by trapped gases - amount of gas present and ease with which it escapes are major factors. Explosive eruptions associated with higher viscosity, intermediate-felsic composition lavas and composite volcanoes. Effusive eruptions associated with mafic lavas and shield volcanoes. b. Consequences -- volcanic hazards (lava flows, ash fall, pyroclastic flows, dome collapses, sector collapses, lahars, bombs) c. Andesite-rhyolite domes d. Basaltic lava floods
What are Metamorphic Rocks? Where do they form? Two basic types of metamorphic rocks? Examples of Each type?
been modified by heat, pressure, and chemical processes. Deep below the earths surface. Foliated metamorphic rocks have a layered or banded appearance by heat and direct pressure. Non-foliated metamorphic rocks do not have a layered or banded appearance. Foliated rocks are gneiss, phyllite, schist, and slate. Non-foliated rocks are hornfels, marble, novaculite, quartzite, and skarn.
Sedimentary Formations
bodies of rock of considerable thickness with a regional extent large enough to be "mappable". Named based on a "type" locality where it is well-exposed, along with the rock type.
Common eruption type for each
effusive (lava flows) at shields, explosive (pyroclastics) at composite, cinder cones, & domes
What are Igneous Rocks? Where do Igneous Rocks Form? What are examples of Extrusive Igneous Rocks? What are examples of Intrusive Igneous Rocks?
formed from the solidification of molten rock material. Above or below earths surface. Diabase, diorite, gabbro, granite, pegmatite, and peridotite. Andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff.
Mt. St. Helens
large active composite volcano - 1980 lateral blast and explosive eruption
Definition of sediment
loose, solid particles of minerals/rocks that originate from weathering and erosion of preexisting rocks (detrital sediments) or from precipitation from solution in water (chemical sediments). a. Main sediment size fractions - from smallest to largest: clay, silt, sand (sand is 1/16 to2 mm in diameter), gravels (pebble, cobble, boulder). b. Relationship to clastic rock types: clay - shale or mudstone, silt - siltstone, sand - sandstone, gravel - breccia or conglomerate
Correlation
principles and use in relative dating a. Physical Continuity: can trace a particular formation continuously b. Similarity of Lithology and Fossils: must be careful using similar lithology, fossil assemblages can be used to correlate formations of similar age. c. Methods i. Faunal Succession: fossils change in predictable manner over geologic time -- William Smith ii. Use of Index Fossils: fossils from short-lived wide-spread species iii. Fossil Assemblages: groups of fossils from species that coexisted can be diagnostic of a particular geological time
Weathering
processes that physically or chemically alter rocks at or near the Earth's surface
Relationship to tectonics
sedimentary basins are often associated with convergent plate boundaries and thickening of the crust in mountain belts. In the early stages of continental rifting and the development of a divergent plate boundary there is a progression through course gravel and sand deposits to shallow lake sediments and evaporites along with basaltic rocks. This sequence is often preserved at depth along passive continental margins.
Types of volcanoes
shield, composite, cinder cone, dome
Morphology of different types of volcanoes
shields: largest, gentle slopes, basaltic lava flows composite: large, steep, lava flows and pyroclastics cinder cones: steepest, small, pyroclastics domes: steep, viscous, plug vents
Erosion
the picking up or physical removal of rock particles. Weathering and erosion produce the sediments that form sedimentary rocks.
Lithification
the processes that convert loose sediments into solid sedimentary rocks. a. Compaction due to overburden: sediment grains are packed closer together with deeper burial. b. Cementation: minerals precipitate from water moving through the pore space between sediments and cement the grains together producing a solid rock. Most commonly, these cements are composed of calcite or silica.
porphyritic texture
An igneous rock texture in which large crystals are scattered on a background of much smaller crystals.
Unconformities
An unconformity is a contact that represents a significant gap in the geologic record, with the rock unit above being considerably younger than the unit below. a. Disconformity: unconformity between parallel sedimentary layers - can be difficult to detect - need to 5 examine fossil record. b. Angular Unconformity: unconformity between sedimentary layers with different bedding orientations (usually inclined beds beneath near-horizontal layers above) c. Nonconformity: unconformity between sedimentary layers sitting on top of older metamorphic or igneous rocks
Principles of Relative Time
a. Contacts and formations - contacts are surfaces between layers/formations b. Original Horizontality - sedimentary rocks initially form in horizontal layers c. Superposition - younger sedimentary rocks are on top of older sedimentary rocks d. Lateral Continuity - sedimentary rock layers extend until they thin at edges e. Cross-cutting Relationships - younger rocks/features cut across fabrics or layering in older rocks/ features f. Inclusions - parts of country rocks included within intrusive bodies are older than the intrusion
Types and characteristics of metamorphic environments
a. Contact: high temperatures without differential stress in small areas surrounding magma bodies - produces non-foliated metamorphic rocks b. Regional: large areas exposed to high temperatures and large differential stresses associated with tectonic motions (convergent plate boundaries). c. Subduction Zone: high pressures, low temperatures in subducting slab d. Shock: very high pressures resulting from impact of asteroids or comets. e. Hydrothermal: circulation of fluids near magma bodies or deep within the earth, produces hydrothermally altered rocks f. Metamorphic grade: higher metamorphic grade means rocks formed at higher temperatures and/ or pressures, prograde metamorphism means increasing in grade with deeper burial, retrograde metamorphism refers to decreasing grade with decreasing depth (not seen as much due to loss of water) g. Relationship to tectonics: regional metamorphism associated with convergent plate boundaries. Depressed isotherms in subduction zones leads to high pressure, low-temperature metamorphic rocks (greenschists/blueschists)
Sedimentary structures
a. Bedding: layers that form due to some change in sediment deposition b. Principle of original horizontality: most sediment deposited in water is deposited in near-horizontal layers c. Superposition: younger layers are deposited on top of older layers d. Cross-bedding - thinner inclined beds within a thicker bed of rock that form due to deposition of sand in water or air as migrating ripples and dunes. Cross-beds can be used to derive paleocurrent directions. e. Graded bedding - a layer with a vertical change in particle size. Usually getting finer toward the top of the bed, due to deposition of sediment from a turbidity current. f. Mud cracks: polygonal patterns of cracks formed due to dying mud - indicate exposure of the finegrained sediment to air. g. Ripple marks: preservation of ripples - small dune like structures can also give paleocurrent directions. h. Fossils and imprints: preservation of the remains of organisms or their imprints in sedimentary rocks. i. Principle of Cross-Cutting Relationships: a geologic object can not be altered until it exists, meaning, the change to the object must be younger than the object itself.
Clastic (Detrital) rocks
a. Breccia and Conglomerate: both have large gravel-sized rock fragments. Breccias have angular clasts, while conglomerates have rounded clasts, indicating that the sediments were transported farther from their source. b. Sandstone types: sand-sized particles. Quartz sandstone contains more than 90% quartz grains, Arkosic sandstone contains more than 25% feldspar grains, Graywacke is a sandstone with a fine-grained matrix mixed in with sand-sized particles. c. Siltstone, Mudstone, & Shale: clastic rocks consisting of fine-grained silt and clay-sized particles. Shale is most common and splits into thin layers.
Main sedimentary rock types
a. Clastic (detrital): form from fragments of preexisting rocks b. Chemical: form from minerals that have precipitated from water (mostly seawater). c. Organic: composed of organic carbon compounds
Factors controlling metamorphic characteristics
a. Composition of parent rock (protolith): starting material provides large control on what type of metamorphic rock is produced b. Temperature: high temperatures contribute to metamorphic reactions c. Pressure: high pressures contribute to recrystallization and development of fabrics d. Fluids: fluids help accelerate exchange of ions and speed metamorphic processes e. Time: long periods of time are required for most metamorphic processes
