Geo 120 1st Edition Exam 2 Study Guide Sedimentary Rocks Study Guide Exam 2 1 Sedimentary rocks are made from the products of chemical and physical weathering i e the break down of other rocks 2 Sedimentary rocks are classified according to what they are made of and what their texture is 3 Sedimentary rocks record information about the environment in which they are formed So we can use them to reconstruct past environments and environmental change Chemical Weathering Dissolution Dissolve Oxidation Oxygen breaks down Biological Hydrolysis Add Water Physical Weathering Fracturing Frost and mineral wedging Roots and other biologic activity Types of sedimentary rocks Clastic Composed of broken pieces or Clasts of older weathered eroded rocks Sediment of grains fragmented rocks minerals Shale Siltstone SandstoneConglomerate Biochemical Organic matter fragments shells of organisms of fossils plant matter or charcoal Chemical Precipitated formed out of liquid solution like rock salt or rock gypsum can be microcrystalline Chert Clastic Clast Size Boulders cobbles pebbles Sand Coarse medium fine Silt and Clay Clast Shape Angular Partially rounded rounded Range of Clast Size Poorly Sorted moderately sorted well sorted Father from the source means rounder smaller better sorted more energy to move it i e steepness of slope more steep can move bigger material Stronger current larger pieces Agent Water Wind Ice Methods of Transfer Wind Can only transport narrow size range small and well rounded Iron oxides must be from oxygen Layered in several directions Cross Bedding Beach Environment Sandstone well sorted well rounded Symmetrical ripples Shale and Sandstone Can have dinosaur tracks plant fossils Wetland Environment Lakes swamps wetlands Can have dinosaur tracks Shale sandstone and freshwater limestone Fern Fossils Colorful red yellow purple Rivers draining mountains Poorly sorted Conglomerate breccia Mixed with sandstone Layering Angular clasts quartz potassium feldspar mica Close to source Deep Ocean Very fine grained mass of clay that needs a calm environment to settle out Shale sand intermixed layers Shells shark teeth fish bones Reconstructing changes in environments form sequence of rocks Limestone also common in clear warm shallow marine waters Cross bedding Groups of inclined layers sloping layers Tells us the depositional environment was water or wind Ripple marks or dunes Environmental Changes When a beach moves in Transgression rise in sea level Layers 1 Limestone 2 Mudstone 3 Sandstone When a beach moves out Regression decrease in sea level Layers 1 Sandstone 2 Mudstone 3 Limestone Metamorphism and Deformation 1 When rocks are subjected to high heat and or pressure they may become altered and deformed 2 Rocks subjected to stress may deform brittlely or ductilely depending on their depth in the crust and how rapidly the deformation happens 3 During metamorphism the minerals in a rock can recrystallize into other minerals migrate within the rock and be deformed to become aligned in a preferred direction Force A push or pull expressed as amount of acceleration experienced by a mass Stress Force per area increase by increase force applied and or decrease area applied to Stress Force divided by area Brittle Deformation When a rock breaks Ductile Deformation When a rock bends folds flows Types of Stress Confining Pressure Same amount of stress from all directions Differential Stress Different amounts of stress from different directions Deformation Too much stress greater than the strength of the rock causing failure Strength of the Continental crust increases with depth then weakens 3 Main Types of Stress Compression Convergent fault Continental collision or subduction zone Tension Divergent fault rifting seafloor spreading Shear Transform faults Types of Rock Fractures Shallow levels Brittle Normal Fault Hanging wall slides under the footwall Reverse Fault Footwall slides under the hanging wall Shallow depths low temps minerals may not be affected Joint Crack where rock pulled apart Fault Rocks have slipped past one another Strike Slip Fault Left or right lateral Standing on one side where the other side is moving Ductile Deformation causes Folds Synclines Anticlines Domes and Basins Deeper Levels At deeper levels in the crust there is Compression Stretching Shearing deformation Deeper depths higher temps may recrystallize new crystals Foliation Result of Ductile Deformation Anytime you get a plain or texture mineral alignment Or flattened Pebbles Or Smearing from shearing Chemical Processes during Metamorphism Recrystallization Pressure Solution Remobilization crystals lining up grouping together Physical Processes During Metamorphism Deformation of objects Rotation Shearing What causes Physical or Chemical Processes 2 types Contact Contacts something hot area around magma chamber Regional Large area like a burial Subduction or circulation of hot fluid Cleavage In metamorphic rocks fractures that are perpendicular to stress Earthquakes 1 Earthquakes are the energy released during movement along faults 2 Energy released in earthquakes travels in seismic waves 3 Earthquakes can be located using the arrival times of seismic waves at different locations 4 The magnitude of an earthquake represents the amount of energy that is released 5 We can assess the risk of earthquakes a particular location by examining the tectonic setting and the history of earthquakes for that location P waves Primary wave compresses material fastest S wave Secondary body wave shears Horizontal surface wave shears material Vertical surface wave compresses material these are called l waves they cause the ground to move back and fourth Stress and earthquakes 1 Before stress increases rock experiences elastic strain 2 During fault slips causing an earthquake 3 After stress drops during quake followed by post slip recovery of elastic strain cycle starts again stick slip behavior because fault sticks and then slips Magnitude Richter scale how much energy is released Intensity How much shaking severity or strength or shaking How do we determine location 1 Earthquake recorded by seismic network 2 Select earthquake records to find P and S waves 3 Calculate distance from P S times 4 Triangulate epicenter Describing an earthquake The hypocenter the point within the earth where an earthquake rupture starts The epicenter the point directly above the hypocenter on the surface of the earth Seismic waves radiate outward from