106 Course Notes (post midterm).doc

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Geological Sciences and Geological Engineering
GEOL 106
Rob Beamish

Lecture #19 Exam: BOT B143 - 50 M/C (lose 0.1 marks for wrong answers) - 30 marks for fill in the blank/short answer 1. Try “controlling” earthquakes - Pump water down the fault to lubricate the fault (big risk in case you trigger another earthquake MIDTERM***** Introduction to the ROCK CYCLE and the PLATE TECTONIC CYCLE and VOLCANOES! What causes earthquakes? Movement on Faults What causes the faults? Stress on the rocks in earth What causes stress inside the earth? Release of earths internal heat energy? - Release of earths internal heat energy (PLATE TECTONIC CYCLE) Energy acting on the earth 1. External energy: wears downs the earth’s surface (hydrologic cycle) 2. Internal Energy: builds up the earth’s surface (volcanoes and mountains, earth acts as a HEAT ENGINE) The Rock Cycle: Summarizes the interaction of external and internal energy - Rocks are in constant change from one type into another - Igneous (external energy wears it down)sedimentary (internal energy)Metamorphic (heat ups enough and cycle continues to form igneous again) Igneous: formed when liquid (magma) cools and crystallizes Sedimentary: formed when weathered particles get deposited and lithified Metamorphic: formed when a pre-existing rock is changed under high temperature and pressure in the solid state Lets concentrate on the release of INTERNAL energy right now! Plate tectonic cycle: summarizes how internal energy is released from the earths interior 1. Conduction: bumping together of atoms 2. Convection: large-scale movement of hot material **Internal heat is the combo of these two** Does the earth get rid of its internal heat uniformly over the surface of the earth? NO - There are discrete zones of higher heat release (volcanoes, earthquakes, mountains, belts) - 880% of heat reaching surface comes through these zones - These zones reveal the patterns of large scale CONVECTION in the mantle The plate tectonic theory: summarizes this convection process - The main way that heat is brought to the surface or gets rid of internal energy Plate Tectonics - Outer 100km of earth is like a cracked egg shell - The earthquake zones are where the cracks are - The rigid egg shell pieces (called plates) can slide around over the convecting mantle beneath BUT…We have a problem - The mantle is solid so how can it convect like a pot of soup? How do we know the mantle is solid? - By the study of seismic waves passing through the earth - There are three parts 1. Crust 2. Mantle 3. Outer core (liquid) 4. Solid core (solid) - Crust and mantle are solid and go about 3000km down There is a difference between the ocean and continent crust 1. Thin part is the oceanic crust and is about 5km thick with 5km water on top - Black rock 2. Thicker part is continental crust about 30km thick - Pinkish white rock but highly variable A. Lithosphere: 100km very brittle or rigid (egg shell) (includes upper mantle and crust) - Slides on the asthenosphere B. Asthenosphere: 100-500km things are quite plastic C. Mesosphere th Lecture 20 (Feb. 27 ) The “silly putty” mantle • The mantle, although solid, convects plasticallyit behaves like “silly putty” (elastico-plastic behavior) • With slow stress application over long geologic timemantle is “liquid” Theory of plate tectonics • Outer part of earth consists of a number of rigid lithosphere platesplate boundaries are marked by earthquakes and volcanoesplates are 100km thick • The plates slide about on the plastic asthenosphereat rates of several centimeters per year (same rate of fingernail growth) • Almost all igneous and “tectonic” activity is localized along plate margins o Constructivepull apart  Upwelling part of the convective cyclenew lithospheric plates are produced and move apartnew ocean crust produced o Destructivecome together (one slides below the other)  Down-going part of the convective cyclelithospheric plates are destroyed as plates collide o Transformslide past one another (side by side) • Trenches: where oceanic lithospheric plates are subducted Destructive plate earthquakes • Creates: volcanic island arcs, continental volcanic arcs, volcanic mountain belts (fold + fault), continental crust • Can: close oceans, cause continents to collide (the Himalayas) th Lecture 21 (Feb. 29 ) Where do earthquakes occur? -99% occur at plate margins. -90% occur at depths than less than 100km -10% occur at depths between 100km to 700km -Destructive margins have the greatest potential to create large earthquakes  reverse dip slip (Like two trucks colliding into each other, lots of energy) Intraplate earthquakes: -Earthquakes that occur away from plate margins -represent only about 1% of all earthquakes -and most are along former plate margins -E.g. St. Lawrence/Ottawa Valley rift zone, Mississippi river rift zone -20,000 years ago NorthAmerica was covered in ice sheets -This placed a huge amount of stress on the ground beneath, pushing it down -The ice melted quickly, and the earth is still “rebounding” from the pressure Volcanic Risk Read Chapter 4: -Connected to igneous rocks -Rocks formed when liquid (magma) cools and “crystallizes” Magma  Liquid “rock” -where does it originate? -mainly in the upper mantle (down to 200km) -Sometimes in the crust -Most (but not all, ex Hawaii) is closely related to margins -Magma general rises due to its lower density -Some magma reaches earth’s surface (extrusive) extrusive rocks -Most magma crystallizes beneath the earth’s surface (intrusive)  intrusive rocks -The size of the crystals in an igneous rock depends mainly on the rate of cooling of the magma -The slower the cooling, the larger the crystals -Chemical composition of magma: (rather important for risk analysis) -Magmas differ in the proportion of elements -Results in rocks of different colors -and magmas with different properties Main elements in magma: 3 most important Silicon  “sic” Iron  “fic” Magnesium  “ma” -Rocks rich in iron (and magnesium) will be black -Mafic rocks (Ma for magnesium, fic for iron) -Gabbro (intrusive) -Basalt (Extrusive) -Rocks rich in silicon and feldspar will be pinkish or whitish -Felsic (Fel for feldspar, sic for silicon) -Rhylote (extrusive) -Granite (intrusive) Volcanic Risk: Deal with volcanic risk: -We will mainly be looking at the nature of the hazard -We first need to consider the nature of magma of different volcanoes Magmas: -Magmas contain dissolved gases; H20, c02, s02, F, Cl) - H20 is the dominant one -Pumice is created when gasses are trapped inside the rocks Felsic: Light colored rocks -High viscosity (sticky) -High in water content (more than 5%) Mafic: Dark colored rocks -Low viscosity (fluid) -Low in water content (less than 1%) th Lecture 22 (March 5 ) Volcanic Risk Dealing with Volcanic risk - We will mainly be looking at the nature of the hazard - We first need to consider the nature of MAGMAof different composition - Two major properties are viscosity and water content 1. Felsic Magma (pink.white) - High viscosity (sticky) and High water Content (more than 5%) 2. Mafic Magma (black) - Low Viscosity (fluid) and Low Water Content (less than 1%) Extrusive Rocks - Magma when it comes out of the ground will be in one of two forms - Lava: type of magma (liquid) - Pyroplastic Material: another magma type (pyro=fire, and plastic) (hot particles) Magma reaches the surface by coming through either: 1. Fractures (fissures) 2. Volcanic cone VOLCANIC CONES - Pipe like conduit and vent brings magma to the surface - Eruptions range from calm to violent: depends on the composition of the magma 3 main types of volcanic cones 1. Cinder cone: least important - Pyroplastic - Cone shape with steep angles (angle of …) built up for several years 2. Stratovolcano: “composite cone” - Lava and pyroplastic - Felsic to intermediate material - Very large with steep slopes (much larger than cinder cone) - Called strato because it’s almost like sedimentary rock but it is igneous 3. Shield volcano - Lava mafic material: very fluid - Calm eruptions - Not very steep slopes because the magma flows further and faster Which volcano type is the most dangerous? - Stratovolcanoes are the most dangerous - Because their magma composition is felsic and intermediate - Therefore the magma has: a. High viscosity b. High water content This leads to: a. High, steep-sided volcanoes - Because of the pyroplastic ash that piles up with steep sides - And because the sticky magma flows slowly down the sides maintaining steep slopes of the volcano b. Volcanoes can erupt violently - The magma is water rich and sticky - As magma rises to the surface there is a decrease in pressure - The gas (steam) bubbles try to expand and get out but the sticky magma prevents this - Eventually explosive expansion occurs ***ALSO - Stratovolcanoes occur along continental margins where people like to live (subduction zones) destructive plate margins Anatomy of Stratovolcanoes - Left hand side of volcano is safe - To the right is dangerous as the pyroplastic flow flows down the side before becoming lahar - As well there is an ash cloud and air fall ash 1. Air fall “Ash” (Tephra) andAsh Clouds a. Air fall tephra (bomb, pumice, ash) can bury cities (mount Vesuvius) - Buried roman city of Pompeii - Air fall ash driven by the wind (important when thinking of risk) - Mount Pinatubo Indonesia - Long valley eruption 700,000 years ago region covered in ash was massive th Lecture 23 (Mar. 6 ) How do we go about determining the recurrence history for stratovolcano eruptions? - Carbon dating of buried and burnt wood - If they don’t erupt frequently they tend to erupt big Ash Hazards a. Airborne ash effects airplanes b. Thin ash fall layer 10-15cm can cause roof collapse c. Very thing ash fall layers <1cm can disrupt agriculture d. Intake of ash effects industry e. Human health hazard f. Atmospheric effects of ash cloud Ash Clouds - Ash and sulfuric acid can block the sun’s rays - Lower global temperature for years Effects of Tambora eruption in 1815 a. Famine in Europe as flour price goes up b. Mary Shelley writes Frankenstein c. Bryan composes his most miserable poem darkness d. Joseph turner paints dramatic sunsets e. Massive emigration to California 2. Ashflow a. Nuee ardent - Glowing cloud - Travel downslope at up to 150km/hr - Can travel more than 10km - Example: Mt. Pelee 30,000 killed b. Pyroplastic surge (or lateral blast) - At mount St. Helen pressure built up and burst out the side of the volcano - Took the whole top of the mountain off - Trees were chopped off at the base and stripped all needles and branches off Lecture 24 (Mar. 8 )th 3. Lahar - Debris flow or volcanic related mud flows - Ash on flanks of volcano mix with melted snow - Flow down steep flanks of volcano and river valleys over distances > 100km - Ex) Nevada del Tuiz 1985 more than 20,000 dead - Ex) Mt. St. Helens 1980 4. Tsunami - Most famous are the Krakatoa in 1883 - For island arc volcanoes - Sudden explosion can trigger a tsunami 5. Gases - Various gases can create problems (sulfuric acid) - Can generate SO2 in atmosphere - Direct result from gases close to volcano is not as big of a deal - CO2 heavier than air (2000 killed at lake nyes Cameroon 1986) 6. Caldera Collapse and Supervolcanoes - Crater lake formed - When volcano collapses on itself and very dangerous when collapse is enormous in scale - Rare explosive supereruption (ex: Santorini 3600 years ago) (Toba 74,000 years ago) - Can cover whole continents with ash - Can have truly global catastrophic effects (climate and weather effect) Supereruptions: greater than 1000 to 5000 cubic kilometers of volcanic ash deposits (Toba and Yellowstone) What is the Global Long term RECURRENCE interval for supereruptions? - Like magnitude 9 earthquakes, they are rare - Average about 1 everyone 100,000 years (very rare) - Probably present greatest natural hazard to mankind- S. Self and S. Blake 1. Understand the hazard 2. Analyze the risk in the area of concern 3. Determine ways to manage risk 4. Cost-benefit 5. Implement management techniques Assessing the risk of Volcanoes a. Locate and determine natures of volcanoes in the area - Volcanoes generally easy to find - Look at world volcano maps (plate tectonics) - What kind of volcano? Shape and composition of the rock are important (mafic or felsic) - Fissure, shield and stratovolcano Volcano Zones 1. Constructive: (divergent) plate margins 2. Destructive (convergent) plate margins, subduction zones 3. Hot spots: both oceanic and continental Lecture 25 (Mar. 12 ) th b. Study history of volcanoes in the area - Past behaviour is key to future behaviour (usually) - Nature of the eruptive materials (plastic deposits? Thick? Mudflows? Tephra? Lahars?) - Assess the magnitude of events - Determine the recurrence intervals (use carbon dating on the trapped wood (charcoal) and K-A dating (potassium argon) - Produce frequency magnitude curve (same as earthquake magnitude on x-axis and number of events on y-axis)Alot of small eruptions with few large eruptions (power-log relationship) - Construct probability maps (what’s the probability over the next thirty years produce x amount of ash) Active: erupted in last x amount of years Dormant: Erupts every x amount of years Dead: No eruption in last x amount of years c. Determine geological and geographic factors in the area - Where are the valleys and river channels (routes for pyroplastic flow and lahars - What are the prevailing wind directions - What are the risks of the tsunami (volcanoes in island areas) ***Put all this together to produce HAZARD MAPS d. Determine potential human interaction with the volcanoes in the area - Population distribution - Nature of human infrastructure (ability of roof to withstand ash) - Proximity of people to high-risk zones Carry out preventative measures!! After cost benefit analysis - Think about Ph very difficult to prevent it from happening 1. Apply land use planning and zoning (avoid high risk areas) 2. Apply building codes/build structures to minimize damage (not as easy to do as far as seismic risk) - Roof design to withstand ashfall - Concrete channel ways in valleys to control lahars and pyroplastic flows 3. Set up emergency response plans - Education of public - Evacuation plans (critical because there is usually plenty of warning time) - Have emergency services in place 4. Try to predict volcanic eruption - Generally quite successful due to nature of volcanic behaviour a. LONG TERM: past history is key to future behaviour b. SHORT TERM - Monitoring the volcano generally quite successful - Magma rises up from 5km deep (or more) over a lengthy time (weeks months years) (use methods to detect this) Monitoring Methods 1. Seismology (most important method) - Magma pressure on rocks causes them to shift, creating earthquakes - Locate earthquake foci to determine footprint 2. Measure surface ground deformation (movement) - Bulges etc. - Determined using something such as a tilt meter, telling you magma is coming in - Use GPS, RADAR 3. Gas monitoring - Volume of CO2and SO2 - The more you measure in the steam the closer its getting to eruptive event Lecture 26 (Mar. 13 )th How the Magma gets to the surface! 1. Through a crack and spreads out on the surface (fracture and fissures) 2. Up a pipeline in volcano stratovolcano ***DID these backwards Fracture (Fissures) - I.e. cracks - Where a dyke reaches the surface - Mostly involve MAFIC lava (basalt) - Results in extensive LAVASHEETS due to low viscosity (ex Iceland) - These are generally calm eruptions Dangers from Fissure Eruptions - Generally quite safe BUT… - Major eruptions can generate atmospheric effects from a. SO2 Gas: creating sulfuric acid, causing cooling (ex) Laki eruption in Iceland) b. CO2 Gas: over the longer term this can cause global warming In particular: enormous basalt events can be catastrophic - They have been linked to mass extinctions (65 million years ago) - 250 million years ago (90% of species went extinct) What is GOOD about volcanoes? 1. Builds up lands (makes continents) 2. They are beautiful 3. Geothermal energy 4. Good soil for agriculture 5. Generate ore deposits (copper, gold,, diamonds) 6. Atmosphere and hydrosphere (responsible for water, CO2 producer) - Snowball earth event (750 million years ago): big ball of white we get out of this by volcanoes producing carbon dioxide to warm things back up again 7. Involved in life development (black smokers) RiskAnalysis and Management of LANDSLIDES - Something we have a little more control over - More scientific term is mass wasting and slope stability Mass wasting: the down slope of solid earth materials under just the influence of gravity ***Earthquakes and volcanoes are predominantly at plate tectonic boundaries*** - Landslides are anywhere, everywhere you look there are slopes where landslides can happen How do we minimize damage from landslides? 1. Understand the hazard What determines if a landslide occurs or not? a. Slope steepness (steeper the slope the greater the risk) b. Nature of the material (soil/aggregates vs. solid rock and ice) - How much friction? And how much internal strength? (tape on block analogy) - Leads to wide variety of down slope movements Lecture 27 (Mar. 15 ) th c. Triggering effects” rainfall, earthquakes, human action and others Classification Scheme for Mass wasting Textbook has 4 variables 1. Mechanism of movement 2. Type of movement 3. Amount of water present 4. Rate of movement ***True classification scheme incorporates all of them: my scheme is a bit different Broken into two parts - Mechanism on the y-axis (fall slip flow) - Velocity on the x-axis (low to high) a) Fall: rockfall/debris fall - All about stuff falling down and the high velocity it goes down - The material breaks off and falls virtually straight down on these steep slopes at high speeds b) Slip: stuff is slipping along the surface (Black on board analogy) - Discrete surfaces of slip 1. Translational slip: rock on board and slip surface is a plane or surface - High velocity and lower friction - Primarily rock material 2. Rotational Slip: rotates moves on part of a circle down and out - Earth materials such as soils - Generates its own break in the slope making a circular slope because it requires low energy - South nation river slip c) Flow 1. High velocity are rock or debris avalanches >5km/hour (cannot outrun them) (Yungay, Peru) 2. Mudflows (water saturated debris) 1-5km/hour 3. Earthflow (no slump) 1m/day to 10m/hour 4. Creep and soil-fluction 1cm/year - Creep: Really slow mission kind of event where material gradually moves down a slope Permafrost: occurs up north where its permanently frozen land.Any water in the ground is in the form of ice - You have the ground and then 1-3 meters of an active zone (ice melts in the summer) - Below that is permafrost (100’s of meters thick) - This is what SOLIFLUCTION is Lecture 28 (Mar. 19 ) th Now we are assessing the risk from a landslide hazard in an area (Sh and Ph) 1. Locate and determine the nature of the potentially dangerous slopes - Studies on the ground (look around): you can see scars from old landslides that suggest higher risk - Use of air photographs: determine if it is relatively recent or ancient landslide (this is very valuable when building something like a pipeline instead of walking) 2. Study history of landslides in the area - What kind of mass movement was there in the past? (liquefaction, rock slips? Etc.) - What is the recurrence interval for events, can use C14 dating of dead organic matter 3. What is are the geologic and geographic controls - Nature of rocks and soils - Slope steepness, etc. 4. What are the interactions of people with the slopes? - Population distribution - Infrastructure (roads, buildings, dams, etc.) ***Map out the high risk areas on a physical map or landslide incidence and susceptibility map*** 5. For a particular slope that you are worried about carry out a SAFETY FACTOR analysis (Can’t be done for the other hazards) - This determines the probability of failure of that slope (Ph) How safe really is the slope? Remember: minimizing risk in and area you try to reduce Ph and Sh - For earthquakes and volcanoes, we have little or no control on Ph, so most effot goes towards reducing Sh. - For landslides, we can actually reduce the probability of the hazard (Ph) happening Slope Stability and the Safety Factor Concept - How do we exactly determine the probability of failure of the slope - Use the example of an open pit mine (making slopes that aren’t there in a mine< increasing risk, so how do you make these slopes safe?) The open pit mine - How do we decide
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