Geology Notes

Earthquakes and Subduction Zones

• Earthquakes are often linked to subduction zones, where one plate dives under another.
• Subduction doesn't always guarantee an earthquake.
• Subduction is an ongoing movement, while earthquakes are a sudden release of built-up stress.
• Magnitude is the measure of energy released at the source.
• It's tied directly to energy release, using a logarithmic scale, so small number changes indicate huge energy differences.
• Intensity measures shaking at a location.
• Aftershocks don't have to be smaller; even small earthquakes can cause landslides and tsunamis if the conditions are right (unstable slopes near water, local geology).
• It is false that less than 1M earthquakes happen per year. It's much more frequent due to constant tectonic movement.

Early Warning Systems

• Based on seismic wave speeds.
• P waves are faster but less damaging and detected first.
• The system sends an alert before the arrival of S waves, which are slower and more destructive.

Isostacy

• Isostacy is the equal standing or balance, even if objects have different heights.
• At a certain depth, they exert the same pressure.
• Analogy: Wooden blocks in water. A big, thick block sinks deeper but floats higher than a thin one. Underwater, at the same depth level, the pressure of the weight of either block plus the water above it is equal to the surrounding water pressure.
• The crust floats on the denser mantle below, constantly adjusting to maintain balance.
• Mountains have deep roots extending down into the mantle.
• Continents are usually higher than ocean basins because continental crust is generally thicker but less dense than oceanic crust.
• Isostasy responds to changes constantly. As the crust thickens, its root grows deeper to compensate for the weight above, and vice versa.
• If the crust stretches and thins (like in rift valleys) due to gravity pulling things apart, it becomes lighter overall for its area, more buoyant, and tends to rise up relative to its surroundings.
• Mountain height limits and gravity: A smaller planet with higher gravity could actually support taller mountains because stronger gravity pulls harder on the mountain's base, increasing pressure and making the underlying rock stronger and more resistant to collapse, which limits mountain height.

Cratons and Dynamic Topography

• Cratons are old, stable cores of continents with low elevation that haven't seen tectonic action in ages; they're ancient foundations with incredible stability.
• Dynamic topography: Surface elevation changes caused not just by the crust's thickness and density but by movements in the hot mantle beneath the crust.
• Examples: Mantle plumes pushing up, drag from subducting plates pulling the surface down.
• These mantle stirring effects influence surface shape.

Sea Level Changes

• Tied to average surface temperature.
• Warmer planet: Thermal expansion of seawater and melting glaciers and ice sheets add more water volume, raising sea level.
• Crucial link to climate change.

Water Cycle

• Water evaporates from surfaces, transpires from plants (evapotranspiration), goes into the atmosphere, and comes back down as rain or snow.
• The tiny fraction of accessible and usable fresh water for humans is becoming more precious due to climate change, population growth, and pollution overuse.

Stream Erosion and Transport

• Streams work to erode, transport, and deposit, continually reshaping the landscape.
• Types of erosion:
  • Quarrying/Plucking: High-energy water forces itself into cracks in bedrock or plucks out chunks of rock; very effective in fractured rock with fast flow.
  • Abrasion: Grinding and scraping of sediment and sand gravel carried by the stream against the bed and banks; swirling pebbles can even drill potholes into the rock.
  • Corrosion: Chemical dissolution of rock minerals by the water itself, especially effective in limestone (calcium carbonate) which dissolves easily in slightly acidic water; the stream chemically eats away at the rock.
• How streams move material:
  • Dissolved Load: Invisible ions/minerals dissolved in the water itself.
  • Suspended Load: Fine particles (silt, clay) that make the water look muddy, light enough to be carried along with the flow.
  • Bed Load: Heavier stuff (sand, gravel, boulders) that roll, bounce, or slide along the stream bottom, most occurring in higher flows.
• Muddy water after rain is mostly suspended load.
• Settling velocity determines if a particle is suspended or stays on the bed.
  • Finer, lighter particles settle slowly and remain suspended longer.
  • Heavier, coarser particles settle faster and stay near the bed.
  • Depends on size, density, water viscosity, and particle shape.

Stream Types

• Bedrock Channels: Cut into solid rock in steeper areas.
• Alluvial Channels: Flow through sediments the stream itself deposited.
  • Meandering Channels: S-curves, single looping channel winding across a floodplain.
  • Braided Channels: Networks of smaller, interwoven channels with lots of sediment bars in between.
• Meander Formation:
  • Even a slight bend causes water to flow faster on the outside curve, causing erosion, and slower on the inside curve, causing deposition.
  • Over time, this exaggerates the bend, making it loop more and more until an oxbow lake is formed.

Deltas and Floodplains

• Deltas form where a river enters still water, velocity drops, and sediments deposit, often in a fan or triangle shape, building outwards.
• Natural levees: Raised banks along rivers, built up by coarser sediments dropped during floods when water overtops the channel.
• Alluvial Fans: Fan-shaped deposits at the base of mountains where a fast stream hits a flat plain, slows down suddenly, and dumps its sediment load.

Floods

• Regional Floods: Slow, large area, often from snowmelt or prolonged rain.
• Flash Floods: Rapid, deadly, intense, little to no warning from local rain.
• Ice Jam Floods: Ice blocks dam a river, failing and releasing floods.
• Glacial Outburst Floods (Jökulhlaups): Catastrophic releases of water can result in a flash flood.
• 100-year flood: 1 \% chance of happening in any given year.
  • Theoretically, can happen two years in a row but unlikely.
  • Much worse than a 50-year flood, which has a 2 \% chance of happening.

Flood Control

• Artificial Levees: Man-made embankments built along rivers to keep water inside the channel.
• Flood Control Dams: Hold excess water.
• Channelization: Straightening or deepening a river to move water faster.
• Floodplain Management: Trying not to build things in floodplain areas; engineering and urban planning.

Shorelines and Ocean Waves

• Shorelines are dynamic, always being reshaped by wave action.
• In deep water, water moves in a circular orbit (analogous to seismic Rayleigh waves).
• As water reaches shallow water near the coast, the bottom of the wave starts to feel the sea bed.
• Friction slows the bottom down, but the top keeps going faster, the wave gets taller and steeper, velocity decreases, but height increases, eventually becoming unstable and breaks with tremendous force during storms causing erosion through direct impact and pressure of the water.

Rip Currents

• Very dangerous, strong, narrow channels of water flowing back out to sea away from the shore.

Hurricanes

• Ultimate coastal hazard (also known as cyclones and typhoons).
• Massive low-pressure systems; pressure drops as you near the center, fueled by warm, moist tropical air forming late summer/early fall.
• Needs very warm ocean water above 80^\circ F.
• Coriolis effect is key to making hurricanes spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
• Hurricanes do not form right at the equator (within 5^\circ latitude) because the Coriolis effect is too weak to initiate that spin.
• What makes one hurricane more destructive depends on size, population density where it hits, the shape of the ocean floor near the shore (shallow slopes amplify storm surge), and the strength of the storm.
• The Saffir-Simpson Hurricane Scale measures storm strength based on wind speed.
• Storm surge is the biggest killer, especially in low areas; abnormal rise in sea level pushed to shore by winds and low pressure.
• In the Northern Hemisphere, it's worse on the storm's right side of the path because the counterclockwise winds and the forward motion combine on the right side.
• Satellites are key in watching the formation and tracking movement of hurricanes.
• Forecasters monitor storm wind speed and look for signs of strengthening, like hot towers (very tall, warm cloud formations inside the hurricane) which can signal that the storm is strengthening.

Groundwater

• A hidden but vital resource.
• The largest readily available freshwater reservoir for humans.
• Glaciers hold more freshwater when melted, but groundwater is more accessible.
• Irrigation is groundwater's biggest use; agriculture is the main consumer globally.
• Conservation and efficiency are key to reducing how much we take, such as by fixing leaks and using better irrigation methods.
• Zone of Saturation (Phreatic Zone): Below a certain level, all the pore spaces in the rock and soil are completely filled with water; the top of that zone is the water table.
• Above the water table is the Unsaturated Zone (Vadose Zone), where pores have both air and water.
• The water table can move; it rises in wet seasons and falls in dry seasons, generally higher under valleys and lower under hills.
• Overpumping by humans can drastically lower it, drying up wells and causing land to sink (subsidence).

Monitoring and Interaction with Surface Streams

• Observation wells (drilled holes) measure the water level inside over time to map its shape and changes.
• Streams get water from runoff but also interact with groundwater.
• Gaining Streams: Receive groundwater when the water table is higher than the stream bed; water flows into the stream from the ground.
• Losing Streams: Lose water to the groundwater system because the water table is below the stream bed; can be unconnected or connected with a saturation zone in between.
• Artificial Recharge: Adding water back into an aquifer using pumping water into injection wells using infiltration ponds (where surface water soaks down).
• Trying to replenish what we take out, but contamination is a huge risk; prevention is best.

Water Movement Through the Ground

• Porosity: Amount of empty space; how much the rock can hold.
• Permeability: How easily water flows through those spaces.
• High porosity doesn't automatically mean high permeability (e.g., clay can be very porous but has low permeability because the pores are tiny and disconnected).
• We need both for a good aquifer.
• Aquifers (e.g., sandstone and gravel) have both high porosity and high permeability, meaning they store and transmit water well.
• Aquitards (e.g., clay, solid shale) have low permeability and block or slow water flow.
• Darcy's Law measures the quantity of this flow: Q = KA \frac{hi - ha}{d}
  • Q = Hydraulic Discharge.
  • K = Coefficient that represents hydraulics conductivity.
  • A = cross sectional areas of the aquifer.
  • hydraulic Gradient = \frac{hi - ha}{d}
    • hydraulic gradient = difference in water table height over distance like the slope that pushes water
    • hydraulic conductivity = how easily water moves through a material
    • cross section area = size of the area water moves through
  • Darcy's Law: the actual volume of water that flows through an aquifer in a specified time

Perched Water Tables and Artesian Systems

• Perched Water Table: A small, localized saturated zone sitting on top of the main water table, usually trapped by an underlying impermeable layer.
• Artesian systems: Water is confined to an aquifer that is inclined so that one end can receive water, and aquitards both above and below an aquifer must be present to prevent the water from escaping. *Confined aquifer self-rising wells: water enters at high elevation creating pressure
  • Water enters at a high elevation, creating pressure. Drill a well into the lower part, and water rises on its own right to the surface and even gushes out without pumping.

Springs and Hot Springs

• Springs: Where the water table intersects the ground surface, letting groundwater flow out naturally.
• Hot Springs: The water is significantly warmer than the local average air temperature, usually heated by circulating deep underground.
• Geysers: Intermittent hot springs that erupt water and steam; needs specific underground plumbing (narrow channels that trap heat), more common in the western US due to more volcanic activity.
  • Leave deposits that precipitate out as hot water cools at the surface.
    • Geyserite: Siliceous sinter; water contains dissolved silica.
    • Travertine: Calcareous tufa; water contains dissolved calcium carbonate.
  • Not all hot springs are good to drink; some contain sulfur.

Groundwater Erosion and Deposition

• Groundwater acts as a powerful agent of erosion and deposition.
• Groundwater dissolves rock (limestone in particular) and forms caverns underground.
• Sinkholes form on the surface.
• It deposits minerals like calcium carbonate to form dripstone features.
  • Stalactites: Hang from the ceiling of caves, forming as mineral-rich water drips down and deposits calcite over time.
  • Stalagmites: Grow up from the floor, forming from droplets falling from stalactites; may connect to form columns.

Karst Topography

• Characterized by sinkholes, caves, and disappearing streams; how it looks depends partly on the water table level.
  • High water table level: Scattered sinkholes and disappearing streams.
  • Intermediate water table level: Large sinkholes and surface subsidence.
  • Low water table level: Rock towers and insoluble leftovers; features include sinkholes, collapsed or dissolved depressions.
• Karst Towers: Isolated steep limestone hills left behind as surrounding rock is dissolved away.
• Tower Karst: The final landscape evolution of karst topography; most soluble rocks are completely gone, leaving a landscape close to the water table peppered with rock towers of less soluble materials.

Climate Change - Past Climates

• Proxy data (Indirect evidence of climate change):
  • Ice Cores: Analyze trapped air bubbles and oxygen isotopes in the ice, giving past temperature and atmosphere composition.
  • Fossil Plant Data: Shows past vegetation, which reflects climate.
  • Tree Rings: Wider rings mean good growing years; narrower rings mean harsh conditions.

Natural Causes of Climate Change

• Greenhouse Gases: Earth gets energy from the sun; the surface radiates heat, and greenhouse gases trap some of it.
  • The most important and abundant greenhouse gas is water vapor, responsible for 50 \% of Earth's greenhouse effect.
  • Although some man-made gasses are way more powerful per molecule and CO2 is critical because of how much we're adding and how long it lasts.
• Long-term natural causes:
  • Plate Tectonics: Moving continents and changing ocean currents.
  • Milankovitch Cycles: Long-term wobbles in Earth's orbit.
    • Eccentricity: Shape of Earth's orbit around the sun.
    • Obliquity: Angle at which Earth's axis is tilted.
    • Precession: The slow wobble of Earth's axis (like a spinning top), which changes the direction it points, altering how solar energy hits the earth over 100s of years.
• Fast natural causes:
  • Volcanoes: Dual effect; big eruptions blast aerosols high up, causing short-term cooling but long-term warming by contributing CO2.
  • Solar Variability: The sun itself; the effect on current climate change is small compared to greenhouse gases, tectonics, and Milankovitch cycles.

Human Causes of Climate Change

• Deforestation: Releases stored carbon.
• Agriculture: Releases CO2 and methane.
• Burning Fossil Fuels: Releases CO2.
• Global warming does not mean every year is warmer than the last; year-to-year fluctuations happen, but the overall trend is up.

Climate Change - Future Predictions

• What we don't know: How long this transitional phase will last.
• Predicting future climate change is very difficult.
• Finding engineering methods to store and absorb CO2 is a goal.
• Positive feedback loops are in action.