g101

The principle of isostasy

adjacent blocks of crust are equal in terms of pressure beneath them even if they are at different elevations. The crust "floats" on the denser mantle, with thicker crust sinking deeper and thinner crust floating higher.

Airy's Model

Oceanic crust is lower than continental crust only because oceanic crust is thinner (both crust types are assumed to have the same density). Same density, different thickness.

Why is Airy's model wrong?

Continental and oceanic crusts have different densities.

Pratt's Model

Oceanic crust is lower than continental crust only because oceanic crust is denser. Both crust types are assumed to have different densities but the same thickness.

Why is Pratt’s model wrong?

Requires the depth to the base of the crust to be much thicker than indicated by seismic data.

Heiskanen hypothesis (correct model)

Continents are higher than areas underlain by oceanic crust because continental crust is both thicker and less dense than oceanic crust. This causes continental crust to "float" because it is less dense.

The thicker, less dense continental crust floats higher on the Earth's mantle than the thinner, denser oceanic crust, resulting in the landmasses being above sea level. This phenomenon is due to isostasy, where crustal blocks are in equilibrium with the mantle below

Less mass leads to ____

mountain root move to shallower depth

More mass leads to ____

development of deeper mountain root.

Thickening of crust leads to ____

1. Elevation increases

2. Thick root

Thinning of crust leads to _____

1. Rift valley

a. Elevation decreases

2. Thinning decreases root

What does gravitational collapse cause?

Stretching and thinning of the crust. Gravitational forces dominate

What do mountains rising cause?

Horizontal compressional forces dominate. Compression causes shortening and thickening of the crust.

Which planet can form larger mountains and why? Earth v Mars.

With planets that have less gravity, larger, vertical mountains are able to form. Mountains on Mars can be more vertical than mountains on Earth. This is because Mars has lower gravitational pull, allowing for taller structures without collapsing under their own weight.

Shield

basement Precambrian rock exposed at the surface

a large area of ancient, exposed crystalline rocks (primarily igneous and metamorphic) that form a stable part of a continent.

Platform

basement Precambrian basement overlaid by sediments and sedimentary rock

is a region where Precambrian rocks are covered by a layer of relatively flat, horizontal sedimentary rocks.

Dynamic topography

elevation change on Earth's surface caused by mantle convection, where rising or sinking mantle material pushes the crust upward or pulls it downward over geologic time.

- Plume push up the continent; subduction drag down the continent

Older ocean

deeper ocean basin - sea level decrease

Younger ocean

(at mid ocean ridge) - shallower ocean basin - sea level increase

Erosion (removal of material from mountains) relates to isostasy how

Thins the crust at the surface, reducing the weight.

The mountain root rises in response (isostatic rebound), like a boat rising when cargo is removed.

Deposition (adding sediment in basins or on crust): relates to isostasy how

Adds weight to the surface, especially in low areas.

The crust sinks under the added load (subsidence).

Tectonic Shortening (compression at plate boundaries): relates to isostasy how

Thickens the crust and builds mountains.

Creates a deeper mountain root to support the added height (like pushing more of the iceberg underwater).

Tectonic Stretching (extension at rift zones): relates to isostasy how?

Thins the crust, leading to uplift due to loss of mass and reduced lithospheric support.

Often forms rift valleys or high plateaus.

Glaciation relates to isostasy how?

Ice sheets add enormous weight → crust is pushed down.

After melting, isostatic rebound occurs, and the land rises again (ongoing today in places like Canada and Scandinavia)

What is the most accessible source of water for humans?

Groundwater

Stream water is the surface runoff from

1. Rainfall

2. Snowmelt

3. Infiltrated ground water where the water table intersects steam channels.

Quarrying (plucking)

a form of stream erosion where water dislodges and removes chunks of bedrock from the stream bed, especially in fractured rock under high flow conditions.

Abrasion

the bed and banks of a bedrock channel are bombarded by particles carried by the flow. The rotational motion of swirling pebbles acts like a drill to create pot holes.

Corrosion

Rock is gradually dissolved by the flowing water. Mostly takes place when streams passing through limestone rocks made of calcium carbonate CaCO3.

What types of load and how much load depend on

Sediment supply (bedrock type, vegetation; precipitation); Flow velocity(Relief, precipitation; water depth)

Bedload

large grains that cannot be picked up, but still are able to be moved. They roll, bounce, and slide along the bottom.

Suspended load:

small sediment grains that mix with the flowing water are transported above the bed, rarely touch bottom, and can make the water cloudy or muddy.

Settling velocity

Deposition of suspended particles occurs when the stream flows slower than their settling velocities. Transported through Bedload or deposited.

Bedrock channels

stream cutting into solid rock – near headwaters

Alluvial channels

bed and banks are composed largely of unconsolidated sediment (alluvium) that was previously deposited in the valley.

Meandering

a single main channel that has various loops and/or curves down its length.

more suspended load, finer grained, increasing stability, low variability, low slope

Braided

a series of small channels that are interwoven into a larger one.

More bedload, coarser grained, decreasing stability, high variability, high slope

Delta

A landform produced by a sudden deposition of a lot of sediment where a stream enters a lake, or sea.

What landforms do rivers create when flowing through broad floodplains or exiting mountainous terrain?

Alluvial fans and floodplains

Dissolved load

○ Minerals dissolved in water (e.g., salts, calcium, bicarbonate).

Invisible; transported in solution.

Groundwater Discharge

○ In areas where the water table (upper level of saturated ground) meets the surface, groundwater can re-emerge into streams.

water table

the top level of groundwater in the soil or rock underground where all the spaces (pores) are completely filled with water.

Zone of Aeration (Unsaturated Zone)

• The area above the water table.

• Contains both air and water in the pores.

• Water here is held loosely by soil — it’s not part of groundwater.

Zone of Saturation

• The area below the water table.

• All the pores and spaces are completely filled with water.

• This is where groundwater is stored and flows slowly.

Why does the water table matter?

When the water table intersects the surface, you get streams, springs, lakes, and wetlands. Becomes surface water.

If it drops too low (e.g. during drought): wells can dry up and streams may disappear

Water Table Behavior

• Rises with heavy rain or snowmelt (recharge)

• Falls during dry periods or when humans over pump wells

• Its not flat, it mimics the shape of the land but is more gently sloped

gaining stream

When a stream flows because of groundwater.

the water table is above the streambed.

losing stream

If a stream loses water to the ground. the water table is below the streambed.

Bedrock Channels:

○ Cut into solid rock.

○ Common in steep, mountainous areas.

○ Erosion dominates.

Alluvial Channels:

○ Formed in loose sediment (alluvium).

○ Common in lowland areas.

○ Can change shape easily (dynamic).

Formation of Meanders & Oxbow Lakes:

1. Stream curves due to erosion and deposition.

2. Over time, meanders become more pronounced.

3. During floods, the river may cut across a meander neck.

4. The abandoned bend becomes an oxbow lake.

Regional Floods

Long-lasting, widespread flooding from prolonged rain or snowmelt.

Flash Floods

○ Sudden, short-term flooding from intense rain.

○ Very dangerous, especially in narrow valleys or cities.

Ice-Jam Floods

Occur when ice blocks a river, causing water to back up

Shoreline

the line that marks the contact between the land and sea

The coastal zone

- The shoreline is constantly modified by waves

- Extremely short term

What Are Breaking Waves?

occurs when a wave's energy causes the water to collapse as it reaches shallow areas, like the shoreline.

This happens when the wave's bottom slows down due to friction with the seafloor, while the top continues moving faster, creating instability.

exert great force, and that force is even greater during storms

What is erosion caused by?

natural forces like water, wind, and ice; wave impact and pressure

Rip currents

flow in the opposite direction of breaking waves; concentrated movement of backwash that move along the surface inside of along the bottom as normal.

occur when waves break near the shoreline and push water towards the beach, causing a build-up of water in the surf zone. This excess water then flows back out to sea through narrow channels

Hurricanes

• Intense low-pressure centers which form a steep pressure gradient

• Sea surface temperatures must reach 27ºC to provide heat and moisture to the storm.

• do not develop within 5º of the equator because the Coriolis effect is too weak there (cannot form spiraling motion)

• anywhere poleward of 20º latitude cannot form

• rare in south atlantic and south pacific

Storm surge

• most devastating damage in the coastal zone, especially in the low-lying coastal areas

• - a dome of water 65–80 km wide that sweeps across the coast near the point where the eye make landfall

  - In the Northern Hemisphere, it is more intense on the right side of the storm.

  - In the Southern Hemisphere, it is more intense on the left side of the storm

Coastal Zone Instability

• The coastal zone is not stable — it's constantly reshaped by wave action.

• Erosion, deposition, and storms all contribute to coastal changes

As Waves Approach Shoreline; wave shoaling

○ The sea floor interferes with the wave base.

○ The wave slows down, wavelength shortens, and height increases → causes wave breaking.

Primary use of ground water

irrigation of crops, followed by drinking water, and then for industrial and domestic purposes

Has water usage increased or decreased since 1980?

Decreased due to conservation efforts and efficiency increases.

Is groundwater a renewable resource?

Depends on the replenishment/recharge of the system. If recharge is less than use, it will decrease over time, potentially leading to scarcity.

What is the vadose zone?

unsaturated zone - pore spaces contain both air and water.

What is the phreatic zone?

zone of saturation - Water in the zone of saturation is called groundwater.

Artificial recharge

the practice of using injection wells or infiltration ponds to assist ground water recharge.

Saltwater intrusion

the movement of saline (salt) water, typically seawater, into areas of freshwater, particularly in coastal aquifers. This can contaminate freshwater sources, making them unsuitable for drinking or irrigation, and damaging coastal ecosystems.

Water hardness is caused by what two elements?

Magnesium (Mg) and Calcium (Ca) ions.

Hard water

a. Precipitate in your water pipe in the form of calcite or dolomite

b. They decrease effectiveness of soaps

c. Clog, showerheads, irons, and coffee makers.

d. Linkage between kidney stones and hard water is debated - Mg, Caa are good for health. Continous consumption of really hard water associated with kidney stones.

Soft water

not as tasty; leaches ions from metal pipes, potential lead poisoning

Water Table Variation

• Seasonal changes (rises in wet season, lowers in dry season).

• Spatial variation (varies by geology/topography).

• Human usage can lower the water table (e.g., overpumping).

🔍 Observation wells monitor groundwater levels and help map the water table.

aquifer

geological formation that readily stores and transmits groundwater; high porosity and high permeability

aquitard

formation that restricts groundwater flow; low permeability

Well

Hole drilled into saturated zone to extract water.

Perched water table

Localized zone of saturation above the main water table, due to an impermeable layer.

Artesian System

○ Requires a confined aquifer under pressure.

○ Water rises in a well without pumping.

○ Flowing wells = water flows to surface naturally.

self-rising wells, 1. Water is confined to an aquifer that is inclined so that one end can receive water;

2. Aquitards, both above and below the aquifer, must be present to prevent the water from escaping – confined aquifer.

Porosity

the total volume of pore spaces in the rock divided by the total rock volume

Permeability

the easiness for the water to flow through/penetrate/infiltrate through the rock (linkage between the pores is more important)

If an aquitard is vertical then _____

it means the low-permeability layer runs up and down rather than horizontally. This can still act as a barrier to groundwater flow,

Cone of depression

decrease in the groundwater table or potentiometric surface that develops around a well from which water is being withdrawn

Water tower

provide a steady and reliable water supply by using gravity to create pressure and storing excess water for peak demand periods.

Can a skyscraper receive water if the skyscraper is taller than the nearby water tower?

No, because water pressure is required to move water up to higher elevations.

Water towers are elevated to use gravity to create enough pressure to push water into buildings.

the gravity-fed pressure from the tower may not be sufficient to reach the building's higher floors, unless additional pumping systems are used.

spring

natural outflow of groundwater that occurs when the water table intersects the surface.

hot springs

6°–9°C (10°–15°F) warmer than the mean annual air temperature for the locality where it occurs

hot springs heat source

- Temperature increases with depth: When groundwater circulates at great depths, it becomes heated. If the hot water rises rapidly to the surface.

- Ground water is located near magma bodies and hot recently formed igneous rocks.

geyser

can form if the underground plumbing does not allow heat to be readily distributed by convection. erupts periodically!

Where are hot springs and geysers primarily located in the United States?

Western United States because of more volcanic activities there.

Geyserite

Siliceous sinter: water containing dissolved silica

Travertine

calcareous tufa: water containing dissolved calcium carbonate.

What can some hot springs contain?

sulfur, which gives water a poor taste and unpleasant odor (Rotten Egg Spring, Nevada.)

NOT all natural spring water is tasty or even can be drinked…

Stalactite

a tapering structure hanging like an icicle from the roof of a cave, formed of calcium salts deposited by dripping water.

grow from the cave ceiling, T for top

Stalagmite

a rock formation that grows upwards from the floor of a cave; grow from the ground. G for ground.

Geologic Work of Groundwater

• Groundwater is a powerful erosive agent, especially in rocks like:

○ Limestone (mainly)

○ Gypsum

○ Marble

• Water + carbon dioxide → carbonic acid → dissolves calcite in limestone.

How do caverns form?

form in limestone through dissolution by slightly acidic groundwater.

How do dripstone form?

form when dissolved calcite is redeposited

Karst Topography

type of landscape shaped by the dissolution of soluble bedrock, primarily limestone, by groundwater. It features distinctive features like caves, sinkholes, and underground drainage systems.

Do we primarily use direct temp measurements or indirect temp measurements for climate?

Indirect temperature measurements like glacial ice cores, oxygen isotope concentration, water molecules, and tree rings.

Ozone (O3)

Absorbs UV light, keeps lives safe