quiz 6 mountains, landslides, groundwater

What is isostasy? (in terms of geo layers)

the floating of the lithosphere on the asthenosphere, which changes depending on the weight of the lithosphere

Which of the following are ways mountains can form at convergent plate boundaries?

 

addition of igneous rocks to the crust 

compression wrinkles the crust into folds and thrusts

What are ways mountains can form at divergent plate boundaries?

 

addition of igneous rocks to the crust 

blocks of crust remain high as other blocks slide down around them 

the asthenosphere pushes upward, forming a bulge of high terrain

When will mountains grow? sink?

 

Mountains will grow if uplift is faster than erosion, otherwise they will shrink

If mountains are growing, isostasy will cause the crust to ____________. If mountains are being eroded down, isostasy will cause the crust to ____________.

 

sink, rise

What limits the maximum height of mountains on Earth?

 

the thickness of the lithosphere and the height at which glaciers form

Which of the following apply to continental accretion?

 

describes the process by which continents grow 

occurs at subduction zones 

has contributed to the makeup of North America

mountains can form at which plate boundaries

all of dem

the plate above doesn’t move as much as the subducting plate because

some of the motion is being taken up compressing and therefore thickening the crust

crustal shortening and thickening

• crust is compressed horizontally, so it grows vertically

• creates fold and thrust belts

  • erosion continually removes material from mountains, we don’t see the full thickness of fold and thrust belt

volcanic mountain building

• adding igneous rocks thickens the crust

• can happen at subductions zones

• can occur via asthenospheric melt at rifts

divergent mountain building

• asthenosphere rises under divergent boundaries creating bulge in the crust

  • as crust collapse, rift valleys (grabens) slide down along normal faults, leaving high blocks of crust (horsts) in between

Uplift vs. Isostasy vs. Erosion

• thickened crust is heavy, so it sinks due to isostasy

• if uplift(mountain building) is faster than erosion the mountains will grow

• if uplift shuts down or erosion is faster, the mountains will shrink

how high can mountains be?

• lithosphere can’t support tall weight of tall mountains, so they are collapsing as they grow

• due to thickness of earth’s lithosphere, mountains on this planet can’t get more than about 9km above sea level.

• high altitude and high steepness increases erosion so bigger mountains erode faster

glacial buzzsaw

glaciers are so effective at eroding mountains that they control the elevations of mountains where they exists

accreted terranes

• materials aren’t subducted but get sheared off the oceanic plate and stuck onto the upper plate, making it larger

continental breakup

continents break up from rifting

gondwana broken into many pieces and is still breaking up

porosity

The measure of how much empty space is in a rock, and thus how much fluid the rock can hold

permeability

The measure of how easily a fluid can flow through a rock

How can porosity develop in a rock?

 

space between sedimentary clasts that don't fit together perfectly 

pressure on a rock creates fractures that water can fill 

acidic water dissolves limestone, creating large spaces for water to fill

aquitard

An impermeable layer that acts as a barrier to groundwater flow

aquifer

A porous, permeable layer in which water can flow and collect

confined aquifer

An aquifer that is surrounded by impermeable layers so that water cannot easily escape

unconfined

An aquifer that is open to the surface so that water can freely flow in and out

Which of the following is true about the water table?

 

the water table mimics the shape of the ground surface 

the water table is the top of the saturated zone 

the water table changes depending on the weather conditions 

the water table can be above the ground 

the water table is not as high as the highest parts of the surface, or as low as the lowest parts

what is true about the potentiometric surface?

 

it is a measure of the pressure in a confined aquifer

Which of the following is true concerning groundwater flow?

 

groundwater flows are mainly driven by pressure differences 

gravity influences groundwater flow 

large amounts of water enter the ground in recharge areas 

groundwater flows through aquitards, but does so extremely slowly 

groundwater comes out onto the surface in discharge areas 

groundwater can flow into or out of rivers depending on the depth to the water table

characteristics of ephemeral river

the river is high above the water table

the river loses water to the ground

the river usually does not flow all year round

characteristics of perennial river

the river gains water from the ground

the river usually flows all year round

the river is at or below the level of the water table

Which of the following are ways in which springs can form?

 

fractures in the rock intersect the ground surface 

faults provide a conduit to the surface 

sudden changes in slope or elevation cause the water table to breach the surface 

an impermeable rock layer diverts groundwater to the surface

Where are the two sources of heat for water in hot springs discussed in the powerpoint slides?

 

magma chambers and a high geothermal gradient

What are the consequences of overpumping an aquifer?

 

a drop in the water table in a cone shape around the well

saltwater intrusion into wells

dust storms 

drying up of surface water 

ground subsidence 

contamination of well water by pollutants

Put the four steps in the development of karst topography in the correct order.

1. caves form as groundwater dissolves rock underground

2. sinks and sinkholes form as the ground collapses into caves

3. natural arches and disappearing rivers form as the ground collapses

4. most of the rock layer dissolves away, leaving behind a few pillars or hills

Springs: how does water move from the ground onto the surface?

• surface intersects with water table

• emerges at seep through porous layers

• if water encounters impermeable fault breccia, it will be forced to travel up along it eventually reaching surface

rocks can high porosity but terrible permeability if

pores are abundant but not connected, preventing flow

unsaturated zone also called

vadose zone

water table

the top of the saturated zone

Pressure drives groundwater flow

water flows from high pressure (high water table) to low pressure(low water table) gravity also influences flow

wells to confined aquifers are

artesian wells

groundwater flow rate:

4-500 m/year very slowly

well test

lower the water table and see how long it takes to recover

cone of depression

pumping rate is greater than recharge rate (overpumping)

groundwater pumping reduces …

the hydraulic pressure holding up the ground - ground sinks (almost 30 feet in the san joaquin valley!)

mass wasting

is the downhill movement of rock and soil material due to gravity. Landslides are a type of mass wasting

What is the angle of repose?

 

the maximum stable angle of a slope.

depends on grain size and water content

friction vs gravity - the steeper the slope the stronger the effect of gravity until gravity overcomes friction.

what is oversteepening? How can a slope become oversteepened? Select all that apply.

 

oversteepening - exceeding local angle of repose

erosion from rivers or other sources can eat into the bottom of a slope 

construction of roads, buildings, or other infrastructure can eat into the bottom of a slope

How does water saturation affect friction on a slope?

 

Hydraulic pressure from excessive groundwater pushes grains in the ground apart, decreasing friction

pre-existing weakness

jointing creates cracks along which the surface material can collapse

weak layers

weak and easily deformed rock layers provide little support

i.e. - volcanic ash beds, clay-rich rocks

What are all the factors that can weaken a slope?

 

oversteepening

devegetation

devegetation

pre-existing weaknesses (cracks and joints)

weak layers

shaking from earthquakes

Which of the following are techniques that can be used to strengthen slopes?

revegetation 

regrading and terracing 

cementing and fencing 

draining excess groundwater

anchoring 

protection against erosion

creep

very slow movement of soil over long periods of time

slump

ground breaks into blocks that slide downhill (like a series of cute little normal faults)

flow

fast landslides where there is enough groundwater for the soil to flow like a liquid

• earthflows - sand and mud

• mudflows - just mud

• debris flows - boulders

• classification depends on particle size

• velocity depends on water content, all have enough water to flow like liquid

slide

fast landslides where the soil or rock face is dry

solifluction (a type of creep)

freeze-thaw expansion of soil moves it downhill

hilina slump

hawaii dropping off into the ocean