Geology Midterm 2: James Kerr

Causes of mountain building (list)

Subduction (oceanic-oceanic and oceanic-continental), continental collision, and hot-spot volcanism

Mountain building: oceanic-oceanic and oceanic-continental

Both are subduction-driven; uplift plus volcanism along the overriding plate

Mountain building: continental collision

Two continents converge and neither subducts easily, so crust is crumpled and uplifted (e.g., Mount Everest, Nepal/China)

Mountain building: hot-spot volcanism

Volcano builds up over a mantle hot spot (e.g., Mauna Kea, Hawaii)

Why is 'tallest mountain' ambiguous? (Mauna Kea vs Everest vs Chimborazo)

It depends how you measure: Mauna Kea = base-to-summit (hotspot volcanism); Everest = highest above sea level (continent-continent collision); Chimborazo = farthest from Earth's center (subduction uplift and volcanism)

Common trait of Everest, Chimborazo, and Mauna Kea

All are relatively young mountains from recent uplift, though by very different processes

Youngest mountains in North America

Alaska and western North America (current active margin)

Appalachians + northeastern US: age and tectonic setting

Older mountains; active margin in the Paleozoic

Cratons / Archean shields (Greenland, northern Canada)

Very old stable continental cores; active margin back in the Archean

Highest point in western vs eastern USA

West: Mount Whitney, CA, 4,421 m. East: Mount Mitchell, NC, 2,037 m

Old mountains: features

Little or no active uplift; erosion dominates; mountain is shrinking; rounded mountains; gentle slopes

Young mountains: features

Active uplift; height set by uplift vs erosion; jagged peaks; steep slopes

What determines the height of a young mountain?

The balance of uplift vs erosion

Karst (eroding limestone) and mountain steepness

Can form very steep-sided mountains or chimneys under acidic conditions; steepness is NOT strictly a function of age

What controls the shape of mountains?

Composition of the rock and weathering conditions

Limestone in arid vs humid environments

Arid: limestone is more resistant than many other sedimentary rocks (forms ledges). Humid, forested: limestone weathers faster than other sedimentary rocks (forms valleys, e.g., Appalachian Valley)

Evidence of old mountains under the surface: ductile deformation

Folding from tectonic collision (can show as tilted beds), metamorphism consistent with mountain building, strongly foliated rock (directed pressure), high metamorphic grade

Evidence of old mountains: brittle deformation

Faulting

In old mountains, how does topography relate to ancient compression?

Topography is perpendicular to the ancient compression direction; rivers and streams follow the topography

Connecticut metamorphic rocks: what do they represent?

A highly eroded mountain range built in phases during the Ordovician and Devonian (continental collision and subduction zone)

Largest mountains in the world during the Devonian

Likely on the US east coast (the ancestral Appalachians)

Modern analog for the Devonian Appalachians

Ecuador today (Andes): very tall, subduction-related, tropical latitudes, calderas/stratovolcanoes (but the biota would have been very different)

Evolution of East Coast geography: the sequence

Paleozoic active margin with mountain-building (Ordovician and Devonian phases; diagram labels Taconic, Acadian, Alleghanian orogenies), then Mesozoic rifting of Pangea, then a passive margin (no active plate boundary since the Jurassic)

Why is the East Coast now a passive margin?

Pangea rifted apart; the coast has been in the middle of a plate (no active boundary) since the Jurassic, with thermal subsidence and no uplift

Fault

A fracture along which blocks of rock have moved; a product of brittle deformation

Hanging wall vs footwall

Hanging wall = the block above the fault surface; footwall = the block below it

Strike vs dip of a fault

Strike = the direction of the fault's line on the surface (trace); dip = the direction/angle the fault surface slopes down

Normal fault

Caused by EXTENSION; blocks move apart; hanging wall moves DOWN along the fault surface

Reverse fault

Caused by COMPRESSION; blocks move together; hanging wall moves UP along the fault surface

Thrust fault vs reverse fault

Thrust = a reverse fault with a gentle dip (low angle); reverse = steep dip

Strike-slip fault

Blocks slide horizontally past each other (left-lateral or right-lateral); no up-and-down motion; caused by shear

Oblique-slip fault

Combines dip-slip and strike-slip motion; one block moves diagonally relative to the other

Dip-slip faults (include which types?)

Normal and reverse (thrust) faults; motion parallel to the dip of the fault

Why are there earthquakes on the passive East Coast (e.g., 2024 NJ M4.8)?

Intraplate earthquakes: reactivation of old faults in deformed bedrock where ancient plate boundaries used to be

Mass movement (definition)

Downslope or downward movement of a solid, continuous or discontinuous mass under the influence of gravity

How does mass movement smooth Earth's surface?

Removes material far from Earth's center and deposits it closer to Earth's center

Scar vs toe (of a mass movement)

Scar = the exposed surface where material broke away (top). Toe = the deposit at the bottom of the movement

Talus / scree slope

Accumulation of rocks or sediment at the bottom of a slope or cliff

Angle of repose

The steepest angle at which a pile of loose material is stable (example slide: 32 degrees)

Slope vs angle of repose: stable or unstable?

Slope at or above the angle of repose = unstable. Slope at or below = stable

Can the angle of repose change?

It is inherent to the material; it only changes if something about the material changes

Factors that influence the angle of repose

Grain size, cohesion of grains, composition, wet vs dry

Why do mass movements happen?

Slope angle exceeds the angle of repose; uplift; removing matter at the bottom/adding at the top; changing water content; shocks or jolts (e.g., earthquakes)

Effect of adding water on slope stability

A little water can stabilize a slope; a lot destabilizes it

Four main types of mass movement

Falls, flows, slides, subsidence

Two axes used to classify mass movements

Speed and water content

Liquefaction

Brief fluid-like behavior of sediment; happens when pore space is saturated with water and a sudden force (e.g., earthquake) is applied

Rockfall

Rock falls freely from cliff faces (natural cliffs, quarry walls, road cuts); rapid and dry

Debris flow

Fast landslide (can exceed 35 mph) from intense rainfall or rapid snowmelt; carries boulders, cars; can travel miles in a narrow channel; recent burn areas are especially susceptible

Mudflow

Rapid, wet; like a debris flow but a mix of water and fine-grained sediment

Lahar

Flow of water, volcanic ash, and sometimes ice; can exceed 120 mph (200 km/hr)

Slumping

Loosely consolidated material moves downslope; can lead to more rapid mass wasting events

Soil creep

Slow downhill movement of unconsolidated rock and soil; slowly damages structures (loss of foundation support, interrupted roads, lateral stress)

Subsidence (as a mass movement type)

Ground sinking (downward movement)

Catchment (drainage) basin

The area where any water that falls will drain to the ocean via a particular river

Discharge (Q) definition and formula

Volume of water passing a point per unit time; Q = depth x width x velocity (d x w x v)

Relationship between catchment basin size and river discharge

Larger catchment basins collect more water, so they generally have larger discharge (e.g., Mississippi drains everything from the Rockies eastward and the Appalachians westward)

Why is the Mississippi basin so large?

It drains everything east of the Rockies and west of the Appalachians; determined by plate tectonics

Three sources of water that feed rivers

Rainfall, snowmelt, groundwater

Hydrological (water) cycle: why it matters

Moves water between ocean, atmosphere, and land; determines weathering rates and is crucial in the carbon cycle (Venus: no liquid water, runaway greenhouse)

Source zone of a river: features

High energy; steep gradient; erosion > deposition; coarse, poorly sorted, angular sediment; braided rivers; flash flooding; waterfalls

Floodplain zone of a river: features

Low energy; shallow gradient; deposition > erosion; fine, well-sorted, rounded sediment; meandering rivers; broad floodplains; regional flooding

Braided river

Low discharge relative to sediment load; near sediment sources; steep gradient and coarse sediment; often tied to rapid uplift; immature sediment

Waterfall

Forms near the source where a more erosion-resistant rock layer overlies a less resistant one

How a meandering river behaves

Flows slowly; erodes the outside bend (cutbank) and deposits on the inside (point bar); meanders migrate, building the floodplain; can be cut off to form oxbow lakes

Point bar

Deposit on the inside of a meander where water flows slower and drops sediment

Cutbank

Eroded outer bank of a meander; where meanders reach the floodplain edge they widen the floodplain

Oxbow lake

A meander that has been cut off from the main channel

Floodplain

Relatively flat area surrounding a meandering river, formed by the migration of meanders

Gradient of floodplain rivers

Extremely shallow (Mississippi, Memphis to New Orleans: ~220 ft over 500 miles, under half a foot per mile)

Delta

Where a river meets the ocean and deposits sediment

Three delta types

River (current)-dominated, tide-dominated, wave-dominated

Current (river)-dominated delta

River current stays strong at the mouth; sediment builds outward; many narrow distributaries (e.g., Mississippi)

Tide-dominated delta

Tides strong enough to reverse river flow at high tide; broad, funnel-shaped distributaries; elongate channel bars/islands between them

Wave-dominated delta

Limited river current and tides, so wave action dominates; sediment is moved by longshore drift

Three options for a stream with increased discharge

1) Erode the stream bottom, 2) increase meandering, 3) flood

Flash flood

Quick (minutes to hours); source areas; topographically variable; localized downpours; very unpredictable; poor drainage areas

Regional flood

Slow (days to weeks); floodplain; flat places; large accumulated discharge over a large area; more predictable; usually on soft sediments

Recurrence interval (RI)

Time between a flood and the next flood of the same size or larger; small floods are frequent, large floods have a longer RI

Passive margin

Boundary between continent and ocean with NO tectonic plate boundary; coasts on either side of an oceanic spreading center (e.g., US East Coast)

Active margin

Boundary between continent and ocean WITH a tectonic plate boundary (e.g., Pacific Ring of Fire)

Passive margin coast features

Long sediment buildup (deposition > erosion); beaches, dunes, marshes, reefs, barrier islands; broad continental shelf; sandy shores

Active margin coast features

Sediment can't build up (erosion > deposition); cliffs, bluffs, sea stacks, arches, terraces, pocket beaches; narrow continental shelf; rocky shores

Stages of ocean basin rifting (examples)

East African Rift (continental rift basins) then Red Sea (new ocean basin) then Atlantic (mature basin, rifting ~175 Ma)

Deep ocean zones, deep to shallow

Trench, abyssal plain, ridges, seamounts / shelves

Deep ocean sedimentation rate (general)

Extremely slow; the deep ocean is dark, deserted, and depositing sediment really slowly

Where is deep-ocean sedimentation lowest vs highest? (SAR map)

Lowest: open-ocean basins far from land; highest: near continental margins and river mouths (map-based)

Sediment accumulation rate (SAR)

How fast sediment accumulates on the seafloor (cm per thousand years on the slide's map)

Manganese nodules

Form in the slowest-sedimentation deep ocean areas by precipitation of metals from seawater; growth ~1 cm per million years

Conditions for manganese nodule formation

Low sedimentation rates, polar water currents, deep-water flow removing excess sediment, oxygen supply, sediment with metal-rich pore fluid

Why does sediment get thick on passive margins?

Sediment piles up if it isn't subducted or eroded; no plate boundary means it stays

Isostatic rebound

Crust rising after the weight of glacial ice is removed (glacial down-weighing then uplift)

Isostatic rebound and New England coasts

Rocky coasts in New England result from post-ice-age uplift (erosion > deposition)

Glacial forebulge

Area just south of the ice sheet that subsides (sinks) while the northern half of North America rebounds upward; affects DC

Longshore current

Current produced as waves hit the shore and refract/bend; travels parallel to the coastline

Longshore drift

Transport of sediment along the shore by the longshore current; makes sediment migrate along the coast

Beach

Sediment deposit along the coastline; local sediment; size and shape set by sediment abundance and wave energy

Barrier island / spit / marsh: what shapes each?

Barrier islands: waves; marshes: tides (calm, low energy, fine sediment, organic matter); spits: sediment redistributed by longshore currents

What determines East Coast coastal geomorphology?

Tectonics (passive margin), rivers and sediment supply, glaciers and isostatic rebound, and shoreline processes (waves, tides, currents)

Relationship between mountains and deserts

Mountains force moist air up (orographic precipitation) so it drops moisture on the windward side; the leeward side is a dry rain shadow where deserts form