Earth 201 Final

earthquake

vibration of Earth produced by the rapid release of energy

largely associated with faults

energy occurs in the form of waves

focus

energy released radiates in all directions from its source

epicenter

location on the surface directly above the surface

elastic rebound

rocks on both sides of an existing fault are deformed by tectonic forces

rocks bend and store elastic energy

frictional resistance holding the rocks together is overcome

rocks elastically rebound to their original shape and release energy

surface waves

travel along Earth’s interior

cause the greatest destruction

body waves

travel through Earth’s interior

two types of body waves

primary and secondary waves

primary waves

push-pull (compress and expand) motion, changing the volume of the intervening material

can travel through solids, liquids, and gases

P waves travel faster than S waves

S waves

shake motion at right angles to their direction of travel

only travel through solids

slower velocity than P waves

seismographs

record the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape

more than one type of seismograph is needed to record both vertical and horizontal ground motion

resulting record is called a seismogram

used to determine velocity of earthquake waves and location of epicenter

travel time curve

Graph that shows the relationship between the distance of an earthquake and the arrival time of seismic waves. It is used to determine the epicenter of an earthquake based on the arrival times of P and S waves.

draw horizontal line to calibration curve from the arrival times —> draw vertical line down from that point to determine the distance to the epicenter

locating an epicenter

triangulation: requires 3 seismic stations

earthquake energy release

moment magnitude is proportional to slip (displacement) on the fault times the area of the fault surface

earthquakes at divergent plate boundaries

are typically characterized by shallow-focus earthquakes and are caused by the tectonic plates moving apart, allowing magma to rise and create new crust.

tension and normal faulting at divergent boundaries

shallow earthquakes

earthquakes at convergent plate boundaries

are typically characterized by deep-focus earthquakes and are associated with the subduction of one tectonic plate beneath another, leading to intense pressure buildup and release.

downgong lithosphere gives shallow, intermediate, and deep focus earthquakes.

tsunamis

seismic sea waves

result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake

2011 Japan earthquake

lack of M9s in human record consistent with model that M9s only occur for young lithosphere with fast convergence rates - belief

short human record does not include rarer, larger multi-segment ruptures

thus: large eqs can still happen with very old lithosphere

liquefaction

loosely packed/water-logged sediments near the ground surface lose through strength in response to ground shaking and behave like a liquid, causing buildings and other structures to sink or tilt.

can earthquakes be predicted?

Currently, earthquakes cannot be accurately predicted due to the complex nature of tectonic processes, although researchers continue to study patterns and indicators to improve forecasting methods.

no short-range predictions

long-range predictions are possible based on historical data and tectonic stress accumulation (huge uncertainty)

hazard maps fail because of

bad physics, bad assumptions, bad data, bad luck

seismic hazard map of USA

high hazard on west coast

high hazard near Yellowstone

high hazard new New Madrid

high hazard near Charleston

basic premise of probing earth’s interior

travel times (velocities) of body waves (P and S) through Earth’s interior vary depending on the properties of the materials encountered and can provide insights into its composition and structure.

how seismic waves propagate through the planet

seismic waves velocity increases with depth

P waves travel faster than S waves

P waves propagate through solids and liquids

shear waves only propagate through solids

P-wave velocity of various Earth materials

low to high velocity

air, water, sandstone, salt, iron, granite, basalt, peridotite

Role of pressure

P wave velocity increases with increasing pressure

role of temperature

P-wave velocity decreases with increasing temperature

when seismic waves pass from one material to another they

reflect

refract (bend)

snell’s law

sin(theta₁)/sin(theta₂) = v₁/v₂

waves that travel far and deep have

fast velocities

deeper wave arrives before shallow wave

what would seismic wave paths look like if Earth had uniform properties?

They would be straight, predictable lines.

what would seismic wave paths look like if velocity increased with depth?

They would curve and refract, following a non-linear path.

take home message of seismic wave velocity changes

abrupt changes in seismic wave velocities that occur at particular depths demonstrate that the Earth is composed of distinct layers with unique chemical and physical properties

reality of seismic wave paths

refracted paths

reflected paths

refracted paths that refract again

solid to liquid back to solid → evidence of liquid outer core

Moho (Mohorovicic discontinuity)

P wave velocity increases

crust-mantle boundary

due to peridotite

5-20 km below the oceanic crust

30-40 km below continental crust

mantle low velocity zone (MLVZ)

P and S wave velocities decrease

lithosphere-asthenosphere boundary

peridotite near melting point

10-75 km below oceanic crust

100-300 km below continental crust

mantle transition zone (MTZ)

P and S wave velocities increase

410-660 km

atoms in minerals that compose peridotite reconfigure to accommodate high pressure

olivine transforms to spinel

core-mantle boundary (CMB)

P waves refract and produce the “P wave shadow zone”

S waves disappear and produce the “S wave shadow zone:

evidence that the outer core is liquid

inner core-outer core boundary (ICOCB)

P wave velocity increases

evidence that inner core is solid

seismic tomography

use of seismic waves to construct 3D images of Earth’s interior

map regions of slow and fast seismic velocity

principal is identical to medical CAT scans

seismic waves and Earth’s geothermal gradient

near surface geothermal gradient calculated from temperature measurements in mines and wells

extrapolation of these gradients suggests that peridotite should be entirely molten at 50 km (not the case)

center of earth should be 200,000 degrees C (not the case)

how do we know earth’s geothermal gradient?

geothermal gradient below 15 km is estimated based on seismic evidence as well as high P-T lab experiments

MLVZ indicates partial melting at the top of the asthenosphere

geothermal gradient must approach melting T of peridotite

travel of S waves in the mantle indicate that the mantle is mostly solid

geothermal gradient in the mantle must be less steep relative to the near surface gradient. otherwise, mantle would be molten

seismic wave velocities still increase through most of the mantle due to increased pressure

geothermal gradient in the mantle must still modestly increase because T increases as P increases

S waves do not pass through the liquid outer core

To melt liquid Fe, geothermal gradient must increase

seismic data indicate that the inner core is solid, but the melting temperature of Fe increases as P increases

hence, geothermal gradient could be as high as 6000 degrees C in the center of the Earth

continental rifting

birth of a new ocean basin

splits landmasses into two or more smaller segments

ex. East African Rift, Baikal Rift, Rhine Valley, Rio Grand Rift

two mechanisms for continental rifting

mantle plumes and hotspots

slab pull and slab suction

mantle plumes and hotspots

continental landmasses insulate the mantle and lead to the formation of a plume

hot mantle plumes cause the overlying crust to dome and weaken

lifting and stretching of the crust causes continental rifting

decompression melting produces large volumes of basaltic magma

slab pull and slab suction

subduction of oceanic lithosphere may pull a continent attached to an overriding slab and create a rift

sinking of a cold slab causes the trench to retreat or roll back due to flow in the asthenosphere

exerts tension on the overriding plate

not all rift valleys develop into full-fledged spreading centers

Some rift valleys may become inactive or only form local basins instead of leading to the continuous divergence of tectonic plates.

sea level

widespread marine sedimentary rocks in continental interiors indicate that whole continents were once submerged by seawater

sea level has changed by 200 meters or more over the past 500 Myr

plate tectonics provides the best explanation for changing the shape of ocean basins

accelerated sea floor spreading

shallow ocean basins

young oceanic lithosphere rides high in the asthenosphere

creates sea level high stand

decreased sea floor spreading

deep ocean basins

old oceanic lithosphere rides low in the asthenosphere in the asthenosphere

creates sea level low stand

two types of margins for ocean basins

passive and active

passive margins

not associated with plate boundaries

experience little volcanism and few earthquakes

found along most coastal areas that surround the Atlantic ocean

active margins

subduction zones

located primarily around the Pacific Ocean

abyssal plains

all oceans basins have them

likely the most level places on Earth

sites of thick accumulations of sediment

continental rise

found in regions where trenches are absent

thick sediment deposits

grades into continental slope

location where turbidity current deposit sediment to form deep-sea fans (turbidites)

passive

continental slope

marks the seaward edge of the continental shelf

relatively steep structure

boundary between continental crust and oceanic crust

passive

continental shelf

passive

flooded extension of the continent

varies greatly in width

gently sloping

contains oil and natural gas deposits

some areas are mantled by extensive glacial deposits

volcanic arc

active

built on overlying plate

• island arc if ocean-ocean boundary

• continental volcanic arc if ocean-continent boundary

forearc region

active

area between trench and volcanic arc

backarc region

active

located on the far side of the volcanic arc

deep-ocean trench

active

long, relatively narrow features

sites where subduction occurs

deepest parts of ocean

most located in the Pacific Ocean

accretionary wedge

active

chaotic accumulation of deformed and faulted sediments scraped off subducting plate

prolonged subduction may thicken an accretionary wedge enough, causing it to protrude above sea level

orogenesis

processes that collectively produce a mountain belt

mainly occurs at convergent plate boundaries

• ocean-continent

  • sierra nevada

  • andes

• continent-continent

  • himalaya

continent-continent collisions

involves two plates carrying continental crust

collisions produces compressional mountain rages that possess shortened and thickened crust

zone where two continents collide is called the suture

terrane accretion

small crustal fragments collide and merge with continental margins

accreted crustal fragments are called terranes

terranes are too buoyant to subduct and may have been microcontinents, island arcs, etc before accretion

today’s continents are the products of mountain building and terrane accretion at ancient plate boundaries

isostasy

gravitational equilibrium between the lithosphere and asthenosphere

that allows the Earth's crust to float at an elevation corresponding to its thickness and density.

add lead weight → block sinks

remove lead weight → block rises

glacier

thick mass of ice that originates on land from the accumulation, compaction, and recrystallization of snow

part of hydrologic and rock cycles

valley (alpine) glaciers

exist in mountainous areas

flow down a valley from an accumulation zone at the headwaters

ice sheets

exist on a larger scale than valley glaciers

greenland and antarctica

ice flows out in all directions from one or more snow accumulation centers

how much water is in glaciers?

contain greater than 2% of the world’s water

antarctic ice sheet:

• 80& of world’s ice

• 2/3 of earth’s freshwater

• if melted, sea level would rise 60-70 m

formation of glacial ice

glaciers form in areas where more snow falls in winter than melts during the summer

Steps

1. air infiltrates snow

2. snowflakes become smaller, thicker, and more spherical

3. air is forced out

4. snow recrystallizes into a denser mass of small grains called firn

5. firn fuses into glacial ice when thickness exceeds 50 m

accumulation zone

area where glacial ice forms

ablation zone

area where there is net ice loss due to melting, calving (large pieces of ice break off where a glacier meets ocean)

accumulation equals ablation

glacier is stationary

accumulation exceeds ablation

glacier advances

accumulation decreases and/or ablation increases

glacier retreats

plastic flow

occurs within ice

under pressure, ice behaves like a plastic material

deforms under its own weight

basal slip

entire ice mass slips along the ground due to the presence of water between ice and bedrock

rates of glacial movement

rates are variable

several meters per year are common

some glaciers exhibit extremely rapid movements called surges

glacial erosion

glaciers are capable of enormous erosion and sediment transport

avg. erosion rates

• ice sheets: 0.01 mm/yr

• valley glaciers: 10-100 mm/yr; sediment load 100x the world’s largest rivers

abrasion

rocks within ice act like sandpaper

produces rock flour (pulverized rock), glacial striations (grooves in bedrock), glacial polish

plucking

lifting of rocks

2 mechanisms of glacial erosion

abrasion and plucking

5 landforms that are particularly distinctive of current of past valley glaciers

U-shaped valleys (erosion)

knife-edge ridges and pointed peaks (erosion)

overdeepened valleys containing lakes (erosion)

hanging valleys (erosion)

moraines (deposition)

glacial drift

all sediments of glacial origin

till

drift deposited directly by ice

poorly sorted mixture of gravel, sand, and mud that lacks bedding

stratified drift or outwash

drift deposited by glacial meltwater

sorted and distinctly bedded

moraines

layers or ridges of till deposited at the margin of ice

• lateral, medial, end

moraine formation

upward and forward flow of ice carries sediment to the front of the glaciers

melting releases sediment

characteristics of depositional landforms related to ice sheets

• larger versions of those created by valley glaciers

• large areas of scoured, plucked, and abraded bedrock

• large regions covered with thick till

• streamlined regions of till that parallel the direction of ice movemnt

• large and small lakes formed by erosional and depositional processes

drumlins

smooth, elongate, parallel hills of till

eskers

deposited by meltwater flowing over, within, and at the base of motionless ice

kames

mounds of till deposited within openings, or depressions on top of, ice

kettles

depressions in till left behind when a block of ice melts

last glacial maximum and effect on North American hydrology

21000 years ago

the last significant advance of ice that shaped rivers and lakes.

Missoula Floods

long island, cape cod, martha’s vineyard, nantucket are end moraine formed during the LGM

indoor water usage

toilets and showers/baths have the greatest percentage of daily use

dishwashers have the least percentage of daily use