Evolution of Earth’s Structure and Plate Tectonics
First World Atlas – 1570, Abraham Ortelius
North America and Europe can be fitted together. Biblical Flood.1600, Francis Bacon
An ancestral continent.
Mobilism – mountains made by shifting continents.
Permanentism – Continents are fixed in position connected by land bridges which sink to create oceans.
1858, Antonio Snider Pellegrini
Earth’s plan -Continents move as sheets by tidal centripetal forces.1910, Frank Taylor
Continental drift and Pangea
Used distribution of coal mines and glacial deposits, match of rocks, shape of coastlines.
1915, Alfred Wegener
Mantle convection currents
Convection currents driving the drift of continents
Mantles way hotter than possible, so it’s more malleable and can move, temp gradience as it’s hotter lower and cooler upper, mantle can diverge and hit the surface.1928, Arthur Holmes
Mid-ocean ridges and trenches
1957, Marie Tharp and Bruce Heezen
Sea floor spreading
Sea floor created at MOR’s recycled into mantle at trenches.1962, Harold Hess
Wander paths for continents; magnetic strips the ocean floor
Magnetic reversals – the polarity of the magnetic field switches from N to S.1965 Ted Irving; Fred Vine
Computer reconstruction of Pangea
1965 Edward Bullard
Transform faults and hot spots, plate tectonics
1965, Tuzo Wilson
Supercontinent cycle
1974, John Dewey
Geoconservation – ensuring there is enough of different rocks around protecting sites of importance
Geological heritage – tries to bring stories forward of its history and why we would care
Geotourism – tourism that’s associated with geological attractions and ensuring the tourists are visiting in an educated and responsible
Geodiversity – place that has lots of different rock types
Geochemist– analyze chemistry of rocks or minerals
Petrologist – study the makeup of rocks and how they form
Geophysicist – employ high-tech equipment to learn under Earth’s surface
Seismologist – measure and mitigate earthquake activity
Paleontologist – study fossilized remains of ancient organisms
Glacial geologist – study landscape left behind by ice sheets
Paleoclimatology – study of ancient climates and how we understand what has happened in the past
Geomorphology – study shapes how it shifts over time
What occurred during final stages of Earth’s formation?
Earth swept up chunks of space debris and was bombarded by huge meteorites
Why did the planet melt?
- heat generated by accretion and gravitational compression
- decay of radioactive isotopes
What is differentiation?
Process of zonation of different materials w/in a planet; heavier materials settled towards centre while lighter materials rose toward surface
- What was the result of differentiation?
Generation of large quantities of magma that rose to surface along fractures - What is the innermost core composed of?
Iron alloy (Fe w/ Ni and Si) - What is the mantle composed of?
Fe-Mg silicates (forming a rock called peridotite) - What is the outer crust composed of?
“Lighter” rocks such as basalt and granite - Describe the movement w/in the mantle
Hot rock behaves like wax as it moves in enormous convection cells > create stress in crust > breaks crust and uppermost rigid mantle into large pieces (lithospheric plates) - Lithospheric plates get pushed around on weak layer called asthenosphere
- What is the rock cycle?
Theoretical concept relating tectonism, erosion, and various rock-forming processes to common rock types - What is magma?
Molten rock - How do igneous rocks form?
When magma solidifies - Describe igneous rocks
– extrusive or intrusive
– may undergo weathering and erosion - What occurs when debris from igneous rocks are deposited on sea floor as sediment?
Can become sedimentary rock if lithified (cemented or consolidated into a rock) which can then recrystallize into metamorphic rock if temperature and pressure increases enough - What is plate tectonics?
Earth’s surface is divided into a few large, thick plates that move slowly and change in size
- What is continental drift?
Idea that continents move freely over Earth’s surface, changing their positions relative to one another
What is subduction?
Sliding of sea floor beneath a continent or island arc
What is convection?
Circulation pattern drive by rising of hot material and/or sinking of cold material b/c of its higher density
Describe convection as a driving factor
Hot mantle rock rises (a) basalt eruptions occur and (b) circulation pattern splits > mantle rock moves away (a) tension forms rift valleys and (b) sea floor is carried > hot rock sinks > Benioff zones of earthquakes and adesitic volcanism
What is the cause for the young age of sea-floor rocks?
Sea-floor spreading forms young sea floor from basalt eruptions at ridge crest which are then carried sideways by convection and is subducted into the mantle - What are plates?
Large, mobile slab of rock that is part of Earth’s surface (may be made up entirely of sea floor, or both continental and oceanic rock) - What is the lithosphere?
Relatively rigid outershell of Earth that contains rocks of crust and uppermost mantle
– oceanic lithosphere averages 70km thick
– continental lithosphere varies from 125 to 250km - What are plates a part of?
Lithosphere - What is below the lithosphere?
Asthenosphere
- What is the asthenosphere?
Zone of low-seismic-wave velocity that behaves plastically b/c of increased temp and pressure > acts as lubricating layer to allow plates to move - What is the asthenosphere made up of?
Upper-mantle rock - How thick is the asthenosphere?
70 to 200km - What can be found under the asthenosphere?
More rigid mantle rock - What is the result of a plate moving as a unit?
Interior of plate is relatively inactive tectonically - What is tectonic activity caused by?
Plate interactions at plate boundaries - What is a divergent plate boundary?
Boundary btwn. plates that are moving apart - What is a convergent plate boundary?
Boundary btwn. plates moving toward each other - What is a transform plate boundary?
Boundary at which 2 plates move horizontally past each other - How do we know that plates moves?
Paleomagnetic evidence - What are magnetic reversals?
Changes in polarity of the magnetic field - What is normal polarity?
Magnetic lines of force flow from south to north pole and our compass needles point N - What is reversed polarity?
Lines of magnetic force run the other way around and compass needles point S What is the magnetic polarity time scale?
It records the pattern of magnetic reversals over time
- How does the magnetic polarity time scale record patterns?
It uses stacked continental lava that contains magnetic minerals which allows for isotopic dating - How often does Earth’s magnetic field reverse?
Every 500,000 years - How long does it take for reversals to develop?
10,000 years - What is an anomaly?
Any deviation of magnetic strength from average readings - What is a magnetometer?
Instrument that measures strength of Earth’s magnetic field
Image: What is a magnetometer? - What is the Morley-Vine-Matthews hypothesis?
Pattern of magnetic anomalies symmetrical about the ridge crest (mirror image on either side of MOR) matches magnetic reversals - How did the pattern for the MVM hypothesis develop?
Continual opening of tensional cracks w/in rift valley on MOR crest > cracks filled w/ basaltic magma > cools to form dikes which records Earth’s magnetism at time of crystallization > moving sea floor cracks dike in 2 > halves carried in opposite directions > new magma intrudes newly opened fracture - How does the MVM hypothesis allow us to determine how fast plates move?
– measures rate of sea-floor motion
– predicts age of sea floor - How can we predict the age of the sea-floor?
We need to match a measured anomaly pattern at sea surface w/ known pattern of anomalies (using cores by fossil dating)
- What is the result of divergent plate boundaries?
Creates of opens new ocean basins - What is the divergent boundary marked by?
Rifting, basaltic volcanism, uplift, subsidence of continents, slow and fast spreading rates - What occurs during rifting?
– continental crust is stretched and thinned
– produces shallow-focus earthquakes on normal faults – rift valley forms as a central graben - What is basaltic volcanism?
When basaltic magma rises from the mantle to erupt on the surface as cinder cones and basalt flows - What causes uplift?
Upwelling of hot mantle beneath crust elevates the surface - How is the passive continental margin formed?
From marine sediment which forms a shallow continental shelf - What occurs when a spreading rate is slow?
Crest has rift valley - What occurs when a spreading rate is fast?
Rift is prevented from forming - What are mid-oceanic ridges?
Giant undersea mountain ranges that lie under the ocean and extends around the world - What are mid-oceanic ridges made of?
Mostly basalt - Describe mid-oceanic ridges
– more than 80,000 km long and 1,500 to 2,500 km
wide
– rise 2 to 3 km above adjacent ocean floor What is a rift valley?
Tensional valley bounded by normal faults found at diverging plate boundaries and along crest of MOR
- Describe heat flow on crest of MORs
High heat flow on crest that decreases as it moves away - What is the result of basalt eruptions at the rift valley on ridge crests?
Volcanoes that protrude above sea level as oceanic islands - What is the result of a continuous rift?
Sporadic volcanic activity - Where do fresh pillow basalts occur?
Narrow band along bottom of rift valley - Describe how hot springs carry and precipitate metal sulphides
Circulation pattern in which cold sea water is drawn downward, moves horizontally toward a rift valley, re-emerges on sea floor after being heated. This causes metals and sulfur to dissolve when it comes into contact w/ cold sea water which then precipitate at black smokers - How are exotic organisms able to survive in extreme conditions?
Feed on thermophyllic bacteria - What are ridges and ore deposits made of?
Predominantly iron, copper, and zinc w/ smaller amounts of manganese, gold and silver - What are transform boundaries marked by?
Shallow-focus earthquakes in a narrow zone (single fault) or broad zone (group faults) - What do transform boundaries indicate?
Strike-slip movement parallel to faults - What does the character of convergent plate boundary depend on?
Types of converging plates - What occurs during ocean-ocean convergence?
Subducting plate bends downward, forming the outer wall of an oceanic trench, which usually forms a broad curve convex to subducting plate - What is an oceanic trench?trough parallel to edge of a continent or an island arc
- What is an island arc?
A curved line of volcanoes that form a string of islands parallel to oceanic trench - How are island arcs formed?
During ocean-ocean convergence, a plate reaches depths of at least 100 km and generates magma which works upward to erupt - What does the distance btwn. an island arc and a trench depend on?
Where subducting plate reaches 100 km depth
– if subducting angle is steep, the distance is shorter
– if subducting angle is gentle, the distance is greater - What does an inner wall of a trench consist of?
Accretionary wedge (subduction complex) of thrust-faulted and folded marine sediment and pieces of oceanic crust - What occurs during ocean-continent convergence?
Accretionary wedge and forearc basin forms an active continental margin btwn. a trench and continent - What is a magmatic arc?
Term used both for island arcs at sea and for belts for igneous activity on edges of continent - What is the surface expression of a magmatic arc?
Either a line of andesitic islands or continental volcanoes - What are beneath magmatic arcs?
Large plutons in thickened crust which is seen as batholiths on land - How does hot magma rising affect continental crust?
It thickens to cause uplift and makes it weaker and more mobile than cold crust > regional metamorphism takes place
- What causes a mountain belt to grow?
– Benioff zone of earthquakes dips under edge of
continent during ocean-continent divergence
– stacking up of thrust sheets on continental side of
magmatic arc - What happens to a craton inland of a backarc fold-thrust belt?
Subsides to form sedimentary basin (foreland basin) which extends effect of subduction - What is a craton?
Continental interior - What helps form a fold-thrust belt?
Underthrusting of craton beneath core of mountain belt - How does ocean-continent convergence become continent-continent divergence?
As sea floor is subducted, ocean narrows until continents collide (one continent may slide a short distance under another but won’t subduct completely) - Where do 2 continents weld together during continent-continent convergence?
Along a dipping suture zone that marks the old site of subduction - What happens to the original magmatic arc during continent-continent convergence?
It’s now inactive and thickens the crust at the region of impact which is furthered by shallow underthrusting of continents and stacking of thrust sheets in the 2 thrust belts >> mountain belt formed in interior of continent - What is backarc spreading?
When regional extension tears an arc in two, and moves the two halves in opposite directions >> creates new oceanic crust
- What happens to ore deposits at convergent boundaries?
Metallic ores carried away from ridge crests and subducted - How does volcanism at island arcs produce hot-spring deposits?
Rich ore collects above local bodies of magma and sometimes distributed as sedimentary layers in shallow basins - What is an orogeny?
Episodes of mountain building characterized by intense deformation of rocks in a region - How does ocean-continent convergence form mountain ranges?
Accretionary wedge develops where newly formed layers of marine sediment are folded and faulted as they are snowploughed off subducting ocean plate - Where and how do fold-and-thrust belts form?
On craton (backarc) side of mountain belt; largely due to crustal shortening by convergence - What occurs during arc-continent convergence?
Intervening ocean is destroyed by subduction and remaining sea floor breaks away from arc to create new site of subduction and trench - How does arc-continent convergence form mountain ranges?
Arc welds to continent - What is the Wilson Cycle?
Cycle of splitting of a supercontinnent, opening of an ocean, closing of the basins, and collision of the continents - How do plate boundaries change plates over time?
Allow plates to move and jump to new positions - How do plate sizes change over time?
May grow due to addition of new sea floor or shrink due to subduction - What causes plate motions? Mantle convection
- What are the 3 proposals for why plates diverge and sink?
1) ridge-push 2) slab-pull 3) trench-suction - What is the ridge-push proposal?
Cooling sea floor subsides to form side slopes of a MOR - What is the slab-pull proposal?
Cold lithosphere sinking at a steep angle through hot mantle should pull the surface away from a ridge crest and down into the mantle - What is the trench-suction proposal?
If the angle is steeper than the dip then the trenches and plates are pulled horizontally seaward toward subducting plate - What are mantle plumes?
Narrow columns of hot mantle rock that rise through the mantle from thermal boundary layers at the base of the mantle - How are mantle plumes and hot spots related?
Plumes may form “hot spots” of active volcanism - How can plumes form “hot spots”?
When large head of a plume nears a surface, it causes uplift and eruption of vast fields of flood basalts. As head widens, the flood-basalt area widens and stretches the crust. The tail that follows produces a narrow spot of volcanic activity. - How do continents break up?
Mantle plume heats land and bulges it upward to form a dome marked by volcanic eruptions > crust fractures into a 3-pronged pattern: 2 fractures become continental edges as new sea floor forms, the 3rd is a failed rift that fills with sediment (aulacogen)
- How can 2 plumes split a continent and begin plate divergence?
Local uplift causes rifting which lengthens until land is torn in two and they begin to diverge - What occurs when plumes rise beneath centres of oceanic crust?
A line of volcanoes form, creating an aseismic ridge > volcanoes sink as they are carried away from the eruptive centre b/c of cooling >> line of extinct volcanoes increasing in age away from active volcano - What are seamounts?
Conical undersea mountains that rise 1000 m or more above the sea floor - What are guyots?
Flat-topped seamounts - What are aseismic ridges?
Submarine ridges that aren’t associated w/ earthquakes - What is an earthquake?
Trembling or shaking of ground caused by sudden release of energy in rocks beneath Earth’s surface - Describe the pathway of earthquakes
Earth activity > creates stress > rock eventually breaks > waves of energy released and sent out - What are seismic waves?
Waves of energy produced by an earthquake - What do seismic waves cause?
Causes ground to tremble and shake during an earthquake - What is a fault?
Break between 2 rock masses - What is the elastic rebound theory?
Sudden release of progressively stored elastic strain energy in rocks, causing movement along a fault
- Describe the pathway of elastic rebound
Tectonic forces cause rock to bend, lift, or stretch > continuous forces store more energy > stored energy exceeds breaking strength of rock > rock breaks > earthquake > energy from strain moves rock into new positions and creates seismic waves - What are the 2 models for fault behaviour?
– existing faults are strong > need large amount of stress for earthquake to occur
– existing faults are weak > need small amount of stress
for earthquake to occur - What are the limits to the depth where faults can occur?
– rocks near Earth’s surface are brittle
– rocks below are ductile b/c in presence of increased
temp and pressure - What are the suggested causes for deep quakes?
– minerals transform into dense forms as they subduct
– dehydration of water-containing serpentine and its conversion into glass - What is a focus (hypocentre)?
Point w/in Earth where seismic waves first originate; where rupture begins and spreads rapidly along fault plane - What is an epicentre?
Point on Earth’s surface directly above focus - What are the 2 types of seismic waves generated during earthquakes?
Body and surface waves - Describe body waves
Waves that travel through Earth’s interior, spreading outward from the focus in all directions
- Describe surface waves in general
Waves that travel on Earth’s surface away from epicentre; rock movement dies out w/ depth into Earth - What are the 2 types of body waves?
P and S waves - Describe P waves
– compressional (or longitudinal) wave in which rock
vibrates back and forth parallel to direction of wave propagation
– v. fast – 4-7km/s
– first wave to arrive at recording station
– can pass through a fluid (gas or liquid) and solid rock - Describe S waves
– transverse wave
– travels at 2-5km
– propagated by shearing motion
– rock vibrates perpendicular to the direction of wave
propagation; crosswise to direction that waves are
moving
– can travel easily through solid rock - Describe surface waves
– slowest waves set off by earthquakes
– produce more ground movement
– travel more slowly, so they take longer to pass
– love and rayleigh waves - Describe love waves
Ground moves side to side in horizontal plane that’s perpendicular to the direction the wave is travelling or propagating - How are love waves most like S waves?
– have no vertical displacement
– don’t travel through liquids
- Describe rayleigh waves
– behave like rolling ocean waves
– cause ground to move in elliptical path opposite to the
direction the wave passes - What is a seismometer?
Instrument used to measure seismic waves - Describe how a seismometer works
Its principle is to keep a heavily suspended mass as motionless as possible – suspending it using springs or hanging it as a pendulum. The mass acts as a point of reference in determining the amount of ground motion - What is a seismograph?
Recording device that produces permanent record of Earth motion detected by seismometer, in digital format that can be processed and displayed on computer terminals - What is a seismogram?
Paper record of Earth’s vibration - What are seismograms used for?
To measure the strength of earthquakes - What is a travel-time curve?
Plots seismic-wave arrival time against distance - How can P and S waves be used to determine the location of earthquakes?
Both waves start out from the focus at the same time and gradually separate b/c of their different travel speeds. Their time intervals can be used to determine the distance from a seismograph to a quake - What is the depth of focus?
Distance between the focus and epicentre; the deeper the focus, the further the distance - What is the 1st method to measure the size of an earthquake?
Find out how much and what kind of damage the quake has caused to determine the intensity
- What is the 2nd method to measure the size of an earthquake?
Calculate the amount of energy released by quake by measuring the height (amplitude) of one of the wiggles on a seismogram to assign a magnitude - What is intensity?
A measure of an earthquake’s effect on people and buildings that is expressed as roman numerals on modified Mecalli scale - What is a magnitude?
A measure of energy released during an earthquake - What is the Richter scale?
Numerical scale of magnitudes - What is a moment magnitude?
Most objective way of measuring energy released by large earthquake; determined from strength of rock, surface area of a rupture, and amount of rock displacement along fault - What kinds of damage can earthquakes cause?
– ground motion
– fire
– landslides
– liquefaction
– permanent displacement of land surface - What is ground motion?
The trembling and shaking of land that can cause buildings to vibrate - What is liquefaction?
When water-saturated soil or sediment turns from a solid to a liquid - What are aftershocks?
Small earthquakes that follow the main shock - What are foreshocks?
Small quakes that precede the main shock - What’s another name for a tsunami?
Seismic sea waves
- What causes tsunamis?
Great earthquakes that disturb the sea floor, submarine landslides or volcanic explosions - How is a tsunami formed?
Large section of sea floor suddenly rises or falls during a quake > all the water over the moving area is lifted or dropped for an instant > long, low waves spread rapidly over ocean - Where do earthquakes occur on a global scale?
Circum-Pacific belt and Mediterranean-Himalayan belt - Describe the Circum-Pacific belt
– encircles the rim of the Pacific Ocean
– w/in belt occurs approx: 80% of world’s shallow-focus
quakes, 90% of intermediate, nearly 100% of deep - Where do a number of shallow-focus earthquakes occur?
– along summit or crest of MORs, huge underwater
mountain ranges that run through all the world’s oceans
– isolated spots associated w/ basaltic volcanoes - What are Benioff zones?
Zones of inclined seismic activity - What is the relationship beetween earthquakes and plate tectonics?
Plate boundaries are identified and defined by earthquakes - Describe earthquakes at divergent plate boundaries
– shallow, restricted to narrow band
– lower magnitude than those that occur at other boundaries
– located along sides of rift valley and beneath its floor
– normal faults, parallel to rift valley
- Describe earthquakes at transform plate boundaries
– shallow
– strike-slip motion occurs on faults parallel to boundary
– aligned in narrow band along transform fault
– transform faults occur on ocean floor and offset ridge
segments, sometimes in continental crust - Describe earthquakes at convergent plate boundaries
– collision boundaries characterized by broad zones of
shallow earthquakes on complex system of faults
– underthrusting - How is a subduction angle determined?
Using horizontal and vertical distributions of earthquakes - What are subduction angles controlled by?
Plate density and rate of plate convergence - Where do intraplate earthquakes occur?
In areas of thinned or weakened crust and in distinct zones which are underlain by ancient rift systems and/or suture zones that record collisions of former continents - What are intraplate earthquakes triggered by?
Buildup of stress btwn. lithosphere and underlying asthenosphere or isostatic adjustments of crust due to loading or unloading by ice or sediment - How can we predict when earthquakes will occur?
monitor slight changes that occur in rock next to a fault before the rock breaks and moves (increase in microseisms due to openings of small rocks, rock magnetism, electrical resistivity, seismic velocity, rock porosity [water levels in wells], increase in radon emission)
- What is seismic reflection?
Return of some of the energy of seismic waves to the Earth’s surface after the waves bounce off a rock boundary; data from seismogram used to calculate depth of boundary - What is seismic refraction?
The bending of seismic waves as they pass from one material to another - When does seismic refraction occur?
Only if velocity of seismic waves is different in each layer - From what pathway does seismograph station 1 receive seismic waves?
Waves that pass directly through upper layer A - What are the 2 pathways in which seismograph station 2 receives seismic waves?
1) direct path straight through layer A
2) refracted path through layer A to layer B then back to layer A (receives the same wave twice) — may arrive first b/c layer B is of higher velocity - What is a seismic shot?
Seismographs set up in a line away from explosion - What is the function of a seismic shot?
It finds the distance between the point of transformation and epicentre of earthquake which can be used to calculate the depth to a boundary - Why do waves return to the surface?
– advancing waves give off energy in all directions and small part of it “leaks” upward into layer A
– waves follow curved paths b/c of small changes in directin as it passes through many layers w/ slightly higher velocities
- What is the crust?
Outer layer of rock which forms a thin skin on Earth’s surface - What is the mantle?
Thick shell of rock that separates the crust above from the core below - What is the core?
Central zone of Earth; probably metallic; source of Earth’s magnetic field - Describe the crust
– thinner beneath oceans than beneath continents
– seismic waves travel faster in oceanic crust than in continental crust
– Mohorovicic discontinuity - What are the 2 types of crust?
Oceanic and continental - What is the Mohorovicic discontinuity (Moro)?
Boundary that separates the crust from the mantle beneath it - Describe oceanic crust
– P waves travel through it at ~7 km/s
– upper part is basalt, lower part is grabbo (coarse-grained equivalent of basalt)
– 7 km in thickness
– Mafic (rocks high in Mg and Fe) - Describe continental crust
– P waves travel through it at ~6 km/s
– consists of crystalline basement composed of granite, other plutonic rocks, gneisses, and schists, all capped by layer of sedimentary rocks
– Felsic (rocks high in feldspar and silicon)
– avg. 30-50 km in thickness
– thickest under geologically young mountain ranges bulging downward as a mountain root into the mantle
– less dense than oceanic crust
- What does the upper mantle consist of?
Ultramafic rock which is dense igneous rock made up chiefly of ferromagnesian minerals; lack feldspar - What composes the lithosphere?
Crust and uppermost mantle - Describe rocks in asthenosphere
– closer to melting point than rocks above or below
— may represent zone where magma is likely to be generated
— rocks may have relatively little strength and therefore are likely to flow - What is the P-wave shadow zone?
Region btwn. 103 and 142 degrees that lacks P waves b/c of refraction of P wavese when they encounter the core boundary - What is the S-wave shadow zone?
Direct S waves not recorded in entire region more than 103 degrees away from the epicentre; larger than the P-wave shadow zone - WHat does the s-wave shadow imply?
Since S waves can travel only through solids, the core must act like a liquid >> core is made up of liquid outer core and a solid inner core - What is the composition of the core?
Metal – probably iron mixed w/ small amount of a lighter element, such as oxygen, sulphur, or silicon - What is the D” layer?
Transition zone up to 200 km thick at the base of the mantle where P-wave velocities decrease dramatically
- What is the core-mantle boundary formed by?
Ultra-low-velocity zone (ULVZ): iron silicate “sediment” accumulates unevenly and causes some liquid iron to be squeezed out of pore spaces to form electrically conductive layer that connects the core and the mantle - What is the core-mantle boundary undergoing?
Convection: circulation pattern in which low-density material rises and high-density material sinks - What is isostasy?
A balance or equilibrium of adjacent blocks of brittle crust “floating” on upper mantle - What is isostatic adjustment?
Concept of vertical movement to reach equilibrium; once at balance, a tall block (mountain range) extends deep into the mantle (mountain root) - What is crustal rebound?
The rise of the crust after the removal of ice - What is a gravity meter?
Measures gravitational attraction btwn. Earth and a mass w/in the instrument - What are the functions of a gravity meter?
– identify relative changes in gravity that may indicate local variations in rock density
– explore for metallic ore deposits
– discover whether regions are in isostatic equilibrium - What is a positive gravity anomaly?
When the gravity reading is higher than normal regional gravity - What does a positive gravity anomaly indicate?
Tectonic plates are holding a region up out of isostatic equilibrium which is caused by local concentrations of dense rock
- What is a negative gravity anomaly?
When the gravity reading is lower than normal regional gravity - What does a negative gravity anomaly indicate?
A region is being held down out of isostatic equilibrium or that local mass deficiencies exist - What is a magnetic field?
Region of magnetic force that surrounds Earth - What does a magnetic field do?
Deflects magnetized objects that are free to move - Describe the magnetic field
– North and South magnetic poles (dipolar) which are displaced about 11.5 degrees from geographic poles
– generated w/in liquid metal of outer core - Where is the strength of a magnetic field greatest?
At magnetic poles where magnetic lines of force appear to leave and enter Earth vertically - What is the Curie point?
Temp. below which metal becomes magnetized - What may be the causes of magnetic anomalies?
Circulation patterns in liquid outer core or variations in rock type - What is magnetotellurics?
Geophysical approach being used in remote regions of the Canadian Arctic to investigate and map structures w/in underlying crust and mantle - Define geothermal gradient
Rate of temp. increases w/ depth into Earth - Where is a geothermal gradient measured?
In abandoned wells or on sea floor by dropping probes into mud - What is the geothermal gradient?
25 deg. C per km of depth - What is heat flow?
Gradual loss of heat through Earth’s surface
- Where does heat come from?
Heat during formation OR by-product of radioactive isotopes inside Earth - What causes high heat flow?
Presence of magma body or still-cooling pluton near surface or radioactive isotope decay - Describe the heat flow from oceans and heat flow from continents
They are the same - What is a positive magnetic anomaly?
When the reading of a magnetic-field strength is higher than the regional average - What is a negative magnetic anomaly?
When the reading of a magnetic-filed strength is lower than the regional average
-Oceanic crust mainly made up of basalt
-Fissure eruptions r common in iceland
-The spinning outer core creates the magnetic field while the inner core is too hot to be magnetic
-Lahars common in volcanic and magmatic arcs
-Hot or cold mixture of rock and water fragments that flows does slops of a volcano and enters river valley
-Failed arms of triple junctions: many large rivers in NA (eg; st lawrence)
-Magnetic striping on the seafloor is a aproducts of linear basalt eruptions at mid ocean ridges recording changes in the magnetic polarity
-Trench landform can be found in an ocean basin immediately adjacent a subduction zone
-Plate motions can be measured directly with GPS
-Pillow basalt ia evidence of basaltic magma erupted underwater
-NA was called Laurentia
-Shallow-focus earthquakes can be found in MORs
-Decompression melting of mantle rock at plume heads produces flood basalt
-Passive margins are excellent sources of salt and hydrocarbons
-Ertle Ale is a shield volcano made of blood basalt
-Transform faults allows for the ocean floor to spread on spherical surfaces
-The cascade ranges of BC are an example of a magmatic arc
-Famine, tsunami, nuee ardente are hazards near subduction zonews (not lava lakes)
-Found near MOR: black smoker, fissure eruptions, rift valley
-The red sea is likely to grow larger in the coming millenia
-Idea of cont. crust plowing thru oceanic crust is eg of continental drift -What process often precedes the violent eruption of a volcano?
– sector collapse and landslide
-Oceanic crust is destroyed at the subduction zone
-Magmatic arcs do NOT form above oceanic-oceanic subduction zones -Famine is one of the greatest risks for ppl near subduction zones -Effects of seismic damage are amplified for cities built on soft sediments
-Pangea subcontinents = laurasia and gondwana