Ch 5: Tectonics Flashcards
Divisions of the Earth by physical properties
Atmosphere, hydrosphere, lithosphere, asthenosphere, mesosphere, outer core, inner core
Divisions of the Earth by chemical compositions
Atmosphere, hydrosphere, crust, mantle, core
Continental crust is:
less dense rock, more Si and Al
Oceanic crust is:
more dense rock, more Fe and Mg
The three lithospheric plate boundary types are:
Divergent (spreading centres), Convergent (Subduction zones), Transform fault (transcurrent plate)
Divergent boundaries are:
Rifts and ridges
Characteristics of Divergent boundaries
marked by a ridge, rises 2-4 km above surrounding ocean floor, submarine volcanoes and earthquake activity, forms new lithosphere
Convergent Boundaries are:
convergent boundaries
Characteristics of Convergent boundaries
Linear topographic features that parallel boundary: deep ocean trench, volcanic arc (islands), orogens (mountain belts).
Abundant seismicity and magmatic/volcanic activity.(Benioff zone)
Define Orogens
belt of Earth’s crust that forms mountains
Define Benioff zone
A dipping planar (flat) zone of earthquakes that is produced by the interaction of a downgoing oceanic crustal plate with a continental plate.
Transform fault boundaries are:
transcurrent plate boundaries
Characteristics of Transform faults
variable length/size. Many smaller transform faults along mid-ocean ridges. Larger ones cut continental crust. Frequent shallow earthquakes. Sheering motion, lateral motion.
Example of transform fault:
San Andreas Fault: dextral (right lateral).
Plate movement velocities can be measured by:
Magnetic anomalies, GPS, re-occupied sites (via lasers)
Magnetic anomalies & plate movement
magnetic anomalies help predict plate movement velocities.
GPS and plate movement
network of satellites, provides very accurate locations
Re-occupied sites and plate movement
reoccupying sites over a period of years. Measures plate movement directly, usually via laser
Pangaea time
palezoic - mesozoic
Rodinia time
protozoic
Mechanism of plate movement
convection, gravity, wilson cycle, lithosphere drag
Convection & plate movement
hot rock rises and cool rock sinks
Lithosphere drag:
drives convection, as the plate is forced downwards, the heaviness of the plate helps draw the rest of the plate downwards
Debate:
is convection causing drag or is drag causing convection
Gravity
another plate movement mechanism, aids lithospheric drag
Paleomagnetism
pre-1960s geophysicist argued against continental drift. They validated plate movement using magnetism
Earth’s magnetic field
Earth behaves as if there is a bar magnet in the core
Direction of the magnetic field
Inclination (steepness), declination (direction).
Inclination indicates
indicates distance from pole
Declination indicates
indicates direction to pole, orientation
Remanent Magnetism
some ancient rocks were magnetized when formed. If age of rocks is known, remanent magnetism indicates the ancient location of the pole.
Silt orient themselves…
according to compass direction. Only if not disturbed.
Polar wandering
pole appears to have wandered through time.
APWP
apparent polar wander path. Samples taken off one lithospheric plate, either the pole moved or the continent moved.
Magnetic reversals:
N and S magnetic poles appear to have flipped through time. Reversed polarity in the inner core vs outer core. Outer attempting to invade the inner.
Reversal timescale
time scale of magnetic reversals, well established on land, relative dating technique
Linear magnetic anomalies
Field sightly stronger or weaker than normal. What the magnetic was when the rocks were formed
Surveys in the oceans show:
central positive anomaly, symmetric pattern.
Sea-spreading hypothesis:
magnetic anomalies result from remanent magnetism acquired during spreading of ocean-floor while magnetic reversals occured
Vine-Matthews hypothesis
magma would come to the surface and solidify with the current magnetic form
Useful for mapping ocean:
match magnetic anomalies with reversal history, measure growth rates, new ocean floor is found along mid-ocean ridges
Define Focus
(hypocentre) point of rupture on the fault. Energy can be spread in all directions, 360
Define Epicentre
point on the surface directly above the focus. Energy moves only sideways in all directions
majority of Earthquakes happen
majority of earthquakes happen within 70 km of the surface, more within 12 km.
Most earthquakes are on the _____ instead of the _____
subduction zone instead of the divergent zone
Measuring ground shaking
Ancient seismic detector, traditional seismograph, seismometer.
Magnitude:
amount of total energy released by an event.
Richter:
first values after an earthquake, at the epicentre.
Moment:
more accurate, later will be the official measurement, measures energy at the focus
Intensity
qualitative description, strength of ground shaking at a point. Depends on many factors
Seismic waves
Body waves or surface waves
Body waves: P-waves
From focus, primary waves, similar to sound wavs,
compression and expansion (dilation). Vibration direction parallel to propagation. Pass through solid, liquid or gas.
3-7 km/s in the crust
Body waves: S-waves
From focus, secondary waves. Shear waves, vibration direction perpendicular to propagation, solids only. 1.5-5 km/s in the crust
Surface Waves:
surface waves on land, form when body waves reach the surface, slower but larger than body waves, cause most damage
Further away from the source of the energy, the further…
the S-waves from the P-waves
Tsunami waves
Surface waves on the ocean, due to water displacement. Not tidal waves
Tsunami wave heights
low on open ocean, in shallow water, slow down and get higher
Origin of earthquakes
results from elastic strain followed by brittle fracture/failure.
Where do earthquakes occur
in cold rocks, typically less 70 km deep, most are within 12-15 km. Lithosphere
More strain =
more probability of having an earthquake
Finding the distance from an earthquake
distance of focus is found from interval between P and S arrival
Earthquake and the interior of the Earth
P waves from major earthquakes, redirect by something dense in the core, can determine the boundaries of the core.
Evidence from P-waves
evidence from core, P-waves are refracted by something dense in the core, acts as lens. Can determine the boundaries of the core
Interior: evidence from S-waves
S waves from major earthquake
S-waves and the core
S-waves are blocked by liquid core, evidence for core. S-wave shadow zones.
Distortion of plates
Large scale movement stresses material. This results in folds and breaks and extension and compression of crust.
Define isostasy
buoyancy of the lithosphere
Define orogenesis
mountain building process
Geophysics used what to map large scale changes
distortion of plates
Gravity _____ slightly from place to place on the Earth
varies
Gravity measurements are made with a
gravimeter
Gravity is affected by
density, composition, altitude, thickness
Gravity is slightly stronger over
dense rocks
Gravity is weaker over
thicker units of less dense rocks
Gravity and crustal thickness
many negative gravity anomalies are due to thick, low-density crust
Plate reconstructions
Rifting and spreading
Rifting
may eventually lead to opening of a new ocean
Ancient rift
rifting of an ancient continent led to opening of Atlantic ocean
Mountain Belts (orogens)
Plates are not perfectly rigid, large zones of distortion where continental crust is involved in subduction.l
Mountain belts typically
form high mountain ranges (orogens) –> crustal thickening
Rocks within orogens are…
crumpled (deformed)
Cordilleran orogens
Continental and oceanic crust above a subduction zone
Collision orogens
collision orogens form when two continents collide, isostasy prevents subduction of lower density crust
Subduction zone: oceanic and oceanic merging
volcanic islands
Subduction zone: oceanic and continental
cordilleran orogens, classic
Subduction zone: continental and continental
collision orogens
