Final Gel 001 Flashcards
1
Q
Wegener
A
Alfred wegener come up with the idea of continental drifts, was a German meteorologist /geologist
2
Q
Pangea
A
Alfred wegener: continents were once combined into huge content called Pangea
3
Q
Continental drift
A
the movement of continents resulting from the motion of tectonic plates.
4
Q
Exploration of the oceans
A
•WWII & cold war
•echo sounding
5
Q
Bathymetry
A
•The depth and topography of the sea floor
•Avg depth of world ocean ~4km below sea level
6
Q
Mid-Ocean ridges
A
• Occur in the mid-atlantic, East pacific, Indian Ocean, between Australia & Antarctic in the southern Ocean
• roughly symmetrical
7
Q
Abyssal plains
A
Flat regions of Ocean between the Mid-Ocean ridges and the continents
8
Q
Fracture zone
A
Mid-Ocean ridges are bisected at right angles by steep-walled fracture zones that parallel one another, segmenting and off setting ridges into smaller pieces
9
Q
Deep Ocean trenches
A
Regions adjacent to chain of active volcanic islands/ exhibit narrow, deep ocean trenches
• depths 8-11 km
• Marianas trench in the western Pacific) the challenger deep
10
Q
Continental shelf, slope, rise
A
• Shelf: gently steepening underwater continuation of the coastal plain along of edge continents
• shelf → slope: abrupt drop off. Covered by sediment few km thick
• slope: marks true edge of continents. Is the drop off of shelf region
• rise: gentle stope covered with sediment derived from the continental shelf hat acts as a transition zone between the steep continental slope and flat abyssal plains
11
Q
Coastal plain
A
the continental shelf is just the gently steepening underwater continuation of the coastal plain along the seaward edge of the continents. The coastal plain meets the continental shelf at the shoreline.
12
Q
Plate
A
•Most plate boundaries occur on the seafloor and coincide with mid-ocean ridges, deep-ocean trenches, and fracture zones. These bathymetric features of the seafloor are fundamental geologic boundaries that mark the edges of thick slabs of rock called plates.
13
Q
Tectonics
A
“tectonics” refers to large scale movement and deformation of Earth’s outer surface (crust plus upper
mantle)
14
Q
Plate tectonics
A
•the continual motion, creation, and destruction of parts of the planet’s active surface.
• ~100-150 km thick: based on composition = crust, mantle, and core, strength = lithosphere, asthenosphere,
15
Q
earthquake distribution
A
earthquake distribution follow somewhat linear patterns. They tend to be located along the edges of continents or strung out along linear trends down the middle of oceans.
16
Q
volcano distribution
A
volcano distribution follows a similar linear trend as the earthquake locations. The distribution of active volcanoes around the Pacific Ocean is called the “Ring of Fire” because they are among the most violent and deadly volcanoes in the world.
17
Q
locations of earthquakes and volcanoes
A
The locations of earthquakes and volcanoes, in concert with large bathymetric features on the seafloor,
mark the locations of tectonic plate boundaries - they occur irrespective of the geographic boundaries
of continents and oceans based on the arbitrary position of sea level.
18
Q
Lithosphere
A
Lithosphere = consists of the crust and upper mantle down to about 100 to 150 km beneath Earth’s surface
- lithosphere beneath continents is thicker than lithosphere beneath oceans (primarily because
continental crust is thicker than oceanic crust) - lithosphere is a “cool and strong” layer that behaves rigidly (it bends, flexes, and breaks, but does
not flow easily) - plates consist of lithospheric rock
19
Q
Asthenosphere
A
Asthenosphere = upper mantle down to ~ 400-600 km
- boundary between lithosphere and asthenosphere defined by a temperature of ~1280°C, the
temperature at which rock (at high pressures) begins to slowly flow when acted upon by a force
- “hot, weak, semi-plastic” strength properties – the asthenosphere is solid, but mobile
- heat moves by convection (more on this later) in the asthenosphere
So, in sum, rigid lithospheric plates move above weak, slightly molten asthenospheric rock.
20
Q
Divergent plate boundaries
A
Boundary between two plates that contributes to the growth of ocean basins or the break-up of continents.
- divergent boundaries commonly occur along mid-oceanic ridges and contribute to the continual growth of older ocean basins (e.g., Atlantic, Pacific, Indian Oceans) or they may occur within continents where they act to open new ocean basins (continental rifting,
- the primary force at divergent boundaries is extension – ‘stretching’ caused by the motion of the two plates away from each other
- two main types of divergent boundaries: mid-ocean ridges and continental rifts S
21
Q
Extensional stress
A
the primary force at divergent boundaries is extension – ‘stretching’ caused by the motion of the two plates away from each other
22
Q
seafloor spreading
A
•Mid-ocean ridges are commonly called “spreading ridges.
•Seafloor spreading is the process where magma wells up along fractures in the lithosphere near the mid-ocean ridge axis and pours out as lava onto the seafloor
23
Q
Axial rift/ rift valley
A
The lava erupts and solidifies along a narrow, central rift valley (aka ‘axial rift’) that occupies the ridge axis. The axial rift valley has typical dimensions of about 500 m deep and 10 km wide, bordered by
steep cliffs.
As plates are pulled apart along the spreading axis by extensional (divergent) forces, the rocks of the
brittle crust break along faults, with blocks of rock sliding downward to create the axial rift valley and
adjacent ridges
24
Q
Magma vs. Lava
A
•both magma and lava are molten rock. The only difference is that magma is below ground, whereas lava is above.
25
Magma chamber
the underlying rock of the asthenosphere passively rises beneath the thin lithosphere above and begins to melt as the overlying weight of rock is reduced, producing magma (this is called “decompression
melting” and occurs by the release of pressure rather than any increase in temperature)
- magma accumulates in magma chambers beneath the axis of the mid-ocean ridge, sort of a 'holding pen' for the magma supply
- most of the magma solidifies in place beneath the surface, while some finds its way to the surface where it pours out as lava, cools and solidifies, forming brand-new seafloor
26
Pillow basalt
the lava interacts with cold seawater to form bulbous pillow shapes with the lava solidifying to form the main rock of the ocean floor – pillow basalt
27
Seafloor
Because all seafloor forms at mid-ocean ridges and spreads laterally with time, the youngest seafloor lies along the ridge axis and the oldest seafloor lies along the outer margins of the oceans adjacent to continents
- all ocean floor on the planet is less than 200 million years old (extremely young compared to the continents which have rocks ranging back to 4.0 billion years old)
- of all tectonic settings, underwater mid-oceanic spreading ridges produce the greatest sheer volume of volcanic material (essentially produces all of the seafloor rock)
- 80% of all volcanic activity on Earth takes place under water along the mid-ocean ridge axis
- eruptions on the seafloor are generally benign since they occur under the pressure of >2 km of water
28
Marine sediment
Sand!
29
Pelagic rain
- shells of dead plankton float in suspension until gently falling as a 'pelagic rain' to the seafloor
30
seismicity
seismicity (i.e., earthquakes) along mid-ocean ridges is due to the active extensional stress of two plates pulling away from one another. Seafloor spreading breaks seafloor crust by extensional stress, creating faults within the rigid crust and generating earthquakes when the faults rupture. Magma migrates up from below to ‘heal’ the crack and thus create more seafloor within the rift valley
31
Heat flow
Heat flow, the rate of heat release from the Earth's interior, is highest over the crest of mid-ocean ridges and decreases away from the ridge axis.
– the high heat flow above mid-ocean ridges is due to the active magma bodies beneath the surface and active volcanism on the seafloor. Temp as high as 780°F (415°C).
- the elevation of the mid-ocean ridges above the adjacent abyssal plains is due to the hot, expanded, buoyant, partially molten rock just beneath the ridge axis, pushing upward.
- i.e., the youngest lithosphere near the ridge axis is hot and buoyant, so its density is lowered and thus mid-ocean ridges are relatively high regions of the ocean floor
32
Seafloor spreading rates
Through drilling of the seafloor from ship-mounted drilling rigs, geologists determined that the oldest seafloor was only about 200 million years old (very young compared to the age of rock comprising the continents, which may be as old as 4.0 billion years)
- oldest seafloor located in the western Pacific
These maps enabled a determination of spreading rates of the ocean basins.
- Mid-Atlantic Ridge is spreading at a rate of ~2 to 3 cm/yr (~1” per yr) which causes the ridge to build upward. The East Pacific Rise (EPR) (a 'rise' is not as rough and jagged as a 'ridge') is spreading at a rate of
~10 to 17 cm/yr (4-6”/yr).- the faster spreading rate causes the EPR to grow as a broad, low feature.
- at a typical rate of ~2.5 cm/yr, seafloor spreading produced the 5000 km wide Atlantic Ocean in ~200 million years
33
Continental rifting
Ocean basins are born when a continent splits and separates into two divergent continents in a process called continental rifting.
- continental rifts are linear features where continental lithosphere actively stretches and pulls apart, typically driven by upwelling of hot asthenosphere beneath the continent
- new divergent plate boundaries are formed along continental rifts
- continental rifts may (or may not) evolve through time into mid-ocean
ontinental rifts are formed within continental rock as it rifts apart. As the rift widens and the asthenosphere rises into the rift valley, volcanism changes to produce oceanic rock. (continental rock
is different from the basaltic rock created by seafloor spreading)
34
East African rift
the Afar Triangle, in the African countries of Djibouti and Eritrea, marks the location where the three rift arms meet (Gulf of Aden, Red Sea, EA Rift). It’s a hyper-arid land marked by fractures in the surface, common earthquakes and active volcanoes. The crust is sagging and will likely be inundated by the sea in a few million years.
- upwelling (rising) asthenosphere beneath Africa pushes up the overlying lithosphere, stretching it and causing it to break along faults. Collapse of fault blocks creates a rift valley.
- as lithosphere stretches and thins, the underlying asthenosphere rises even further, melts to form magma, and produces volcanism
35
linear sea
as seafloor spreading continues and the rift valley sags downward, seawater may flood in and an elongate linear sea may develop
- - e.g., the Red Sea is a linear sea, a nascent ocean basin
– in perhaps 10 m.y., East Africa may pull away from the rest of Africa, opening up a linear seaway
- a narrow linear sea may widen through time, forming a brand new ocean basin
36
continental margins
With time, the diverging , faulted edges of the continental rift develop into continental margins (shelf-slope-rise) on either side of the growing ocean basin
37
Oceanic crust
oceanic crust is about 7-10 km thick. Oceanic crust is composed of silicate rock dominated by the elements Si and O along with smaller amounts of iron (Fe) and magnesium (Mg).
- the rock composing the oceanic crust is full of iron and magnesium that makes the rock dense, relative to continental rock. Rock composing oceanic crust has an average density of 2.9 g/cc he dominant rock composing both continental and oceanic crust are igneous rocks that form by the solidification of a molten fluid like magma or lava. Both oceanic crust and continental crust “float” above denser rock of the mantle (density of 3.3 g/cc).
Ocean basins exist because denser, Fe-rich oceanic crust sinks deeper into the underlying mantle, relative to continental crust. R
38
Continental crust
the dominant rock composing both continental and oceanic crust ' are igneous rocks that form by the solidification of a molten fluid like magma or lava. continental crust is about 30-70 km thick. Continental crust is formed by a variety of different processes that tend to incorporate lighter elements into the rock such as potassium, sodium and aluminum. (remember that almost all rocks are silicates
– it’s the additional elements that determine the density and other characteristics of the rock.)
Rock composing continental crust has an average density of 2.7 g/cc.
Both oceanic crust and continental crust “float” above denser rock of the mantle (density of 3.3 g/cc). Rock composing lighter continental crust ‘floats’ higher above the underlying mantle (analogous to a buoyant iceberg)
39
Convergent Plate Boundaries
Plate boundary where two plates converge to produce linear mountain belts.
- the primary force at convergent boundaries is compression – squeezing and ‘deformation’ caused by the squeezing motion of the two plates toward each other
- ‘deformation’ refers to the bending and breaking of rock, commonly associated with tectonic compression and the uplift of mountains
- whereas lithosphere is created along mid-ocean ridges, lithosphere is commonly destroyed along convergent margins by burial back into the mantle
- three types of convergent plate boundaries – oceanic/continental convergence, oceanic/oceanic convergence, and continent/continent convergence
40
compression stress
the primary force at convergent boundaries is compression – squeezing and ‘deformation’ caused by the squeezing motion of the two plates toward each other
41
oceanic-continental convergence
Along convergent boundaries where a plate composed of oceanic lithosphere dives beneath a continental part of a plate, lithospheric material is returned to the mantle. The process where oceanic lithosphere descends down into the mantle is called subduction
42
subduction
The process where oceanic lithosphere descends down into the ' mantle is called subduction. oceanic lithosphere is denser than continental lithosphere and thus subducts into the asthenosphere beneath the continental plate at rates of 10-15 cm/yr
- along oceanic-continental convergent boundaries, the process of subduction creates a deep oceanic trench that forms along the arcuate contact between the two plates (e.g., Peru-Chile trench, Middle American trench)
- deep-ocean trenches are the bathymetric expression of a subduction zone
43
magma chambers
As the subducting plate reaches depths of about 100 to 150 km where the temperatures and pressures are just right, the water is released into the overlying rock of the asthenosphere. The infusion of water lowers the melting point of the rock of the mantle just above the subducting plate, causing it to melt, creating magma.
- this buoyant molten rock (magma) rises toward the surface (melting its way upward) where it pools within bodies called magma chambers. The chamber supplies magma to volcanoes on the surface within the mountain belt
44
volcanic arc
The chamber supplies magma to volcanoes on the surface
within the mountain belt.
- the volcanoes align roughly parallel to the convergent margin, forming a linear belt called a continental
volcanic arc
- compression of the rocks along the edge of the continental plate causes them to deform and warp upward into a high mountain chain. The linear belt of volcanoes pierces through the deformed rocks and forms high peaks.
45
inclined zone of seismicity
Earthquakes, ranging in depth from near-surface to ~670 km, are common along the dense subducting plate as it grinds downward against the over-riding plate (earthquake activity is commonly called ‘seismicity’) – this inclined zone of seismicity is characteristic of subduction zones
- of all plate boundaries, subduction-zone earthquakes are commonly the largest in magnitude
46
Oceanic-oceanic convergence
Many deep-ocean trenches occur adjacent to linear chains of islands called volcanic island arcs. (as opposed to continental volcanic arcs) island arcs are formed by convergence of two plates composed of oceanic lithosphere, with the older, 'colder' (and thus denser) plate subducting beneath the younger, 'warmer' less dense plate
- island arc volcanism results when magmas generated along the subduction zone buoyantly reach the overlying seafloor and erupt, with the underwater volcano eventually building up above sea level
through time as a volcanic island
47
Continental-continental convergence
A third type of convergent margin exists where continental plates converge with other continental plates (e.g., Himalayas/Tibetan Plateau) rocks along the continent-continent convergent margin are squeezed and lifted upward to heights of 8+ km along the spine of the Himalayas (Everest is 29,035 ft high)
- since both plates along the convergent margin are composed of relatively low-density continental rock, neither will subduct. (Because there is no subduction involved, continent-continent convergent
boundaries are called collision zones.)
48
Ex. Convergence
Currently the Indian part of the plate is being pushed beneath
the south Asian plate, but both plates are jammed, permitting the accumulation of tremendous amounts of tectonic stress
49
Compressional stresses
high mountains like the Himalayas are created along continent-continent collision zones by the detachment of thick slivers of rock (each maybe a km thick) and their stacking like shingles one atop
the other through “thrust faulting.” Compressional stresses squeeze the “thrust sheets” together, layering them on top and against one another
50
Thrust sheets
Thrust sheets move laterally during earthquakes – maybe 1 to 5 meters per earthquake. So a lot of
earthquakes are necessary over millions of years to produce a high mountain range like the Himalaya
51
Continental transform fault boundaries
Transform faults are a type of plate boundary where two plates slide horizontally past each other along major fault surfaces cutting across continents
52
shear stress
the primary force at transform boundaries is shear – plates on either side of the fault move in opposite directions, sliding past one another
53
fault
A fault is simply a planar fracture along which movement has occurred, offsetting massive blocks of rock and surface features on opposite sides of the fault
- “rupture” may occur along the fault plane, triggering earthquakes; some faults may slip slowly
54
Oceanic transform faults
- mid-ocean ridges are segmented by fracture zones extending perpendicular to the ridge axis
- transform faults are the actively slipping part of a fracture zone between two ridge segments and are capable of generating earthquakes
- most oceanic transform faults connect two segments of a mid-ocean ridge
- fracture zones and transform faults form and evolve at the same time as mid-ocean ridges and accommodate different seafloor spreading rates between mid-ocean ridge segments.
- think of fracture zones and transform faults as the way the surface of a sphere would tear as parts of it rip apart along mid-ocean ridges
55
Hot Spots
bout 100 active volcanoes are located far away from plate boundaries, often in the middle of plates.
- called hot spots, examples include Hawaii, Iceland, Yellowstone, & the Galapagos Islands
- some are located in the middle of plates, while others are located along mid-ocean ridges or near continental rifts
56
mantle plumes
Hot spot volcanoes occur above mantle plumes, columns of very hot rock that rise plastically upward, likely originating at the core-mantle boundary
- mantle plumes are not composed of magma – the pressures within the mantle are far too high to permit rock to melt to form magma. Magma is only generated within the uppermost mantle and crust.
- mantle plumes are one way that heat escapes from the interior of the planet to the surface then ultimately out to space. The heat energy moves upward within the plastically flowing rock of the
mantle.
57
seamounts
Hawaiian Island chain is part of a much longer archipelago of islands (now volcanically inactive), atolls, and submarine seamounts (submerged volcanic mountains that were once islands above the sea but that have since subsided beneath the surface)
58
hot spot trail
thus the Hawaiian-Emperor Seamount chain form a hot spot trail - a volcanic trace of Pacific plate motion over the last 70+ m.y
59
Seamounts form
Seamounts form as the lithosphere beneath the volcanic island becomes ‘colder’ and denser as it moves away from the buoyant heat of the hot spot.. Eventually, the lithosphere and the overlying island become so cool and dense that the island subsides beneath the sea to become a seamount
60
atolls
atolls are ring-shaped islands composed of coralline debris (limestone) surrounding a central lagoon. They form in a variety of ways but the most common is that they originally formed as barrier coral reefs around a volcanic island. As the volcanic island subsided below sea level(with distance from the hot spot) the corals continue to grow upward to maintain their shallow water position so their algal symbionts could photosynthesize. Eventually, the volcanic core subsides deeply enough and the coralline rim grows high enough that a lagoon forms in the center, above the submerged volcano.
Changing sea level exposes the coral islands to the atmosphere and new corals grow in the shallow waters along the outer and inner margins of the ring-shaped island.
61
plate tectonic movement
Earth still has lots of internal heat left over from its original creation, plus heat generated by natural radioactive decay within rocks of the mantle and crust. The heat is continually being lost to space - the clearest evidence of this escaping internal heat is volcanism. This escaping heat provides the driving energy behind plate tectonic movement (and thus, indirectly, mountain-building, ocean-basin creation, earthquakes and volcanoes).
62
convection currents
The heat transfer system within the planet generates convection currents. heat release from the core causes solid rock deep in the mantle to be heated to the point that it expands slightly, becoming less dense than surrounding rock. This hot, solid rock behaves like a
thick plastic and slowly rises as a buoyant mass to the surface. (Some of the heat may rise to the surface via mantle plumes, whereas other convection currents are more diffuse.)
- some of the heat from this rising plastic rock escapes through volcanoes located along major cracks in the Earth’s crust (e.g., mid-oceanic ridge systems) as well as through volcanic islands and land-based volcanoes.
- some of the heated rock flows laterally until it encounters a subduction zone where the rock and its heat energy is returned to the deep mantle. The total cycle of heat energy motion is the convection cycle
63
slab pull
of all forces, the primary force driving plate motion is probably slab pull, the force that down-going plates apply to oceanic lithosphere at convergent margins.
- the sheer weight of the descending plate, driven by gravity, is enormous enough to pull along the entire plate behind it
- slab pull is likely the force that creates extension at some mid-ocean ridges (like the subduction zone off western South America pulling the Nasca plate away from the Pacific plate along the East Pacific Rise), permitting magma to passively well up within the fracture along the ridge axis.
64
Plates move
East Pacific Rise (mid-oceanic ridge) opens at ~10 cm/year on average (4”/yr); the Mid-Atlantic Ridge opens at ~2 cm/yr (<1 “/yr)
- subducting plates may move at rates of 15 cm/yr or so
- over long time spans like 1 million years, plates can move ~100 km (60 mi) at these average rates. Or 1000 km in 10 m.y., and 5000 km in 50 m.y. So given enough time, plates can move significant distances even at those slow rates.
65
Volcano eruption
Igneous rocks form from the progressive cooling and solidification of molten rock.Crystallization can occur either slowly beneath the surface associated with a magma chamber, or it can occur rapidly on the surface of the land or seafloor as lava or volcanic ash ‘freezes’ to become rock. When magma rises up into the crust, it can do two things: it can erupt on the surface from a volcano or it
can solidify at some depth beneath the surface.
66
intrusive igneous rock/plutonism
Igneous rock that solidifies beneath the surface is called intrusive igneous rock and the process is called plutonism. An example is the very common rock called granite
67
extrusive igneous rock/volcanism
igneous rock that solidifies on the surface of the land or along the seafloor is called extrusive igneous rock and the process is called volcanism.
68
silica
Magmas are distinguished on the basis of their silica (SiO2) content, which can range from ~40% to ~85%
- recall how the abundant O and Si in the Earth readily combine to form silicate rock of the mantle and crust
*The shape of a volcano, the style of eruptions, and the composition of the lava or pyroclastic debris that is erupted is directly related to its silica content
69
viscosity
Silica controls the viscosity of the magma within the chamber - viscosity is simply a measure of a fluid’s resistance to flow.
- in turn, viscosity controls the gas content of the magma
70
gases
Magma consists of: abundant gases such as H2O, CO2, SO2, and H2S dissolved in the liquid magma
- up to 9% of a magma's composition (by mass) may consist of gases
- these gases are extremely important since they provide the ‘gas pressure’ that drives the explosive force of volcanic eruptions. (In general, the more gases trapped in the magma, the more powerful
the potential eruption.)
- the higher the viscosity, the greater the concentration of trapped gases and thus the higher the potential explosivity of eruptions
71
Low-silica magma
The lower the silica content, the more “runny” the lava (lower viscosity). The lower viscosity and thinner texture of the lava permits the continual release of gases from the magma, lowering the pressure and thus the potential for explosive eruption.
- the low viscosity contributes to fast-flowing “rivers” of lava that pour out of the central crater or fissures along the flanks
- this type of eruption is described as effusive
72
shield volcanoes
This type of low-silica lava builds broad, gently sloping shield volcanoes, typified by Hawaiian volcanoes.
- the low-viscosity lava flows spread out laterally, maintaining the broad, low-relief profile of shield volcanoes
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fissure eruptions
This low-silica lava is commonly erupted as fissure eruptions, types of lava flows that pour out of long cracks, called fissures, often along the flanks of shield volcanoes.
- these elongate fissures are fed by magma flowing along fractures within the body of the volcano
74
basalt
When this type of lava solidifies on the surface of the Earth, it forms a rock called basalt, containing about 45-50% silica. (your textbook calls these low-silica volcanic rocks “mafic”)
- basalts tend to be dark in color because of abundant iron and magnesium oxides
- basalts are very fine-grained because they cool so quickly due to contact with air or water – mineral crystals have no real time to grow
75
tectonic settings
Magma associated with mid-ocean spreading ridges and oceanic hot spots come directly from the mantle. The rock of the mantle is inherently low in silica (about 50% silica), thus magma derived from the melting of mantle rock is low in silica (basaltic in composition)Of all tectonic settings, submarine mid-oceanic spreading ridges produce the greatest sheer volume of volcanic material (essentially produces all of the ocean-floor rock) - probably 80% of earth’s volcanic activity is hidden from sight under the seas
76
fissure” eruptions
- lava flows along mid-ocean ridges are characterized by “fissure” eruptions (the most common type of volcanic eruption on Earth)
- lava simply pours out of elongate cracks (fissures) in the ocean floor located along the axial rift valley of mid-oceanic ridges
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pillow lavas
- lavas that erupt under water cool very rapidly and form a distinct shape called pillow lavas
- because the ocean floor covers 70% of Earth’s surface, basaltic pillow lavas are the most common type of volcanic rock on Earth’s crust
78
Intermediate- to high-silica magma
As the silica content of a magma increases to ~55 to >85%, the more “sticky” the lava becomes (higher viscosity). The thicker texture of the magma restricts the release of internal gases.
- this build-up of gas pressure often leads to violent pyroclastic eruptions
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pyroclastic eruptions
- pyroclastic eruptions are also known as explosive eruptions (as opposed to ‘effusive’)
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stratovolcanoes
Volcanic material (lava and ash) erupted from silica-enriched magmas tend to build up vertically to form steep-sided, rugged stratovolcanoes (aka “composite volcanoes”) (as opposed to the lateral growth of silica-poor shield volcanoes
- stratovolcanoes are typically dwarfed by the massive size of shield volcanoes
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Pyroclastic debris
Pyroclastic debris produced by an explosive eruption consists of glassy particles of volcanic ash, tiny particles of rock, larger fragments of rock, and frothy pumice, all blasted skyward by the gas pressure suddenly being released from the underlying magma chamber.
- pyroclastic debris ranges in size from powder-sized ash, to pea-sized cinders to boulder- to house-sized blocks
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volcanic ash
- ash is mostly particles of glass because the magma cools so quickly that the atoms don’t have time to organize into a crystal lattice, instead creating a disorganized arrangement that defines “glass”
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pumice
pumice is a common component of pyroclastic debris that consists of solidified gassy ‘froth’ that cooled very quickly, preserving the holes of gases trapped in the molten fluid at the time of solidification
- pumice resembles a sponge because it consists of a network of gas bubbles frozen within fragile volcanic glass
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pyroclastic fall
pyroclastic fall - ash rises in a vertical column during an eruption, then is carried downwind, often for hundreds or even thousands of kilometers. The ash eventually falls back to the surface, forming a layer ranging from hundreds of meters in thickness near the volcano to a few centimeters at the distal edge downwind.
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pyroclastic flows
pyroclastic flows can develop when part of the rising eruption column of pyroclastic debris becomes too dense to rise and as the upward force from gas pressure dissipates, so it collapses back onto the flanks of the volcano
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andesites and rhyolites
Intermediate- to high-silica magma produces a type of lava and pyroclastic debris that forms volcanic rocks called andesite (~60% silica) and rhyolite (~70% silica)
- andesites and rhyolites tend to be lighter in color than basalts due to lesser amounts of dark-colored iron oxides and higher amounts of lighter-colored minerals composed of sodium and potassium
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tuff
Volcanic rock formed by the solidification of pyroclastic ash is called a tuff
- a volcanic ash deposit with ~60% silica would be an “andesitic tuff” A volcanic ash deposit with ~70% silica would be a “rhyolitic tuff”
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continental hot spot
Yellowstone hot spot is characterized by high-silica magma (65-70%) and explosive rhyolitic volcanismdue to extensive melting of high-silica rock of the continental lithosphere into the magma
- as low-silica magma derived from the underlying mantle plume rises into overlying continental crust, heat is transferred outward into the surrounding rock. This infusion of heat partially melts the surrounding rock so that it is incorporated into the magma.
- this process changes the chemistry of the magma from its original low-silica composition to one reflecting the silica-rich composition of the continental rock through which it rises. At the Yellowstone hot spot, high-silica magmatism generated a series of ultra-violent eruptions over time.
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Tectonic setting
* Tectonic setting determines the silica content of magmas, thus the viscosity of lavas, thus the shape and explosiveness of volcanoes.
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Loma Prieta, 1989
The last major earthquake in California was the 1989 Loma Prieta event, with an epicenter in the Santa Cruz Mountains south of San Jose. At 5:04 pm, a 40-km long segment of the San Andreas fault
ruptured with a magnitude of 6.9. It was responsible for 63 deaths, 3700 injuries, and $6 billion in damages.
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fault
fault = plane of weakness within the upper crust along which movement has occurred (i.e., rock on one side of the fault plane moves in some direction - up, down, lateral, oblique - relative to rock on the other side of the fault plane)
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Elastic Rebound Theory
with continued tectonic motion, the rock along the fault plane accumulates strain energy and will deform in response (kind of like a compressed spring)
- the fault remains “locked” as long as the frictional strength holding the two blocks together is greater than the accumulating strain energy
- tectonic strain energy builds until the frictional strength of the rocks is overcome – rocks rupture at a small region along the fault plane (the focus) with the rupture rapidly moving outward along the fault plane at about 2 to 3 km/sec (4000 to 7000 mph)
- upon rupture, strain energy is rapidly released as shock waves (seismic waves) that emanate outward away from the fault plane (this is the earthquake) the rocks on either side of the fault elastically ‘rebound’ to an undeformed, unstressed state after the earthquake
- then the process begins again with the slow accumulation of tectonic stress
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stick-slip motion
the blocks on either side of the fault 'stick' for extended periods of time, as they accumulate strain energy. This may take decades, centuries or even millennia. During this extended phase we say the
fault is “locked.”
- eventually, when the frictional strength is exceeded by the accumulating tectonic stress, the fault will 'slip' in a rapid, violent release of energy called an 'earthquake
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earthquake cycle
repetitive earthquake activity along faults follows the “earthquake cycle” (as yet unpredictable)
- the problem is that stick-slip motion and the resulting earthquake cycle is not periodic in occurrence –in fact, the occurrence of earthquakes may be chaotic and thus inherently unpredictable
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recurrence interval
the recurrence interval is the average time between quakes of a particular magnitude, but is merely a loose estimate within a broad range of times (tens to hundreds to thousands of years) during which a fault may rupture
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earthquake
sudden motion in the Earth caused by abrupt release of accumulated strain energy along a
fault plane
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seismicity
The occurrence of earthquakes in space and time
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seismology
Study of earthquakes, and of the structure of the Earth, by analysis of seismic waves
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focus
focus = location within the Earth where the original rupture along a fault begins. (also called the ‘hypocenter’)
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epicenter
epicenter = point on Earth’s surface that lies vertically above the focus
- i.e., the epicenter doesn’t necessarily occur along the surface trace of the fault . .
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displacement
displacement (aka offset or fault slip) = amount of slip on the fault occurring during an earthquake
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aftershocks
aftershocks may occur for several days to weeks after the main event. Aftershocks are smaller quakes that follow the main shock; they release remnant energy remaining along the partially unlocked fault plane. Or they release new, local stresses developed in response to the main rupture.
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seismic waves
the seismic waves pass outward spherically through the Earth, with the rock vibrating back and forth, transmitting the seismic energy forward until the energy is dissipated
- new seismic waves are generated as the fault rupture travels along the fault plane
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groundshaking
the series of shocks from the initial rupture along the fault and the subsequent vibration create the groundshaking we feel during an earthquake
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P waves
P waves - (primary wave or compressional wave)
- fastest type of seismic wave – first to arrive at the surface
- similar to sound waves in that they alternately push (compress) and pull (dilate) the rock through which they travel
- sound waves (like from your voice) are P waves
- travels through solid rock and liquid material such as magma or water
- almost twice as fast as S waves
- don’t really cause much damage on the surface
- P waves travel quickly, but they do not carry much power. You feel the P waves as a sudden, vertical thump
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S waves -
S waves - (secondary wave or shear wave)
- travel at about half the speed of P waves, but faster than surface waves
- shears rock sideways at right angles to the direction of travel
- only travels through solids because liquids are not elastic, (i.e., they don’t spring back after shearing or twisting)
- when it arrives at the surface, S wave will shake ground with both vertical and side-to-side motion and are responsible for much of the damaging groundshaking.
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Love waves
Love waves - surface wave that is slower than P and S wave
- motion virtually the same as S wave but with no vertical component, only side-to-side (thus do not propagate through water)
- may penetrate downward ~ 10 m below the surface
- horizontal shaking particularly damaging to foundations of structures
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Rayleigh waves
Rayleigh waves - surface wave that is slowest of all seismic waves
- vertical and horizontal displacement in a vertical plane along the Earth’s surface
- occasionally seen by observers as a rolling motion of the Earth’s surface
- may penetrate downward ~ 15 m below the surface
- can also affect bodies of water like lakes (and pools)
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How fast do seismic waves pass through the crust?
the denser and more rigid the material, the faster that seismic waves move through it.
Seismic body waves travel at average speeds of about 4-7 km/sec (~9000 - 16,000 mph) through the
crust
Seismic surface waves travel at ~ 2-4 km/sec (~4500 – 9000 mph)
P waves travel fastest, S waves next, surface waves slowest
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Tectonic Settings of Earthquakes
80% occur along convergent margins of the Ring of Fire surrounding the Pacific
some earthquakes occur away from plate boundaries within the interiors of plates - these are called intraplate earthquakes
85% of all earthquakes originate in upper 20 km of Earth’s crust - called shallow focus earthquakes -rocks are most brittle in the upper 15-20 km of the crust, so the majority of earthquakes occur there.
- shallow-focus earthquakes cause the most damage because of their proximity to the surface and thus man-made structures (wave energy doesn't dissipate much over such short distances to the surface)
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Earthquakes on mid-ocean ridges
Two orientations of faults occur along mid-ocean ridges -
- the newly formed crust is stretched as the two plates separate away from the ridge causing seismically active faults to develop parallel to the ridge axis
- transform faults cut perpendicular to the ridge axis and are seismically active
- both faults generate shallow-focus earthquakes, but are too remote to be of much concern
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Earthquakes at convergent plate boundaries
The largest magnitude earthquakes typically occur along subduction zones composing the Ring of Fire around the Pacific.
Compressional stresses between the two converging plates cause several types of faults to develop and accumulate strain energy
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megathrust
stick-slip motion between the subducting plate and the overriding plate is the fundamental source of seismicity along convergent margins – called ‘megathrust” quakes
- shallow-focus earthquakes occur in both plates, whereas intermediate- to deep-focus quakes occur along the downgoing slab as it sinks into the mantle
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inclined zone of seismicity
inclined zone of seismicity along the top of the subducting plate marks the distribution of these shallow- to deep-focus quakes
- below about 600-700 km, the temperatures are too high for rock to behave brittlely - instead they 'flow' by means of ductile (plastic) deformation
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Earthquakes at transform plate boundaries
Transform faults are dominated by shear stresses oriented lateral to one another
- most transform faults occur along mid-ocean ridges, but some extend for great distances across continents or islands
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Paleoseismology
Paleoseismology involves digging a trench perpendicular to faults to determine the past history of earthquakes (i.e., recurrence interval)
- requires radiocarbon dating of organic material (typically plant matter) in the trench to estimate dates of quakes
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Tsunami defined as “a fast-moving set of ocean waves triggered by an earthquake, underwater volcanic eruption, or submarine landslide”many tsunami are generated along subduction zone megathrusts that vertically offset large masses of rock on the ocean floor - in turn, a large volume of water is displaced, generating a very powerful, fast-moving wave of great force
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Tsunami mechanics
As tectonic strain builds along the fault plane between the two ' plates, the overriding plate ‘sticks’ and slowly flexes upward over time. eventually the tectonic strain exceeds the frictional strength along the fault plane and the fault plane abruptly slips, releasing the pent-up strain energy
- the overriding plate surges forward, (with the seabed abruptly moving 24 m horizontally and 3 m vertically in the case of the Tohoku event). That resulted in billions of cubic meters of water —
trillions of pounds — suddenly being displaced upward. The water above the displaced seafloor abruptly rises, creating potential energy. That potential energy rapidly changes into the kinetic
energy of the tsunami waves as the bulge of water swiftly migrates outward from its area of origin.
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open ocean
In the open ocean (average depth of 4 km), the distance between tsunami waves may be tens of kilometers and the wave height may be <1 m, but it travels at jetliner speeds (500-700 km/hr) (300-
430 mph)
- in the open ocean, the tsunami moves as a ‘wall’ of energy, extending from the surface all the way to the seafloor
As water depth decreases as the waves approach the shoreline, the speed of the wave slows down (~30-60 km/hr) due to frictional drag along the seafloor
- the rear of the wave catches up as the front of the wave slows due to friction, increasing the volume of the wave dramatically
- the mass of water in the open ocean must now squeeze into a smaller volume in shallower water, increasing its wave height of up to 20-30 m and washing inland up to 10 km, depending on the slope
of the coastline
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Earthquake Prediction?
Although geologists and seismologists can make probability
estimates (i.e., forecasts) about future earthquakes on active faults, we cannot even begin to pin down specific dates and magnitudes. Earthquakes appear to be inherently unpredictable
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earthquake early warning system w
as soon as an earthquake begins, a network of >800 sensitive sensors connected to computers would feel the P waves, then instantly calculate the focus and the time till the damaging S waves would arrive.
- because technology transmits information via fiber optic cables much faster than seismic waves travel, we can send advance notice during that interval before the damaging S and surface waves arrive
