Transcript document

CHAPTER 9
Tectonics,
Earthquakes, and
Volcanoes
Earth endogenic systems produce continental
and oceanic surface features through
core-to-crust tectonic activity
- often dramatic; often rapid; often unexpected
- such activity has repeatedly re-built the surface of
the Earth over 4.6 bill yrs
- boundaries of tectonic plates are the most active
landscape building sites
… both form and arrangement of landscape are the
result of the presence [of absence] of tectonic
forces
Earth’s Surface Relief Features
First differentiate a couple of terms:
(1) relief – vertical elevation differences in the
landscape
(2) topography – “the lay of the land”; relief of a
region taken as a whole
As text relates, both have had vital roles in human history
[ex: Eastern N.A: Borchert’s Historical Epochs; the concepts of
Appalachia and the By-Passed East; concentrations of urban
centers along Fall lines]
Crustal Orders of Relief
Geographers group landscape topography into three
orders of relief
(1) first order of relief – broadest category of landform;
includes the continental platforms and the ocean basins
(2) second order of relief – category specific relief forms;
ex: mountain systems, planes, basins, ocean basins,
mid-ocean ridges, etc.
(3) third order of relief – most detailed category; individual
physical features; “local relief”
Text tells us that we can generalize these
three relief orders into six topographic
regions (Fig 9.3)
plains
high tablelands
hills and low tablelands
widely spaced mountains
mountains
depressions
-
-
topographic forms are largely defined by arbitrary
elevation or description
all topographic forms are not spatially distributed
across all continents
[look at Fig.s 9.3 and 9.4]
… look at mountains S/SE Asia; lack of extensive
mountains in Africa/Australia; what would
Antarctica look like?
Crustal Formation Processes
- Internal, nuclear, tectonic activity builds
crustal material
… solar powered erosive processes reduce the
crust
-
What forms do produced continental crust
take?
… text says three categories:
(1)
(2)
(3)
Residual mountains and continental cores (“shields”);
inactive remnants of ancient tectonics
Mountains and landforms derived from active folding and
faulting
Volcanic features formed by accumulation of eruptive
material
Continental Shields
Text: all continents have a nucleus (craton) of ancient
(2 billion yrs / Precambrian) material that has
been heavily eroded to a low elevation and relief
… craton rock is the thickest portion of the
continental Lithosphere
… continental shield (Fig. 9.4) grows with addition
of crustal fragments and sediments
… stable tectonically [except rift region E. Africa]
A continental shield is where a craton is exposed at
the surface and represents large portions of
continental area
Building Continental Crust
- Continental crust is built from processes of
sea-floor spreading, oceanic crust building
and subduction, and recreation as new
magma
[you can read the process for yourself]
--- thumbnail explanation:
dense basaltic rock is created at diverging plate
boundaries;
material migrates from plate boundaries to collide
with continental margins where it is forced
downward under less dense continental rock;
it re-melts to migrate upward and build continental
material
Crustal Deformation Processes
Whatever the form of rock, each is subjected
to stress through tectonic forces, gravity,
and weight of overlying rock [Fig. 9.7]
(1) tension stress – stretching
(2) faulting – breaking
(3) shear stress – tearing and twisting
Strain is how rock responds to stress
(1) folding – bending
(2) faulting - breaking
Folding or Broad Warping
Layered rock subject to compression becomes
“deformed”
… plate convergence compresses layered rock - a
deforming process called folding
[look at Fig 9.a – use these explanations]
(1) anticline – geologic structure in which
strata are bent into an upfold or arch
(2) syncline – a geologic structure in which
strata are bent into a downfold
Aside
I am acquainted [have the misfortune to be acquainted] with
the region shown in Fig 9.b.
The road cut is I-68 east of Cumberland, MD.
The mountain is Sideling Hill.
There is an excellent exhibition center at 9.b.
The good part of Sideling Hill is a State of Maryland natural area.
The rest of the mountain is rocks, rhododendron, and
rattlesnakes
-
-
Forces of compression may create folding patterns
beyond simple synclines and anticlines
In addition to folding, strata may undergo broad
warping
--- the same up-and-down “bending” as strata
undergoes in folding, but over a much greater
spatial extent
… ex: rebounding of portions of the Canadian Shield;
doming over hot spots in Yellowstone Park
Faulting
- Stress on rock may result in fracturing of strata
- Pieces of broken strata may be forced upward or
downward
--- displacement on either side of a fracture is the
process of faulting (Fig 9.11)
(1) normal fault (tension fault); divergent
(2) reverse fault (thrust fault); convergent and
compressional
(3) strike-slip fault (transform fault); transverse
-
Fault zones are areas of crustal movement
At the point of fault movement massive amounts of
energy is quickly released – an earthquake
Orogenesis
Orogeny – mountain building on continental crust
over million of years
- This mountain building (orogens) correlates
closely to plate tectonics models
- Collision, capture, and intrusion processes provide
the crustal material
- Uplifting provides the mountain building
[major episodes bottom p. 305 / top of p. 306]
Types of Orogenies
(1) oceanic plate-continental plate collision
[NA/SA]
(2) oceanic plate-oceanic plate collision
[Japan]
(3) continental plate-continental plate collision
[India-Asia]
Earthquakes
- Tectonic plates seldom migrate smoothly
… more frequently, stress builds and plates
move in short, sharp, rock-shearing surges
- May occur anywhere on the Earth
… susceptibility varies spatially
- Potential for damage varies spatially
… most occur in sparsely populated areas
[a function of both geology and human preference]
Earthquake Essentials
Tectonic quakes are those associated with faulting
… intensity of seismic motion associate with
quakes: P-waves; S-waves; L-waves, is
measured through one-of-three intensity
scales:
(1) Mercalli Scale - subjective (Table 9.1)
(2) Richter Scale – logarithmic (10x) measure of wave
amplitude; less accuracy for higher magnitude
quakes; best known
(3) Moment-Magnitude Scale – most accurate scale,
corrects for weaknesses of Richter Scale;
relates amount of fault slippage, the spatial
area impacted, and the compaction of materials
faulted
Seismic wave motion is initiated at a subsurface point - the
focus or hypocenter [under rebound theory greatest stress]
--- the surface area directly above the focus is the epicenter
--- seismic waves radiate outward from the epicenter
(and the focus)
--- foreshocks may precede the main shock wave
… aftershocks may follow the main shock wave
- As a general rule, the greater the distance from epicenter, the
less severe the shock [by 25 mi 1/10th of effect]
-
Earthquake Forecasting and Planning
?Can we predict earthquakes?
- Earthquake prediction is a major challenge for
seismologists
- Text tells us that historical occurrence / absence
provides a common risk determination method
--- a history of quakes provides historical trend data
--- the absence of quakes in an active zone (seismic gaps)
may be read as an over-stressed area of the fault
-
There exists a school-of-thought that believes that
animals have advance knowledge of earthquakes
Issue of Planning and Preparation
- It is widely held that too little is done to prepare for
quakes when they are likely inevitable
--- what planner/city government would want to advertise they
are earthquake-prone
--- expense of retro-fit and quake insurance when “it might
not happen for a hundred years”
--- if quake is big enough it “won’t make any difference”
Volcanism
-
We must overcome a number of stereotypes about
volcanoes and volcanic activity
--- “the Fuji Complex” – snowcapped and mythical; dwelling
place of the gods; climbing it is a supernatural
experience
--- catastrophic event – molten rivers of lava; poisonous
gases; villages buried under mountains of ash
ASIDE
If you think about it, it makes sense… everything
about volcanoes seems larger than life. They are
thus attention grabbing and memorable
… and yes, sometimes large numbers of people
die or are displaced by them
Overall, volcanoes are not the death and destruction
machines that we often make them out to be:
(1) volcanoes frequently give us warning of their actions
(2) many volcanoes are located in rural uninhabited
places
(3) if the eruption produces lava flows rather than
poisonous gas or flaming projectiles, it is more
possible to evacuate and avoid
Formation of Volcanoes
- The most basic requisite for volcanoes to form is
the presence of a molten rock reservoir – magma
chamber – which is under sufficiently high
pressure to force volcanic materials to the surface
--- This volcanic material (ejecta) may be ejected
to the surface through a central conduit
(volcanic vent) into a crater or through fissures
on the flanks of the volcanic cone
*See Fig 9.34
--- The source of magma is generally within 60 mi
of the surface
[in vicinity of undersea ridges magma may be
as little as 15 mi beneath the surface]
- The composition of magma can vary widely,
and is a primary factor in the ultimate:
(1) shape of the volcano formed
(2) the nature of the volcanic activity
[presence of silica or single magma vs. multiple
magma sources]
- Elements such as gas content
(sulfur vs. CO2 or H2O) and pressure are
determinants of ejection violence
Aside: that’s how you can get the quiet outflows of Kilauea and
Mauna Loa and the violent explosions of Vesuvius
and Krakatoa
Interesting: H2O is the leading element in volcanic explosions (about
70%)
… other compounds include carbon monoxide;
carbon dioxide; hydrogen chloride;
sulfur trioxide and give rise to VOG
Geographic Distribution of Volcanoes
- The volcanoes of the Earth must number in the tens-ofthousands
… obviously not all of this estimate are “active” – most are
dormant or extinct
… common estimate of “actives” number 500
-
We can make some generalizations about the
spatial distribution of volcanoes:
--- because of the correlation of volcanic activity
with active tectonics: volcanoes tend to
cluster in regions of subduction; mid-ocean
spreading and ridges; rifting; etc
… observation of the Circum-Pacific Ring of
Fire; the mid-Atlantic Ridge; the East
African Rift Valley; and the Atlas-AlpineCaucasus
--- volcanic “hot spots” such as Yellowstone Park;
Hawai’i
Aside
Interesting: there appears to be a distinct latitudinal distribution
of volcanic activity – “Two-thirds of the world’s volcanoes are
located in the northern hemisphere and only 18 percent are
found between 10o S and the South Pole”
[Ebert, p.21]
Volcano Types and Activities
- Can be classifies in several forms:
(1) shape – cone volcano (Mt. Rainier / Mt. Shasta) and shield
volcano (Hawaiian Islands)
(2) type of ejecta – a major factor in the type of
volcano
formed
ex: basaltic lava low viscosity; remain fluid at
relatively low temperature; gases escape easily;
relatively tranquil eruptions; shield volcanoes
[the effusive eruption of the text]
ex: high silica or aluminum traps and “clogs” vents,
allowing gas and pressure to build; once ejected
violently (explosive eruption) , tends to solidify quickly;
has high structural strength; in conjunction with lava
ejection can build high stratovolcanoes (composite)
… classifications are not necessarily distinct, there are
intermediate versions / mixed characteristics
ex: lava with pyroclastic lava blocks as in Mt. Etna – or
cones of fine ash – to – aerial bombs
(3) eruptive activities – volcanoes may be classed by their
activity or phases
--- some are in fairly constant eruptive state
… ex: Stromboli, Lapri Islands, Tyrrhenian Sea erupts up
to 4x an hour
or
Kilauea, Hawaiian Islands has been in more-or-less
constant eruption since 1983
… conversely, some may be inactive and erupt violently and
unexpectedly
Volcanic Hazards
… Historically, volcanic eruptions have instilled us
with a curious mix of excitement and fear (almost
hypnotic)
--- for the tourist they are innocuous entertainment
“once in a lifetime thrill”
--- for the local resident a daily potential threat
[but still exciting – we will pay to be scared]
Volcanic Forecasting and Planning
- volcanic activity is a significant natural hazard
--- ?why do we continue to inhabit high risk areas?
- Forecasting their expected behavior occupies a
great deal of scientific time… luckily, they are
less likely to catch us unawares
[they ARE hard to miss]
… fixed and mobile seismic equipment; GPS and ground
truthing technology; gas spectrograph analysis; etc