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)plainshigh tablelands mountains

hills and low tablelands depressions

widely spaced mountains

- 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) Residual mountains and continental cores (“shields”); inactive remnants of ancient tectonics

(2) Mountains and landforms derived from active folding and faulting

(3) Volcanic features formed by accumulation of eruptive material

Continental ShieldsText: 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 WarpingLayered 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 beas 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-of-

thousands

… 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-Alpine-Caucasus --- 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