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EARTH’S CRUST PROCESSES
The Earth’s crust is subject to two types of processes:
internal processes - act from inside of the Earth’s surface,
external processes - act from outside of the Earth’s surface.
Erosion-Transportation-Deposition are external leveling processes through which the higher parts of the Earth’s surface are
worn down and the lower parts are filled with the depositing debris. If their work could be carried to completion, the
continents would disappear and the Earth would become a smooth sphere covered with seawater. The fact that the continents
still exist, not to mention the mountain ranges upon them, is in itself evidence that internal processes exist and undo the
effects of gradation. The internal processes are of two kinds (they often occur together):
1 Magmatic processes (volcanism in a broad sense), involving movement of molten rock;
2 Tectonic processes (diastrophism), involving movement of the solid crust.
Magmatic processes; those that occur near and at the surface are called volcanism, those occurring deep within the Earth’s
crust are called plutonic processes. Volcanism can cumulate rocks and form volcanic mountains. Plutonic processes are
associated with intrusions of plutonic igneous rocks.
Tectonic processes frequently cause deformation, which can be of two kinds.
discontinuous deformation such as a fault originating by pressure acting on brittle rocks near the crust surface (under a low
hydrostatic pressure) abruptly, and
continuous deformation such as a fold formed by pressure making rocks plastic deeply beneath the Earth’s crust surface
(under a high hydrostatic pressure) over a long period of time.
The discontinuous deformation may be associated with an abrupt motion & shaking of the Earth’s surface we call
earthquake (eq.).
PLATE TECTONICS
Plate tectonics is the study (principle) of slow movements (few centimeters/year) of (large) lithospheric
plates on a soft (not molten) upper mantle (asthenosphere). It includes continental drift (continents are
carried on the plates) & ocean floor spreading.
The lithosphere consists of about 12 major plates and a number of smaller ones (microplates,
terranes), all of which float on the plastic asthenosphere. Examples of the major plates: Eurasian,
African, Indian-Australian, Antarctic, Pacific, North & South American, Cocos, Nazca (the continental
shelf off the coast of NW South America is underlain by Nazca plate), Philippine, Arabian, Caribbean.
The plates can move relative to one another in 3 ways, and form the following boundaries:
1 Divergent boundary, e.g. ocean (sea) floor spreading at mid-ocean ridges - by moving apart with
molten rock rising to form new ocean floor at the middle seem;
2 Convergent boundary; typically leads to subduction - one plate slides under another; the lower one
may reach the depth of about 700 km, its deeper parts are resorbed by melting; if one oceanic plate
subducts another one, an oceanic trench forms, volcanism forms above the subduced plate. The
deepest end (700km) corresponds to the deepest earthquake focus;
3 Transform (strike-slip) boundary - sliding mostly horizontally along faults (such as San Andreas
Fault) - adjacent plates (usually microplates) slide past each other.
The ocean floor spreading causes a symmetric age distribution of the ocean floor around the midoceanic ridge. The age and sediment thickness increase apart from the recently formed mid-oceanic
ridge up to continental margins, with the maximum age of the ocean floor (196 mya).
Convergent boundary is associated with the formation of big folded mountains, such as the Rockies,
Andes, Alp Mts., Carpathian Mts., Himalayan Mts., etc..
Continental drift is due to plate motion. Today’s continents were joined into a single supercontinent
known as Pangea, which first broke into two supercontinents called Laurasia (North America,
Greenland, Europe, and most of Asia) and Gondwanaland (South America, Africa, Antarctica, India,
and Australia), separated by the Tethys Sea. Pangea’s break up and reassembling seems to occur in
cycles (Scientific American, July 88, p. 72-9) taking about 440 million years.
The Earth’s interior is made up of concentric layers (183) identified by earthquake analysis through
shadow zones (Fig. 4.7). The main concentric layers in the Earth’s interior:
1 Core 3470 km in radius, probably consists of molten iron and nickel alloy (182-3; such as in
metallic meteorites), the inner core is solid, and spins slightly faster than the rest of our planet,
making an additional rotation with respect to the Earth’s surface every 400 years (Astronomy Nov.
96, p. 30, “Motor Planet”);
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Mantle 2900 km thick, a more or less solid ferromagnesian silicate; the almost upper mantle is
called asthenosphere (Greek asthenos = weak, soft);
Thin crust, average thickness 35 km, maximum 70 km under mountain ranges of the continents
(granitic rock), minimum is less than 6 km under the oceans (basaltic rock).
The crust + the outermost mantle together (129, Fig. 4.10) make up a shell of hard rock 50 to 100 km thick called
lithosphere (Greek lithos = rock). The lithosphere has no sharp boundary, as the crust does, but gradually turns into the
softer asthenosphere.
Hot spots (plumes): rising magma from beneath the plates at few (about 80) places (“spots”, 113-126,
181). Because they are not related to the plate movement, strings of volcanoes, formed by them
successively, are stationary mantle markings of the plate movement (age increasing with the distance
from the current hot spot position). For example, Hawaiian & Emperor Islands, Galapagos Islands. The
past velocity & direction of the plate motion can be determined from the distances & age differences
between pairs of the volcano strings.
VOLCANOES
Volcanoes are places where the cosmic body’s heat is escaping from or through a solid crust by
pouring out warmer (and lighter) liquid materials. On the Earth it is lava. The flow is usually
strongly supported by expanding volcanic gases.
Areal distribution of volcanoes is not identical but similar to the earthquake. distribution: it correlates
with some plate boundaries. Most volcanoes are located at mid-oceanic ridges (ex.: Iceland) and above
subduction zones (ex.: Cascades; Ring of Fire rimming the Pacific Ocean), few on hot spots (Hawaiian
Islands, Galapagos Islands). Magmatic chambers [“roots”] of continental volcanoes are located shallow
in the crust, above subduction zones, and at (relatively) “deep” fault zones.
Three basic types of volcanoes according to the volcanic ejecta:
a lava flows; form shield volcanoes, such as Mauna Loa & Kilauea on Hawaii + volcanic domes;
b cinder cones consist of pyroclastic debris; ex.: Mojave Desert, CA; Cerro Negro near Leon,
Nicaragua; Paricutín (SW Mexico, 320km W of Mexico City;
c stratovolcanoes (composite v.) consist of lava flows and pyroclastic debris; the most common
volcanic type; ex.: Mount Etna (Sicily), Mount Vesuvius (Italy), Popocatépetl (central Mexico, 72 km
SE of Mexico City; 5465m high; since 1920 almost silent, started erupting on 30 June 97).
Content of silicon causes high viscosity (stiffness) of magma. This hinders a smooth magma discharge
from the crater and causes explosions (the silicon-rich magma has also more volcanic gases dissolved)
forming pyroclastic debris. The magma viscosity is revealed by the cone steepness.
Steep cones formed from highly viscous, therefore silicon-rich magma, flat ones formed from low
viscosity, therefore silicon-poor magma; the latter magma (the pertinent effusive rock is basalt) has high
content of iron, magnesium and calcium. Examples of flat volcanoes are shield volcanoes such as
Mauna Loa, examples of medium steep volcanoes are Mt. St. Helen’s and Novarupta in Alaska; steepest
volcanoes are known within Lipari Islands, Mediterranean Sea.
Hazards
Two most dangerous volcanic activities:
1 Lahars: mud-flow generated by melting of snow on volcano (frequently mixed with volcanic ash);
may flow very rapidly (>100 km/hour); Mt. St. Helen’s, Vesuvius, Japan volcanoes. Mt. Pinatubo,
Philippines, in 1991. Vesuvius: Pompeii, Herculaneum, A.D. 79.
2 Fiery clouds (“nuée ardente”): glowing heavy clouds from very hot (>1000°C) gas mixed with fine
hot ash. Produced by composite volcanoes. May move faster than 100 km/h. Mt. Pelée on
Martinique, West Indies, 8 May 1902.
Other volcanic hazards:
Toxic gases (including CO2-suffocation “only”), such as H2S;
Phreatic eruption (water suddenly introduced into cone may cause violent explosion; Krakatoa explosion 1883.
Lava is least hazardous because flow downhill is almost always predictable, and usually moves slowly enough so that at least
people and some property might be saved. To deal with it: water, and build dams of lava to divert flow.
Pyroclastic debris (hot ashes): move farther and faster than lava. If still hot can cause fire; if cool, still a hazard as it buries
things, make roads slippery, etc. Also unbreathable.
Volcanic eruptions may strongly influence global climate (cooling effect if the fine volcanic dust reaches stratosphere (a
high altitude layer of atmosphere, above troposphere; the stratosphere starts at 6 km above poles and 16 km above equator,
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and extends up to about 50 km; the Sun radiation is reduced). Whereas the troposphere makes 75% of the whole
atmosphere’s mass (up to 5km: 50%), includes most clouds (humidity) and dust, the stratosphere is very thin, has almost no
clouds but very strong winds (jets) which spread the volcanic ash rapidly across the whole planet. The fine ash from single
eruptions may reside in the stratosphere several years (Mt. St. Helen’s, El Chichón, Krakatoa), from multiple volcanoes
several decades. Ash particles form condensation nuclei and increase the amount of clouds and precipitation.
Methods of protection before some volcanic hazards:
hazard reduction is limited (Ch. 18, 267-80), prediction of volcanic explosion is more reliable than of earthquakes: by
monitoring of volcanic precursors, particularly of the altitude and tilt of the volcano’s slopes; specific seismic active.
Recent: steam + CO2 on Earth, CO2, S+ S-oxides on Venus; S + SO2 on Io;
Volcanism on solar system bodies:
water spraying on Europa & Enceladus; liquid nitrogen eruptions with
admixture of methane on Titan and Triton.
Extinct: strong on the Earth & Mars, weak on Moon.
Most bodies are tidally heated: the strongest volcanism is on Io (satellite of
Tidal heating in the Solar system
Jupiter), no volcanism is yet known on Mercury.
Mutual gravitational influence consumes rotational & orbiting energies by
Tidal flexing & heating of cosmic bodies
flexing of the pertinent bodies causing their internal friction and heating. This
may occur by rotation in a strong gravity field, or changing gravity direction
and/or intensity (elliptical orbits). The tidal heating may be visible as various
types of volcanism (including cryovolcanism), and may terminate by tidal
locking of one or more motions, such as Pluto – Charon system: both objects
are mutually locked.
Venus: 95% carbon dioxide, thick clouds of sulfuric acid, water deficiency;
Atmospheres of the solar system
472±2°C (=745±2 K), 90 bar;
bodies [except for the Jovian planets]:
Earth 78.03% nitrogen, 21% oxygen (0.9% argon), 0.035% carbon dioxide;
water clouds, 15±50°C, 1 bar;
Mars 95% carbon dioxide, 3% nitrogen, 2% argon;
Titan: ,
Enceladus: water vapor;
Triton: nitrogen, unknown admixture of methane, which decomposes by UV
radiation a leaves long black shadows (“smokers”).
AGENDA:
Saturday:
0900-1015
1030-1145
1145-1300
1300-1415
1430-1545
1600-1715
What is volcanism, its forms in the Solar System (incl. cryovolcanism) and on the Earth;
Earth Interior; plate tectonics; major plates, area distribution of volcanoes, hot spots;
Lunch
Ratio of silicon to metals (Fe, Mg, Ca, Na, K) & volcanic gases in the magma controls its viscosity, the volcano
slope and behavior;
Volcanism and its products in the Solar System and on the Earth; lava & pyroclastic debris;
Exotic volcanoes on the Earth: mud volcanoes and carbonatite volcanoes;
Sunday:
0900-1015
1030-1145
1145-1300
1300-1415
1430-1545
1600-1715
Volcanic gases contribute significantly to the sea and air;
Volcanic activity, not only explosions, may influence climate;
Lunch
Main volcanic hazards: fiery clouds and lahars; reduction of volcanic risk.
Quiz Topics are reviewed & discussed. Please download the Quiz Topics.
Quiz (30 minutes): 40 multiple choice questions. An immediate correction will follow so that every
student may get her/his course grade.
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