ISNS 4359 EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 1. Matter, Earth, Plate Tectonics, & Plate Boundaries
Earth Matter: Atoms [neutrons, protons, electrons] of 92 naturally occurring Elements [top 10
(wt%)– Fe, O, Si, Mg, Ni, Ca, Al, S, Cr, Na] mix to form Minerals [silicates, oxides, carbonates,
metals], which mix to form Rocks [igneous-once molten, sedimentary-chemical and clastic (surface
processes), metamorphic-changed/recrystallized (heated/deeply buried)].
Earth is vertically zoned in terms of composition, density, temperature
Si-Al-O Crust [continental (CC-20-80 km thick; average 38 km 'granite'; 80 vol%, 40 area%; Complex
shields surrounded by platforms and mountain belts; age=0-4400 Ma) & oceanic (OC-3-25 km average 6-7
km thick 'basalt'; 20 vol%, 60 area%; Age=0-200 Ma)]: Cold and brittle on top, ductile at base if hot
enough (>500-700 °C), low density (2-2.8 gm/cm3).
{Ma=million years; Ga=billion years}
Si-Mg-O Mantle (~2900 km thick peridotite) (400-3000 °C), intermediate density (3.2-5.5 gm/cm3)
Fe-Ni Core (~3480 km) outer liquid, inner solid (3600->5500 °C), high density (10-13 gm/cm3)
Earth’s thermal structure is related to mechanical properties
Brittle Conductive Lithosphere (0-~120 km below surface); 20-~1300 °C (crust and uppermost
mantle) thickens with age, after ~150 m.y. reaches ~120 km thick; thinnest at oceanic ridges.
Ductile Convective Asthenosphere (~120-~670 km below surface) ~1300-~2000 °C (underlying
upper mantle) contains a small amount of partial melt but is mostly solid; shallowest at oceanic ridges.
Underlying Mesosphere (~670-~2900 km below surface) ~2000-~3000 °C (middle and lower
mantle).
Earthquakes and Volcanoes are not randomly situated on the planet, but are mainly concentrated in
belts along plate boundaries. The Earth breaks where it is brittle and strained; it melts along plate
boundaries and within plates.
Plate Tectonics - Seafloor Spreading - basaltic oceanic crust is formed at divergent margins (midocean ridges) by decompression melting of upwelling mantle; oceanic lithosphere migrates away from
the ridge, subsides & cools, and is consumed at convergent margins (trench, subduction zone, island
arc or mountain belt) and sinks into the mantle because it is dense. Where 2 continents collide,
mountain belts form because continental crust cannot be subducted into the mantle.
Alfred Wegener (1912) proposed continental drift by fitting the continents together like a puzzle
and linking up belts similar rocks and their depositional settings on continents on opposite side of the
Atlantic Ocean; few believed him because no mechanism could explain it. Hess, Vine, and Matthews
proposed the plate tectonic model in the early 1960's based on recognition of crustal differences,
magnetic stripes in ocean floor, and distribution of earthquakes and volcanoes.
Causes of plate motion-Gravity & Buoyancy: Convection in the mantle, slab pull, ridge push
Earth’s lithosphere - composed of 7 major plates (Pacific, Indian-Australian, African, Eurasian, N
American, Antarctic, S American), 9 minor plates (Somalian, Nazca, Philippine, Arabian, Caribbean,
Cocos, Scotia, Juan de Fuca, Rivera) & ~36 microplates; each plate moves at a unique velocity (vector).
3 main types of Plate Boundaries—36% divergent (rift), 35% convergent (collision-subduction), and
28% conservative (transform, strike-slip); plates move at ~1-20 cm/yr (10-200 km/m.y.) average 4 cm/yr.
Wilson Cycle-continents rift (break apart), drift, collide, build mountains, continents stabilize, rift, drift,
collide, etc., each cycle taking hundreds of millions of years.
Large Rigid Lithosphere plates born 2 Ga; Linear belts of subduction started ~2 Ga
4 Wilson Cycles in last 2 billion years: 4 Supercontinents- Columbia, Rodinia, Pannotia–Gondwana and
Pangaea each assembled and then dispersed (@~2.0, 1.0, 0.7 and 0.15 Ga)
Isostasy-Archimedes principle Lower density (2.6-2.8 gm/cm3) continental crust (CC) floats on a higher
density (3.3 gm/cm3) mantle (mean continental elevation 840 m above sea level) and has thick roots
(average 35-50 km), whereas higher density (2.9 gm/cm3) oceanic crust (OC) resides below sea level
(average depth 3.8 km below sea level) and is much thinner (average 6-7 km) than CC.