Chapter 1 - Plate Tectonics

is bounded by two 
major seismic discontinuities at 410 km and 660 
km depth. At each discontinuity new, high-pressure 
phases are formed. 
5 The Earth has two boundary layers with steep tem-
perature gradients: the LVZ beneath the lithosphere, 
and the D" layer at the base of the mantle. Plates 
move about on the LVZ and mantle plumes may be 
generated in the D" layer. 
6 Plate boundaries are of four types: ocean ridges 
where new lithosphere is produced; subduction 
zones where lithosphere descends into the mantle; 
transform faults where plates slide past each other; 
and collisional zones, where continents or arcs have 
collided. 
7 Divergent plate boundaries (ocean ridges) are char-
acterized by small-magnitude, shallow earthquakes 
with vertical motions reflecting formation of new 
lithosphere. Topography in axial rifts varies from 
high relief to little if any relief in going from slow-
to fast-spreading ridges. Ocean ridges grow by lat-
eral propagation. 
8 Transform faults are characterized by shallow, 
variable-magnitude earthquakes exhibiting lateral 
motion. Transform faults may lengthen or shorten 
with time. 
9 Convergent boundaries (subduction zones) are char-
acterized by a dipping seismic zone with variable-
magnitude earthquakes and, in some instances, 
seismic gaps suggestive of plate fragmentation. Fault 
motions vary with depth in the seismic zone and 
seismicity is strongly correlated with the degree of 
coupling of the descending slab and mantle wedge. 
Plates < 50 My in age may be buoyantly subducted 
and slide beneath the overriding plate. 
10 Buoyant subduction is the only recognized tectonic 
force sufficient to trigger nucleation of a new sub-
duction zone. New subduction zones commonly 
form at zones of weakness such as transform faults 
or fracture zones. 
11 Collisional boundaries are characterized by wide 
(up to 3000 km) zones of lateral deformation with 
compressive fault motions dominating near sutures 
and lateral or vertical motions in areas overlying 
partially subducted plates. 
Plate tectonics 35 
12 A Wilson Cycle, the opening and closing of an 
ocean basin, has occurred many times during geo-
logic history. 
13 The motion of one plate relative to another is that 
of a spherical cycloid and is a function of the po-
sition of the pole of rotation, the direction of rela-
tive motion, and the angular velocity of the plate. 
14 Plate velocities can be estimated from magnetic 
anomalies on the sea floor, the azimuths and mo-
tions on transform faults, first-motion studies of 
earthquakes, apparent polar wander paths, and by 
space geodetic measurements. Average relative 
velocities of plates range from 1-20 cm/y and the 
oldest surviving sea floor is about 160 Ma. 
15 Computer models indicate that plates move in re-
sponse chiefly to slab-pull forces, and that ocean-
ridge push forces transmitted from the asthenosphere 
to the lithosphere are very small. 
16 Rocks acquire remanent magnetization in the Earth's 
magnetic field by cooling through the Curie point 
of magnetic minerals (TRM), during deposition or 
diagenesis of a clastic sediments (DRM), and dur-
ing secondary processes if new magnetic minerals 
are formed (CRM). 
17 The Earth's magnetic field has reversed its polarity 
many times in the geologic past. Normal and re-
verse polarity intervals in the stratigraphic record 
allow construction of the Geomagnetic Time Scale. 
Magnetic reversals show periodicity on several 
scales and evidence of reversals exists in rocks as 
old as 3.5 Ga. 
18 Polarity intervals correlate with magnetic anomaly 
distributions on the sea floor allowing seafloor 
spreading rates to be estimated. The magnetic 
anomalies are caused by magnetized basalt injected 
into axial zones of ocean ridges during normal and 
reversed polarity intervals. 
19 During a reversal, which occurs over about 4000 
years, the Earth's dipole field decreases in inten-
sity and rapid changes occur in declination and 
inclination. 
20 The two most important problems in using paleo-
magnetism to reconstruct ancient plate motions are, 
(1) separation of multiple magnetizations in the same 
rock, and (2) isotopic dating of the magnetization(s). 
21 Apparent polar wander paths show distinct charac-
teristics for various plate tectonic scenarios. 
22 Chains of volcanic islands and aseismic ridges on 
the sea floor appear to have formed as oceanic plates 
more over hotspots, which are the shallow manifes-
tations of mantle plumes. Similar, although not as 
well-defined, trajectories are formed when contin-
ents move over hotspots. Lifespans of hotspots are 
< 100 My. 
23 Although hotspots appear to have remained fixed 
beneath a given plate or beneath adjacent plates, 
distant hotspots have not remained fixed, but move 
at velocities approximately an order of magnitude 
less than plate velocities. 
24 A supercontinent is a large continent composed of 
several or all of the existing continents. A super-
continent cycle consists of rifting and break up of 
one supercontinent, followed by reassembly, in 
which dispersed cratons collide to form a new super-
continent, with most or all fragments in different 
configurations from the older supercontinent. 
25 The youngest supercontinent is Pangea, which 
formed between 450 and 320 Ma and includes most 
of the existing continents. Pangea began to frag-
ment about 160 Ma and is still dispersing today. 
Gondwana, comprising Southern Hemisphere con-
tinents, formed at 750-550 Ma. The earliest well-
documented supercontinent is Rodinia, which 
formed about 1.3-1.0 Ga, fragmented at 750-600 
Ma, and appears to have included most of the 
continents in a configuration quite different from 
Pangea. 
26 To prepare for the survival of living systems on 
planet Earth, it is important to understand the na-
ture and causes of interactions between Earth sys-
tems and between Earth and extraterrestrial systems. 
Suggestions for further reading 
Kearey, P. and Vine, F. J. (1996). Global Tectonics (sec-
ond edition). Cambridge, MA, Blackwell Scient., 348 
pp. 
Klein, G. D. (1994). Pangea: Paleoclimate, Tectonics, 
and Sedimentation during Accretion, Zenith, and 
Breakup of a Supercontinent. Geol. Soc. America, 
Spec. Paper 288. 
Moores, E. M. and Twiss, R. J. (1995). Tectonics. New 
York, W. H. Freeman, 415 pp. 
Storey, B. C. (1995). The role of mantle plumes in con-
tinental breakup: Case histories from Gondwanaland. 
Nature, 377, 301-308. 
Windley, B. F. (1995). The Evolving Continents (third 
Edition). New York, J. Wiley, 526 pp.