Table 3. Collective Properties of Plate Boundaries by Class
Total length (km)
Mean velocity (mm/yr)
Area production (m2/s)
-0.000599 ( -0.5%)
+0.001022 ( +1.0%)
-0.007141 ( -6.7%)
-0.086516 ( -80.1%)
The total length of convergent boundaries is 91,762 km, while the total length of divergent boundaries is 94,810. If we neglect the continental convergent and rift boundaries and consider only convergent and divergent boundaries in the oc basins, the total length of subduction zones and other convergent segments is 68,759 km, while the total length of ocean ridges is 67,338 km, again very similar.
The current rate of area increase along the oceanic ridges is 0.095 m2/s is very close to the current rate of area loss along convergent boundaries in the oceans, 0.094 m2/s. While there is no logical or geometrical requirement for the total leng of convergent and divergent boundaries to be identical, they are amazingly similar.
…the buoyancy of thick continental crust keeps it afloat. If continental lithosphere were strong enough to maintain its integrity at a subduction zone, the buoyant continental crust would not only resist being subducted, but the subducting plate would abruptly grind to a halt when the continental “passenger” reached the trench.
The paragraph containing this quote is the first paragraph in a section entitled “Differences between continents and oceans” and focuses on the contrast in buoyancy. The next paragraph focuses on their contrast in strength. It reads,
The strength of the continental lithosphere also contrasts with that of the oceanic lithosphere. The strongest part of the oceanic lithosphere seems to lie in the mantle, between 20 and 60 km depth, between a brittle upper part and above its increasingly ductile lower part, which grades downward into the asthenosphere (Fig. 3). In the same depth range where oceanic lithosphere is strongest, however, continental lithosphere consists of crust, not mantle. At temperatures typical of the lower crust (400-700 °C), the minerals comprising the crust appear to be much weaker than olivine, the strong mineral that comprises most of the upper mantle. Consequently, continental lithosphere could be much weaker than oceanic lithosphere. Oceanic lithosphere behaves as a virtually rigid plate because of its strong core, but, as the late C. Goetze noted in the mid-1970s, continental lithosphere might consist of three layers: a brittle upper-crustal layer, a weak lower crust and a stronger uppermost mantle, which, nevertheless, would not be as strong as the strongest part of the oceanic lithosphere. This jam-sandwich-like rheological profile (Fig. 3) is also suggested by the frequent occurrence of earthquakes (brittle deformation) in the upper crust, their nearly complete absence in the (presumably weak, ductile) lower crust, and their occasional presence in the underlying upper mantle.
The point of this paragraph is that in regard to overall strength continental lithosphere contrasts strongly with the oceanic lithosphere. Indeed, the strength profile of continental lithosphere has frequently been referred to as a “jam sandwich” because of the weakness of the warm lower crust. So the original quote, taken in its context, does not suggest or imply that continental lithosphere is strong enough to maintain its integrity at a subduction zone and therefore that subduction should abruptly grind to a halt. It is just the opposite. The author in the following paragraph is providing reasons why continental lithosphere is weak and deformable and why this grinding to a halt state of affairs does not generally take place. This observation applies equally to UPT and CPT.
Beneath much of the Andes, oceanic lithosphere descends eastward into the mantle at an angle of about 30°. A partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material. This wedge is the ultimate source for magma erupted at the active volcanoes that characterize the Andean margin. But between 28° and 33° S the subducted Nazca Plate appears to be anomalously buoyant, as it levels out at about 100 km depth and extends nearly horizontally under the continent. Above this ‘flat slab’, volcanic activity in the main Andean Cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge. But it is unknown where slab volatiles go once this happens, and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward. Here we present results from a magnetotelluric profile in central Argentina, from which we infer enhanced electrical conductivity along the eastern side of the plunging slab, indicative of the presence of partial melt. This conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth, or that melt is supplied from the 410 km discontinuity, consistent with the transition-zone ‘water-filter’ model of Bercovici and Karato.
Note that beyond that flat zone the slab “finally rolls over to descend steeply into the mantle.”
Note that like the similar zone beneath Chili and Argentina, the slab after moving nearly horizontally, in this case for only about 300 km, then plunges downward at a steep angle. This thesis also includes a map showing the tectonic setting, shown below.
There is also a strong case that the Farallon Plate that subducted beneath the western coast of North America. This was first proposed more than 20 years ago by P. Bird in “Formation of the Rocky Mountains, Western United States: A Continuum Computer Model,” Science 239, 1501-1507. The abstract of this paper is as follows:
One hypothesis for the information of the Rocky Mountain structures in late Cretaceous through Eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the North American lithosphere. The horizontal components of the motion of this plate are known from paleomagnetism, and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism. New techniques of finite-element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the North American Plate. A model that has a realistic temperature-dependent rheology and a simple plane-layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present. Successful prediction of this model include (i) the location, amount, and direction of horizontal shortening that has been inferred from Laramide structures; (ii) massive transport of lower crust from southwest to northeast; (iii) the location and timing of the subsequent extension in metamorphic core complexes and the Rio Grande rift; and (iv) the total area eventually involved in Basin-and-Range style extension.
In a broad sense, this model has predicted the belt of Laramide structures, the transport of crust from the coastal region to the continental interior, the subsequent extension in metamorphic core complexes and the Rio Grande rift, and the geographic region of late Tertiary Basin-and-Range extension. Its principal defects are that (i) many events are predicted about 5 million to 10 million years too late and (ii) the wave of crustal thickening does not travel far enough to the east. Reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects.
The correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the Laramide orogeny is probably correct. The Rocky Mountain thrust and reverse faults formed in an environment of east-west to northeast-southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the North American crust. Nonuniform crustal thickening by simple-shear transport also caused relative uplifts; therefore, this model is consistent with both of the range-forming mechanisms that have been inferred. A new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional Basin-and-Range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away.
This landmark paper has been widely referenced in subsequent work on the geology of the western United States. The inference of a period of flat subduction by the Farallon Plate beneath western North America is prominent in the much more recent paper by Sigloch, McQuarrie, and Nolet (previously mentioned in my response above to question 8 [ Q8 ] ) that presents the 3D seismic tomography image of the strongly contorted Farallon Plate. The author’s interpretation of the 3D tomographic image in terms of the subduction history of the Farallon Plate is provided in the figure below, reproduced from their paper.
Of course, the interpretation in this article as well as in the previous one is in terms of the uniformitarian time scale, which is to be rejected in regard to absolute dates are concerned. In summary, the case for flat subduction of slabs is compelling, not only in the present but also in the past. Numerical models show that it is mechanically plausible. The main driving force for moving the slab is the slab pull arising from the negative buoyancy of the cold dense material that comprises the slab. These conclusions apply equally for UPT and CPT.
12. Question: Just how conclusively has seismic tomography demonstrated the reality of subducted plates in the mantle?
Response: The paper by K. Sigloch, N. McQuarrie, and G. Nolet, “Two-stage subduction history under North America inferred from multiple-frequency tomography,”Nature Geosciences, 1, 458-462, 2008, [link provided by ed.] mentioned above in my response to question 8 ( Q8 ), provides a dramatic example of the ability of current generation seismic tomography methods to convincingly reveal the 3D structure of subducted slabs. Figure 2 from this paper, reproduced above, shows the present shape of the Farallon Plate which, not only subducted beneath the western coast of North America since the earliest Jurassic in the past, but continues to do so as the modern Juan de Fuca Plate along the coasts of Oregon and Washington.
The paper by Miller, M.S., Gorbatov, A., Kennett, B.L.M., “Imaging changes in morphology, geometry, and physical properties of the subducting Pacific Plate along the Izu-Bonin-Mariana arc,” Earth and Planetary Science Letters 235, 331-342, 2004, shows the geometry of the portion of the Pacific Plate that is currently subducting in the Izu-Bonin Trench south of Japan. Two figures from their paper, shown below strongly suggest that the slab is in the process of tearing as it subducts.
11. Question: Why is it that beneath trenches, earthquakes sometimes occur across a much broader region than the width of a plate?
Response: Earthquakes that occur well behind the trench location tend to be deep-focus earthquakes which occur at depths between 300 and 700 km beneath the earth’s surface. Because subducted lithosphere should not exhibit brittle behavior at such depths, the mechanism responsible for these deep earthquakes has stirred controversy since their actual depth was first verified more than 70 years ago. Because mineralogical phase changes occur in the lower part of the upper mantle where these earthquakes are most frequently observed, a possible and leading candidate mechanism has been the catastrophic transformation of metastable olivine into the higher density spinel phase. Because of the low temperatures in the core of the subduction slab, this phase transition likely may not always spontaneously occur as the slab passes through the depth where the phase transition otherwise ought to take place. When this is the case, metastable olivine is transported to greater depths and has the potential to transform rapidly to the spinel phase, provided there is some process to initiate this transformation. However, a simple volumetric implosion of the low density olivine phase to produce the higher density spinel phase does not match the pattern of earthquake waves these earthquakes radiate—a pattern which typically implies a large amount of shear deformation.
However, about 20 years ago H. W. Green and P. C. Burnley in “The failure mechanism for deep-focus earthquakes,” Geological Society, London, Special Publications 54, 133-141, 1990, described the mechanism now generally thought to account for these deep focus earthquakes. In the abstract of this paper they summarize their findings:
Experimental deformation of Mg2GeO4 olivine at pressures between 1 and 2 GPa in the spinel stability field has led to discovery of a faulting instability that develops at the kinetically-controlled threshold of transformation. Very fine-grained olivine and spinel are found in fault zones. Deformation at lower temperatures is ductile; transformation is inhibited and specimens are very strong. Deformation at higher temperatures also is ductile but transformation is rapid and specimens are much weaker. Detailed examination of the microstructures of specimens deformed in the faulting regime lead to an anticrack theory of faulting that explains the experimental data and provides a fundamentally new mechanism for deep-focus earthquakes. The new mechanism is analogous to the Griffith theory of fracture; nucleation and growth of spinel under stress produces spinel-filled microanticracks normal to the maximum compressive stress that link up to produce faulting. The friction paradox for deep earthquakes is resolved because this faulting process provides a fine-grained, superplastic, ‘lubricant’ for faults. The temperature distribution within subducting slabs of lithosphere requires that the conditions of instability are reached as a natural consequence of subduction; metastable olivine in the interior of deep slabs warms to a critical temperature where faulting ensues in the presence of a shear stress.
To summarize, Green and Burnley used the germanium analog mineral, Mg2GeO4, instead of silicate olivine, (Mg,Fe)2SiO4, to investigate the mechanics of this phase transition in the laboratory in a large enough volume to be able to observe and characterize the actual faulting process. The germanium analog is softer and changes to the spinel structure at much lower pressure than the silicate mineral. Their experiment appears to elucidate how this phase transition can unfold extremely rapidly and also generate large-scale shear motions within the core of a subducting slab.
Another observation that points to the likelihood of the mechanism involving the rapid transformation of olivine to spinel and possibly other lower density phases such as pyroxene transform to their higher density phases is that deep focus earthquakes cease abruptly below a depth of about 680-700 km, which represents the boundary between the upper and lower mantle. This is the depth at which the major upper mantle phases are converted to the yet higher density phases perovskite and magnesiowuestite. Hence, whatever the mechanism is, it shuts down when these transitions between upper mantle mineral phases no longer can occur.
These observations and conclusions apply equally to both UPT and CPT.
10. Question: Why is it that in gravity surveys trenches display mass deficiencies, not mass excesses, as subducted slabs would be expected to produce?
Response: Trenches themselves represent huge mass deficiencies, where, instead of rock, there is water. The free-air gravity anomalies observed over trenches are typically in the range of -100 to -300 milligals (one milligal is 10-5 m/s2), depending on the amount of sediment fill. One can verify that mass deficiencies corresponding to those associated with trenches do indeed produce these sorts of free-air gravity signatures. The formula for the gravity anomaly ∆g produced by an infinitely long line of excess mass g per unit length at depth b below the surface and observed directly above the line is given by the definite integral from -∞ to ∞ of the integrand Ggdx/(x2 + b2), which is equal to pGg/b, where G = 6.673 x10-11 is the universal gravitational constant. Approximating the cross-sectional area of a trench as a triangle of height h and width 2h/3, with its center h/3 below the surface, yields the formula ∆g = pGh∆r, where ∆r is the density contrast between what is filling the trench and normal crustal rock. For a trench depth h of 6000 m and a ∆r of -1700 kg/m3, corresponding to the trench being filled with water, we get a resulting gravity anomaly of -214 milligal. The extra density of the slab immediately beneath the trench, because of its cold average temperature, can readily be shown to be negligible in comparison.
However, the higher density of subducted slabs often does produce a discernible gravity signature behind the trenches. This can be seen from a visual inspection of the free-air gravity anomaly map of the world’s ocean floors shown below. Positive gravity anomalies shown in orange are evident behind the Tonga-Kermadec Trench east of Australia and behind the Izu-Bonin and Marianas Trenches south of Japan. Again, this gravity signature is the result of the subducted slabs greater density, because of their lower temperature, relative to the surrounding mantle rock. These principles apply equally well to both UPT and CPT.
9. Question: If subducting plates produce volcanoes, why are there so many volcanic seamounts on the interior of the Pacific Plate?
Response: More than half of the world’s active volcanoes above sea level today encircle the Pacific Ocean to form what is frequently referred to as the circum-Pacific “Ring of Fire.” This horseshoe shaped belt, some 40,000 km long, is associated almost exclusively with nearby ocean trenches, as indicated in the map below produced by the USGS. Almost without exception, the volcanoes are on the side of the trench beneath which the subduction is occurring.
The crustal portion of the subducting slab contains a significant amount of surface water, as well as water contained in hydrated minerals within the seafloor basalt. As the subducting slab descends to greater and greater depths, it progressively encounters greater temperatures and greater pressures which cause the slab to release water into the mantle wedge overlying the descending plate. Water has the effect of lowering the melting temperature of the mantle, thus causing it to melt. The magma produced by this mechanism varies from basalt to andesite in composition. It rises upward to produce a linear belt of volcanoes parallel to the oceanic trench, as exemplified in the above image of the Aleutian Island chain. The chain of volcanoes is called an island arc. If the oceanic lithosphere subducts beneath an adjacent plate of continental lithosphere, then a similar belt of volcanoes will be generated on continental crust. This belt is then called a volcanic arc, examples of which include the Cascade volcanic arc of the U.S. Pacific northwest, and the Andes volcanic arc of South America.
The volcanoes produced by subduction zone volcanism are typically stratovolcanoes. Incipient island arcs tend to be more basaltic in composition, whereas mature continental volcanic arcs tend to be more andesitic in composition.
The point here is that the basic mechanism by which subduction so commonly generates volcanism is well understood. One aspect of the process that the above simple article did not include is the so-called ‘corner flow’ that occurs in the asthenospheric wedge between the subducting plate and the overriding plate. This flow, driven by drag from the subducting plate, brings fresh, hot asthenospheric rock, like a blow torch, into the very zone where the volatiles are being released and partial melting takes place.
What about the volcanoes, mostly inactive and below sea level, in the western and central Pacific? These are almost certainly a consequence of the massive hot thermal anomaly in the lower mantle beneath the south central Pacific known as the Pacific superplume. This feature, as well as a similar feature on the opposite side of the earth beneath Africa, together with a ring of anomalously cold and dense rock beneath the perimeter of the Pacific, shown in the figure below, were some of the most visible and robust features in 3D seismic images of the lower mantle from the earliest days of seismic tomography in the 1980’s.
It is noteworthy that observational evidence is compelling that a huge pulse of volcanism occurred in the central Pacific in the mid-Cretaceous. Some of the evidence is described in the paper by R. L. Larson, “Latest pulse of Earth: Evidence for a mid-Cretaceous superplume,” Geology 19, 547-550, 1991. The abstract for this paper is as follows:
A calculation of Earth’s ocean crustal budget for the past 150 m.y. reveals a 50% to 75% increase in ocean crust formation rate between 120 and 80 Ma. This “pulse” in ocean crust production is seen both in spreading-rate increases from ocean ridges and in the age distribution of oceanic plateaus. It is primarily a Pacific Ocean phenomenon with an abrupt onset, and peak production rates occurred between 120 and 100 Ma. The pulse decreased in intensity from 100 to 80 Ma, and at 80 Ma rates dropped significantly. There was a continued decrease from 80 to 30 Ma with a secondary peak near the Cretaceous/Tertiary boundary at 65 Ma. For the past 30 m.y., ocean crust has formed at a nearly steady rate. Because the pulse is seen primarily in Pacific oceanic plateau and ridge production, and coincides with the long Cretaceous interval of normal magnetic polarity, I interpret it as a “superplume” that originated at about 125 Ma near the core/mantle boundary, rose by convection through the entire mantle, and erupted beneath the mid-Cretaceous Pacific basin. The present-day South Pacific “superswell” under Tahiti is probably the nearly exhausted remnant of the original upwelling. How this superplume stopped magnetic field reversals for 41 m.y. is a matter of speculation, but it probably involved significant alteration of the temperature structure at the core/mantle boundary and the convective behavior of the outer core.
It turns out that most of the seamounts that are so numerous in the western Pacific today were formed precisely in the time window described by Larson. This is documented in the paper by Stepashko, A. A., “Origin of West Pacific seamounts and features of the Cretaceous dynamics of the Pacific Plate,” Oceanology 46, 411-417, 2006, as summarized in the abstract:
A correlation between the age and position of 25 seamounts in the West Pacific Ocean formed, judging from the 40Ar/39Ar data, in the period from 120 to 65 My B.P. was recognized. The seamounts studied are joined into linear zones with extensions up to 5000 km; the age of the seamounts decreases in the southeastern direction. In the interval 93–83 My B.P., the seamount formation was extremely rapid; this interval coincides with the period of acceleration in the Pacific Plate movements. In the middle of this interval, 87 My B.P., an intensification of the magmatic activity accompanying the seamount formation was observed simultaneously with the extinction of the Isanagi Plate and the appearance of the Kula Plate.
What about this south central Pacific region today? A widely referenced paper on this topic is by M. K. McNutt and A. V. Judge, “The Superswell and Mantle Dynamics Beneath the South Pacific,” Science 25, 969 – 975, 1990, summarized in its abstract as follows:
The region of sea floor beneath French Polynesia (the “Superswell”) is anomalous in that its depth is too shallow, flexural strength too weak, seismic velocity too slow, and geoid anomaly too negative for its lithospheric age as determined from magnetic isochrons. These features evidently are the effect of excess heat and extremely low viscosity in the upper mantle that maintain a thin lithospheric plate so easily penetrated by volcanism that 30 percent of the heat flux from all hot spots is liberated in this region, which constitutes only 3 percent of the earth’s surface.
To summarize, (1) seismic topography shows what appears to be a massive hot thermal anomaly in the lower mantle presently beneath the south central Pacific; (2) a huge pulse of volcanism in this very region during the mid-Cretaceous generated the seamounts that currently populate the Pacific Plate in the western Pacific region; (3) a high level of volcanic activity continues in this region today, but at a level greatly diminished from that of the Cretaceous. In regard to the claim, the western Pacific seamounts are the result of this thermal anomaly and not to the subduction-related processes which generate the Pacific Ring of Fire volcanoes.
Finally, how do UPT and CPT compare in accounting for these features? First, partial melting and volcanism in subduction zones is to be expected in both versions of plate tectonics. But the huge volumes of subduction-generated silicic volcanism that formed the Sierra Nevada and related granites, for example, are extremely difficult for UPT to explain but readily accounted for within the CPT framework. Uniformitarianism in general has difficulty with non-uniform phenomena like the Cretaceous pulse in central Pacific volcanism, so in terms of accounting for the Pacific seamounts, CPT again displays superior explanatory power.
8. Question: Is subduction geometrically possible only along a straight line?
Response: According to GPS measurements, subduction is taking place today into the arc-shaped Aleutian Trench, the arc-shaped Sumatra Trench, and many other trenches that are far from straight lines. The error in the belief that subduction can occur only along a straight line is a failure to recognize that a subducting plate deforms as it plunges into the mantle. Many papers in the peer-reviewed literature provide strong seismic evidence that subducted slabs tear and deform dramatically in their journey downward into the mantle. One location where slab tear is almost a certainty is at the Aleutian-Kamchatka corner. In the paper by A. Davaille and J. M. Lees, “Thermal modeling of subducted plates: tear and hotspot at the Kamchatka corner,” Earth Planet. Sci. Lett. 226 293-304, 2004, the authors ask, “How can the Pacific Plate, which is subducting at an oblique angle in the western Aleutians, physically connect to the relatively steeply dipping Kamchatka slab to the west?” They continue, “The surficial manifestation of the connection is the massive Bering transform zone (TZ), extending from Attu Island westward towards Kamchatka (see figure below). In Kamchatka the margin between the Pacific Plate and North America takes a sharp turn southwards, towards the Kurile Trench and Japan. How does the Pacific Plate accommodate this sharp apparent bend? Does the Pacific plate drape over the corner as a tablecloth folds around a table corner, or is the Pacific Plate torn at the corner along the TZ to accommodate the deformation? In this paper we explore evidence and implications for the latter hypothesis.” The authors do make a strong case that there is a tear in the Pacific Plate along the Bering Fracture Zone.
The extreme deformation that subducted slabs undergo is beautifully illustrated in a recent paper by K. Sigloch, N. McQuarrie, and G. Nolet, “Two-stage subduction history under North America inferred from multiple-frequency tomography,” Nature Geosciences, 1, 458-462, 2008. Figure 2 from this paper, reproduced below, shows via seismic tomography the present shape of the Farallon Plate, which has subducted beneath the western coast of North America since the earliest Jurassic and continues to do so as the modern Juan de Fuca Plate along the coasts of Oregon and Washington. The authors infer from the seismic data as well as from numerous surface observations that this plate underwent a large amount of tearing in its complicated history.
Studies on the way slabs deform after they subduct reveal that they can and do undergo extreme deformations, including tears. Most of the studies utilize seismology to provide actual images of these deforming masses of rock inside the earth. The fact that subducted slabs can and do deform means that geometrical constraints that would apply to perfectly rigid slabs do not apply to the real ones. This conclusion is valid not only in the case of the uniformitarian framework but also in the case of CPT.
7. Question: How can a plate even begin its dive under an adjacent plate that is 30-60 miles thick if cliffs cannot be higher than 5 miles?
Response: Because rock at sufficient depth under stress does indeed begin to deform inelastically or plastically, the boundary below a few km depth between a subducting plate and the overriding plate is never vertical but instead is inclined, typically, at an angle of 30-45 degrees. Inelastic deformation of the edge of the overriding plate readily allows this to occur. The fact that plates are subducting today means that one plate diving beneath another plate not only is possible; it is an undeniable reality. Below is a figure from the NASA website here (ed. updated from original) showing velocities of more than 900 GPS stations worldwide. The velocity discontinuities at mid-ocean ridges and also subduction zones is to me indisputable evidence that both seafloor spreading and subduction are occurring on our earth today.
Image Caption: Station velocities determined by integration of their GPS observations over the period 1999-2007. Data were analyzed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
6. Question: How can plates rift apart, and should they do so, how could they move over the underlying rock?
Response: While plates in general display considerable strength, there is impressive observational evidence that they can and do rift, or split apart, under the right circumstances. A good example is the rifting of the Arabian block from Africa with the formation of the Red Sea and the Gulf of Aden. Just what the forces were that initiated the rifting may not be so clear, but the reality that a once coherent plate has split into two pieces and that the two pieces are presently moving away from each other is documented by many lines of evidence, including GPS measurements. Another example of continental rifting is the separation of Baja California from Mexico and the subsequent migration of this block several hundred kilometers to the northwest. The cause in this case seems to be clearer, given that according to present GPS measurements the motion of this block is essentially identical to that of the Pacific Plate. The implication is that some of the western portion of the North American Plate has overridden the ridge that earlier formed the southeastern boundary of the Pacific Plate and that the forces associated with the divergence at this ridge were sufficient to cause the rifting away of this sliver of North American Plate. Careful numerical simulation indicates that lithospheric plates can also fail in compression. The most common circumstance is when a slab of oceanic lithosphere becomes sufficiently thick through cooling, it begins to founder under its own weight, initially producing a broad depression above it and then fracturing and sinking into the weaker and less dense mantle beneath it.
In regard to the issue of a lithospheric plate moving relative to the asthenospheric layer beneath it, I pointed out earlier that the case is compelling that this layer is on the order of a thousand to ten thousand times weaker than normal lithosphere and that the drag forces exerted by the asthenosphere on the base of the lithosphere tend to be extremely small.