Catastrophist Tectonics

The earth's lithosphere is composed of fault-bounded, lithospheric "plates" which move with respect to each other over time (e.g. Condie, 1989; Strahler, 1998). Space geodetic observations taken over the past 2 decades show that these plates move at speeds of about 1 to 15cm per year. Geologic evidence, such as the close fit of the continents on either side of the Atlantic Ocean, show that these plates have moved thousands of kilometers. For reasons discussed below, plate velocities are thought to have been roughly equivalent to those measured today for many millions of years. Thus, the current configuration of the earth's continents is seen as evidence of the passage of many millions of years of geologic time.

Young-earthers, on the other hand, have attempted to explain the geologic evidence for seafloor-spreading as the result of an historically-unique, months-long tectonic catastrophe that occurred about 4500 years ago, at the time of Noah's Flood (Austin et al., 1994). Stuart Nevins (1976) for instance states that "[t]he idea that sea-floor plates form slowly and continuously at a rate of a few centimeters each year as the ocean crust is being rift apart, is not supported by geologic data," and that the "separation of the continents [and the] rifting of the ocean floor . . . were accomplished by rapid processes, not occurring today, initiated by a catastrophic mechanism."

Here I argue that the evidence overwhelmingly supports the accuracy of the spreading rate histories derived from the geomagnetic polarity time scale, and that the catastrophist explanations of sea-floor spreading are 'catastrophically' at odds with the geologic evidence.

The geomagnetic polarity time scale (GPTS) was first constructed, and later refined, on the basis of K-Ar dating of terrestrial, igneous rocks of both normal and reversed remnant magnetization (it is now known that remnant magnetization is also preserved in some fine-grained sediments as well). By dating many different igneous rocks and determining their remnant magnetization (normal or reversed), an absolute time scale of reversal events and magnetic polarity intervals was produced. For instance, K-Ar dates from many igneous rocks worldwide showed the the current normal interval began 0.73Ma, that the interval from 0.73Ma to 0.90Ma was reversed, that the interval from 0.90Ma to 0.97Ma was normal, that the interval from 0.97Ma to 1.67Ma, was reversed, and so on. Several sets of estimates for polarity reversals can be seen on this page.

The interpretation of oceanic magnetic anamoly profiles in terms of the GPTS, independently suggested by Vine and Matthews (1963) and Lawrence Morley (1963 Annual Meeting of the Royal Society of Canada), automatically generates spreading-rate histories. For instance, suppose magnetic anamoly boundaries located at 100, 300, and 500km from the ridge axis are correlated with the top of the Jaramillo Normal (~1Ma), the top of the Kaena Reversed (~3Ma), and the top of the Thvera Normal chron (5Ma) (Hilgen, 1991). This implies that the average rate of spreading on the side of the ridge axis was about 100mm/yr, and that the rate of spreading remained steady over millions of years. This is the so-called half-spreading rate. [The full-spreading rate would be the rate at which two points on opposite sides of the axis seperate from each other, usually twice the half-spreading rate.]

How plate velocities are estimated by correlation of magnetic anamolies to the GPTS.
From Magnetic stripes and isotopic clocks

By the end of the 1960's, paleomagnetic rates of seafloor spreading/plate motion had been estimated for many locations on most of the larger plates (e.g. LePichon, 1968). These rates ranged from about 1cm per year for some sites in the Atlantic to about 10cm per year for some sites in the Pacific. Many additional spreading rates of this kind were subsequently computed, and used to construct detailed models of plate motions over time. For instance, the Nuvel-1 model incorporates 266 spreading rates (e.g. DeMets et al., 1990).

Recently it has become possible to date reversals and polarity intervals by the completely different method of orbital/ astronomical dating (e.g. Shackleton, 1990; Hilgen, 1991a,b; Hilgen et al., 1995). In short, astronomical dating is based on the correlation of sedimentary cycles, or cycles in various paleoclimate proxies (such as 18O/16O ratios in foraminifera tests), with cyclic variations of the earth's orbital geometry. The periods of the Milankovitch cycles are computed to be 19-23ka for precession or 'wobble,' 41ka for obliquity or 'tilt,' and 100ka for eccentricity (Cronin, 1999). The relative effects of each cycle can be combined into a single 'insolation' curve which can then be compared to sedimentary and paleoclimate proxy records.

Putative orbital cycles have been recognized in many sedimentary sections on land, and in deep-sea marine cores as well. The orbital cycles can be used to date events in the sections and cores, including polarity events and biostratigraphic 'events' (e.g. Berggren et al., 1995). For instance, if two normal polarity zones are seperated by 20 precession cycles, then the reversed zone in between must be about 400k years in duration. This type of method has been used to construct a precisely-dated astronomical polarity time scale (APTS), starting from the Recent and going back at least to the Miocene, that is entirely independent of radiometric dating. The validity of astronomical dating and the APTS is strongly supported by 40Ar/39Ar dates of the same reversal events, and by 40Ar/39Ar dates of ash beds in orbitally-dated sedimentary sections (e.g. Hilgen et al., 1997; Renne et al., 1993, 1994; Tauxe et al., 1992; Steenbrink et al., 1999; Wilson, 1993). The dates of magnetic reversals derived from both methods, although agreeing well with each other, are consistently 5% or so older than dates derived earlier from earlier K-Ar dates. For instance, the date of the Bruhnes/Matuyama boundary was estimated at 0.73Ma by K-Ar, but 0.78 by both astronomical and 40Ar/39Ar (e.g. Tauxe et al., 1992). For more information on orbital stratigraphy and astronomical dating, see Astronomical Cycles, EOS: Breakthrough Made in Dating the Geologic Record, Astrochronology, ODP Leg 154 Scientific Results, Calibration of Miocene nannofossil events to orbitally tuned cyclostratigraphies from Ceara Rise (PDF), Sonic and gamma-ray astrochronology: Cycle to cycle calibration of Atlantic climatic records to Mediterranean sapropels (PDF).

Biostratigraphic evidence can be used to test sea-floor spreading. If sea-floor spreading has occurred, then the biostratigraphic age of the sediments immediately overlying the oceanic crust should increase with increasing distance from spreading ridge axes. If on the other hand the ocean basins were created rapidly and 'all at once,' we would not expect any relationship between distance from the ridge axis and biostratigraphic age. One of the classic early papers demonstrating that biostratigraphic age of basal sediments does in fact increase with distance from ridge axes in a manner consistent with sea-floor spreading was that of Maxwell et al. (1970). These authors examined 7 complete sediment cores taken along a transect across the south Atlantic, 5 on the west of the Mid Atlantic Ridge, and 2 on the east of the MAR. Their biostratigraphic analyses showed clearly that the "age of the sediments above the basalt bears a direct relation to the distance from the ridge axis," such that "older sediments were found further from the axis" (p. 1049).

For instance, the basal sediments at site 16, 221km west of the MAR, are of upper Miocene age and the Globorotalia acostaensis foraminiferal zone. The basal sediments at site 15, 422km west of the MAR, are of lower Miocene age and the Globergerinita dissimilis foraminiferal zone. The basal sediments at site 14, 745km west of the MAR, are of lower Oligocene age and the Cribrohantkenina inflata foraminiferal zone. The basal sediments at site 19, 1010km west MAR, are of middle Eocene age and the Hantkenina aragonensis foraminiferal zone. The basal sediments at site 20, 1303km west of the MAR, are of late Cretaceous (Maastrichtian) age and the Abathomphalus mayaroensis foraminiferal zone. Maxwell et al. (1970) also attempted to estimate the rate of seafloor spreading, by using the (at the time) newly available K-Ar absolute ages for the paleontological zones. Even with these early K-Ar-based paleontological time-scales, the data provided good support for relatively steady plate movement over millions of years. The graph below, from Maxwell et al. (1970) illustrates the relationship between age and distance from the MAR. The slope of the line represents a spreading rate of ~2cm/yr, which is in close agreement with the rate derived from the geomagnetic time scale.

K-Ar paleontologic ages of basal sediments in south Atlantic
cores vrs. distance from MAR.
From Maxwell et al. (1970).

In the 1980's, the advent of space geodetic technologies such as satellite laser ranging (SLR), Very Long Baseline Interferometry (VLBI), and Global Positioning Systems (GPS) made it possible for the first time to measure to the rates at which plates are moving with respect to each other on time scales of months to years, and with an accuracy of a few mm. If the radiometric and astronomical data are accurate, then plate motions have been remarkably uniform for milions of years. It would be strange, then, if direct observation revealed motion in the 'wrong' directions, or no motion at all, or rates of motion very much faster than those derived from radiometric dating.

But that's not what happened. As it turned out, the global plate velocities measured by space geodetic techniques are nearly identical to those deduced from the radiometrically-dated magnetic reversal time-scale (Smith et al., 1990; Baksi, 1994; DeMets et al., 1994; Gordon, 1995; Larson et al., 1996). That is, plate motions measured over months and years, by geodetic techniques, match very closely --at the 1-2% level (Baksi, 1994) -- those derived from the polarity time scale!

As an example, Smith et al. (1990) reported estimates of plate velocities from SLR measurements taken from 1978-1988 from 22 tracking stations distributed on North Americas, Eurasian, African, Australian, Pacific, Nazcan, and South American plates. Comparison of the resultant SLR plate velcities with the velocities predicted by the Nuvel-1 plate model shows a correlation of 0.989. Larson et al. (1997) plate velocities based on 204 days of observations of 38 GPS sites taken from 1991-1996. The GPS velocity estimates are compared to the velocities predicted by Nuvel-1a. The comparison shows that for "all but a few sites, the agreement . .. . is better than 95% confidence," and that "sites in North America, Antarctica, South America, Eurasia, Africa, and Australia with long time series agree with [with Nuvel-1a] to better than 3mm/yr" (p. 9979).

Geodetic rates of closure or seperation on 149 lines between 20 VLBI and SLR sites, averaged over a few to a dozen years, compared to rates for the same lines computed from Nuvel-1, which are averaged over 3Ma. Correlation is 0.994. From Robbins et al., 1993.

To summarize, rates of plate movement in the past are estimated from spreading-rate histories, and are not simply assumed to have remained constant over time. It has been demonstrated that the spreading-rate histories derived independently from radiometric dating and astrochronology are concordant with each other, and both are in turn concordant with current plate velocities as measured by space geodetic technologies such as GPS. Baksi (1994) notes that "measurements based on the radioactive decay of 40K (half-life of ~1.3b.y.) and variations in the earth's orbital geometry (periodicities of tens of thousands of years), are in agreement with results obtained by space geodetic techniques (averaged over the last decade)" (p.135 ) and that "since the principles underlying these 'dating' techniques are entirely different, it lends credence to the results obtained for sea-floor spreading, arguably the most fundamental process in plate tectonics" (p. 133).

These observations constitute a robust falsification of 'catastrophist' tectonic hypotheses in which the "separation of the continents [and the] rifting of the ocean floor . . . were accomplished by rapid processes, not occurring today" (Nevins, 1976). In fact, the evidence seems to demand that the seperation of the continents occurred via seafloor spreading operating at roughly the same rates currently being measured via space geodetic techniques. To argue, as young-earthers and some other catastrophists do, that the ocean basins were actually created in a few months and that current spreading rates "just happen by chance to be a near perfect match with the rates derived from the . . . geologic time scale requires complete abandonment of Occam's razor" (Wise, 1998).

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http://www.icr.org/research/as/platetectonics.html

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