Paleontology and Geology as Sciences
Step 1. Converting values to scientific notation for simplicity = 1.96 × 10 3 km/1.545 × 10
y Step 2. Converting km to cm: 1.0 km = 1 × 105 cm/km. Step 3. = 1.96 × 108 cm/1.545 × 107 y = 1.3 cm/y
Realize also that during 155 million years the average rate might have had a range, such as 0.7 to 3.1 cm/year, but these values also can be calculated as long as radio- metric age dates and distances from the spreading center are known. Using this method, spreading rates (rates of plate movement) for plates have been calculated as ranging from less than 1.0 to as much as 11 cm/year.
The fact that the oldest seafloor dates from the Jurassic Period indicates that the ocean floor has been destroyed and recycled since at least the time that Apatosaurus and Allosaurus were on the continents. This brings up another integral hypothesis in plate tectonic theory, subduction, which is supported by points 6 through to 8. Subduction is a proposed process where, as plates move away from divergent boundaries and collide with one another at plate-convergent boundaries, one plate can go underneath the other. The subducted plate undergoes partial melting as it slides further into the hotter asthenosphere, which in turn forms magma for igneous rocks. The force of the collision is sufficient to generate pressures that can bend (fold) rocks at depth, and can also break (fault) rock at shallower depths. This process explains the folding of originally horizontal strata as is seen in the field, as well as fractures that cut across the strata. Stress (also known as pressure), the force applied to a unit area, has these associated formulas,
F = ma (4.8)
σ = F/A (4.9)
where F is force (expressed in newtons; Chapter 1), m is mass, a is acceleration (typically in m/s2), σ is stress, and A is area (m2). A moving plate has a sufficiently large mass so that its slow acceleration or velocity (the latter in m/s, rather than m/s2
) is irrelevant as it hits the other plate, which is moving as well, providing additive force applied to the area of contact between the plates. The result of this stress is strain, which is manifested by folds and faults. This provides an explana- tion for the built-up tensional energy that is periodically released through earth- quakes, the pressures required for the formation of some metamorphic rocks, and the development of mountain ranges that exceed 9000 m above sea level. Such high mountain ranges occur when a thick continental lithospheric plate collides with another continental plate and neither is subducted, which explains why continents are composed primarily of rocks older than those on the seafloor. This circumstance is very fortunate for dinosaur paleontologists, as dinosaur fossils in mountainous areas would have been destroyed if they had entered a subduction zone.
Other phenomena associated with plate tectonics are transform fault movements and hot spots. Transform faults are areas where movement of the lithosphere is
Metamorphism Formation of volcanic rock Weathering Transport of sediments LITHOSPHERE ASTHENOSPHERE Magma Magma Transform fault Ocean crust IGNEOUS Melting Divergent boundary Convergent boundary Weathering He at and Pressure Hea tan d Pre ssur e Mel ting M eltin g W eath erin g Weathering Heat and Pressure METAMORPHIC SEDIMENTARY
interpreted as a result of plates moving laterally against one another without any accompanying volcanism. Evidence for this movement consists of the aforemen- tioned stress eventually resulting in earthquakes; measurable movement along the fault plane can be defined through offset features in the landscape. Hot spots are interpreted as plumes of magma that pierce the lithosphere and rise up consistently in the same place for millions of years. Evidence for hot spots is best exemplified by strings of islands, such as the Hawaiian chain, which have active volcanism on a single island. However, other islands and undersea volcanoes (guyots) in the chain have volcanic rocks that show increasingly older radiometric ages farther away from the island with active volcanism. Plate tectonic theory has an elegant solution for this pattern: the hot spot stays in the same place while the plate moves over it.
This rudimentary knowledge of plate tectonic theory facilitates a better under- standing of the rock cycle (Fig. 4.8). The way of the Earth is constant change and all rocks are in a state of transition, although they appear static to us during our short lifetimes. In one simplistic and linear example, elements composing sedimentary rocks become incorporated into igneous rocks through subduction and melting of the sedimentary rocks; igneous rocks become heated enough to change into metamorphic rocks; then metamorphic rocks, uplifted by plate convergence, are exposed at the surface and weathered so that their broken-down elements are cemented together into sedimentary rocks. Plate tectonics results in the following:
FIGURE 4.8 The rock cycle as explained through plate tectonics, showing relationship of lithosphere and asthenosphere, as well as convergent, divergent, and transform-fault boundaries.
1 creates new seafloor and destroys old seafloor;
2 brings magma from the mantle to the surface in volcanoes, folds, and frac- tured rocks; and
3 provides uplifted areas that later undergo erosion and shed sediments. It is, therefore, the source of the Earth’s constant recycling that makes the planet a dynamic place for life. In fact, the Earth seems to be the only body in the solar system that shows such comprehensive evidence for plate tectonics.
Preview of the Importance of Plate Tectonics to Dinosaur Studies
Why do we need to know about plate tectonics when studying dinosaurs? Because everything on the surface of the Earth is affected by plate tectonics. It determines the location of the continents, mountain ranges, volcanoes, and earthquakes, as well as the configuration of the world’s oceans. The location of the continents and their inherent geographic features affect the distribution of all land plants and ani- mals. The entire global environment (especially climate) is influenced by the place- ment of the oceans relative to the continents because patterns of oceanic and atmospheric circulation are controlled by whether a continent is in an equatorial or polar position (Chapter 6). Local climates are changed by the presence of moun- tains, which are formed by plate tectonics. The amount of volcanism on the seafloor causes the sea level to either rise or fall, and rising sea level can cause landward environments to become more crowded for terrestrial organisms, with associated ecological stresses. Conversely, uplift of a mountain range caused by plate con- vergence causes land that formerly was shoreline to be more emergent, which expands continental areas for animals and plants. Volcanism caused by plate con- vergence places ash in the atmosphere, blocking solar radiation and cooling the Earth, which can negatively affect plant communities that animal communities depend on to live (Chapter 16). Earthquakes alter the course of rivers, change the landscape, or generate tsunamis (seismic sea waves) that drown many people and other living things in coastal communities. For the purposes of exploration for dinosaur fossils, some dinosaur-bearing strata were uplifted by continental colli- sions, and their present surface distribution in folded and faulted rocks is directly attributable to plate tectonics.
Basically, many (if not most) humans are affected by plate tectonics every day in the form of earthquakes, volcanoes, tsunamis, mountains, climates, and shore- lines. All evidence from Mesozoic rocks indicates that plate tectonics was just as active a global process then. Thus, the dinosaurs lived and eventually died at the consent of plate tectonics as well, although as a group they were permitted to thrive for about 165 million years.
Recovery and Preparation of Dinosaur Fossils: How They Are Collected
From the Field to the Institution
Chapter 2 mentioned that searching for fossils can be hard work, but it did not examine the difficulties of properly collecting fossils, especially those of dinosaurs. Fortunately, many dinosaur body fossils are either from relatively small individuals or just a few parts of larger individuals. However, finding a nearly complete speci- men of a large theropod (Chapter 9) or sauropod (Chapter 10) means that the logistics
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