Tab

Formation

Location Isotopic Method

Age Dates (Ma)

Prince Creek El Gallo Two Medicine Judith River Juifotang

Alaska, USA 40K/40Ar, 40Ar/39Ar

68-71 (Late Cretaceous)

73-74 (Late Cretaceous)

74-80 (Late Cretaceous) 76 (Late Cretaceous)

Formation

Location Isotopic Method

Age Dates (Ma)

Prince Creek El Gallo Two Medicine Judith River Juifotang

Alaska, USA 40K/40Ar, 40Ar/39Ar

68-71 (Late Cretaceous)

73-74 (Late Cretaceous)

74-80 (Late Cretaceous) 76 (Late Cretaceous)

Baja, Mexico 40Ar/39Ar

Montana, USA 40Ar/39Ar

Alberta, Canada 40Ar/39Ar

Inner Mongolia, 40Ar/39Ar China

110 (Early Cretaceous)

Ischigualasto

Northwestern 40Ar/39Ar Argentina

22B (Late Triassic)

when they were deposited. The derived age of a sedimentary particle thus is older than the bed that encloses it (meaning that the principle of inclusions can apply equally well to individual sand grains!). The radiometric clock starts with the cooling and subsequent crystallization of a mineral from a major heating event, which fixes the radioactive elements into a small enough place (i.e., mineral grains) where geo-scientists can sample them. For this reason, only rocks formed in high temperatures (greater than 250°C) at the time of their formation can yield accurate radiometric age dates. These rocks include igneous rocks, formed from originally molten rock, called magma, at temperatures of 70-1200°C; and metamorphic rocks, formed through pressure and heat of about 250-700°C.

Igneous rocks are preferable for age dating, especially volcanic ash beds, because their sediments represent the original time of formation, not the "reheating" times that are typical of metamorphic rocks or the ages of reworked sediments. Ash beds are geologically valued because they were formed by airborne ash spewed from a volcano. The ash was originally deposited in widespread, horizontal layers in a short time, which lends well to cross-checking any derived absolute ages with underlying and overlying strata. Fortunately for dinosaur studies, some dinosaur populations were proximal to volcanic areas that produced voluminous amounts of ash; these layers in combination with relative age-dating techniques have provided a good general framework for defining and understanding the timing for evolution and extinction of certain dinosaur species and groups (see Table 4.5). Igneous rocks that cross-cut Mesozoic sedimentary rocks containing dinosaur fossils, as shown in Fig. 4.5, can also be used to calculate ages, although in some instances the igneous rocks are too young for purposes of comparison to other Mesozoic dinosaur-bearing strata.

Plate tectonic theory states that the Earth is differentiated into layers with their own distinctive chemical and physical properties and that the outermost layer, the lithosphere, moves and interacts with an underlying layer, the astheno-sphere. The lithosphere, the 5-120 km thick, outermost, rigid layer, is composed of the Earth's crust and the upper part of its mantle. Directly underneath it is the asthenosphere, the

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