The Geologic Time Scale

Since the water movement will be very slow compared with the rate at which the wastes dissolve, we are concerned first and foremost with equilibrium solubility. Also, if only to relate behaviour on the geological time scale to that on the laboratory time scale, we will need to know about the mechanisms and kinetics of dissolution and leaching. The waste forms envisaged at present are glass blocks containing separated fission products and residual actinides fused into the glass and, alternatively, the uranium dioxide matrix of the used fuel containing unseparated fission products and plutonium. In the... 

Many diverse processes are involved in the transformation of the elements and their compounds in the Earth. Some of the pathways observed are shown in Figure 1, a version of the so-called geochemical cycle . This cycle is very much simplified and is not a closed one. It may also be short-circuited and indicated processes may be very fast on the geological time scale, or, more often as not, occupy very lengthy periods, amounting in some cases to billions of years. 

One may wonder whether recycling processes such as a linear or a nonlinear back reaction exist in relevant autocatalytic systems. So far, we are not aware of their existence, ft is, however, possible that the back reaction rates k or ji are nonzero but too exceedingly small to be detected in laboratory experiments. Concerning the problem of homo chirality in life, very small A. or // are not unimaginable, considering the geological time scale for its establishment on earth. 

In the discussion of the results given in this chapter, correlations are made with respect to coal rank and geothermal stress which, in the case of the five coals studied here, coincides with geological age. Table I gives the latest ranking of coals (2) and Table II (3) gives the geological time scale. 

Hedberg H. D. (1978) Stratotypes and a geochronologic scale. In Contributions to the Geologic Time Scale, Am. Assoc. Petrol. Geol. Studies in Geology (ed. G. V. Cohee), vol. 6, pp. 33-38. 

Naturally occurring nuclides which have short half-lives on the geological time scale, but which are being continuously produced by cosmic-ray radiation. 

The Cenozoic Era (c.65 million years BP to the present) encompasses the Tertiary and the Quaternary Periods. During the Tertiary the Earth s climate began an overall cooling trend of about 12 °C in the past 40 million years. Over the past two and a half million years the climate has varied from cool to warm periods, accompanied by massive expansions and contractions of the polar ice caps. This period of climate fluctuation is termed the Quaternary Period and spans the geologic time scale from the end of the Pliocene Epoch, roughly 1.8-2.6 million years ago, to the present. The Quaternary Period includes the Pleistocene and Holocene Epochs, with the Holocene... 

Carboniferous. The combined Mississippian and Pennsylvanian periods of the upper Paleozoic era of the geologic time scale commenced approximately 345 million years ago. 

Holocene. An epoch of the Quaternary period of the geologic time scale from the end of the Pleistocene to the present commenced approximately 10,000 years ago. 

Biomass burning results in the sequestration of carbon as charcoal, constituting a significant sink for atmospheric carbon dioxide and therefore a source of atmospheric oxygen. The charcoal is not subject to microbial oxidation. On the geological time scale atmospheric oxygen levels may increase, which increases the risk of fires and thus establishes a positive feedback loop for the buildup of atmospheric oxygen. 

Neogene The last 23 million years of the Geologic Time Scale. 

The Geological Time-Scale is hierarchical, consisting of (from smallest to largest units) ages, epochs, periods, eras and eons. Each era, lasting many tens or hundreds of millions of years, is characterized by completely different conditions and unique ecosystems. 

It is followed the geological time scale as determined by the International Commission on Stratigraphy (ICS). The ICS has not finished its job and gaps remain, particularly in the Early Paleozoic. Where gaps occur, it is generally followed the Russian system for the Cambrian and the British system for the Silurian. Epochs are subdivided further into ages not listed here. The periods from Cretaceons and older are subdivided into epochs and ages not shown here. 

The Earth is now estimated to be 4.6 bUlion years old. The different rock types formed at different times have assigned ages of their time of formation from the present time based on the geological time units of the geological time scale. Stratigraphy determines the succession in time of rocks to establish the sequence in which the rocks were deposited or formed. 

The geologic time scale is based on the International Stratigraphic Chart issned by the International Union of Geological Sciences (lUGS) in 2002... 

Sedimentary phosphate rocks occur throughout the geological time scale. Most sedimentary deposits were apparently formed in offshore marine conditions on continental shelves. They exhibit a wide range of chemical compositions and great variations in physical form. Economic deposits usually occur as one or more thick high-grade beds that are unconsolidated and have a uniform granular texture and composition, shallow overburden, and a minimum of structural deformation. 

In order to grasp the information pertinent to the nature of geologic shale formations discussed in this text it is important to have an understanding of the geologic time scale. Table 5.1 is provided on the following pages to help the reader correlate the relationship between each shale gas formation discussed and where each resides within the spectrum of the geologic time scale. 

Source Adapted from USGS, The Geologic Time Scale, U.S. Geologic Service, Washington, DC, 2008 (http //vulcan.wr.usgs.gov/Glossary/geo ... 

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