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The Geosphere and Geochemistry
Published in Stanley E. Manahan, Environmental Chemistry, 2022
At elevated temperatures deep beneath Earth's surface, rocks and mineral matter melt to produce molten magma. Cooling and solidification of magma produce igneous rock. Common igneous rocks include granite, basalt, quartz (SiO2), pyroxene ((Mg,Fe)SiO3), feldspar ((Ca,Na,K)AlSi3O8), olivine ((Mg,Fe)2SiO4), and magnetite (Fe3O4). Igneous rocks are formed under water-deficient, chemically reducing conditions of high temperature and high pressure, whereas exposed igneous rocks are formed under wet, oxidizing, low-temperature, and low-pressure conditions. Since such conditions are opposite those conditions under which igneous rocks were formed, they are not in chemical equilibrium with their surroundings when they become exposed. As a result, such rocks disintegrate by a process called weathering. Weathering tends to be slow because igneous rocks are often hard, nonporous, and of low reactivity. Erosion from wind, water, or glaciers picks up materials from weathering rocks and deposits it as sediments or soil. Lithification is the conversion of sediments to sedimentary rocks. In contrast to the parent igneous rocks, sediments and sedimentary rocks are porous, soft, and chemically reactive. Heat and pressure convert sedimentary rock to metamorphic rock.
Basic characteristics of soils
Published in Jonathan Knappett, R. F. Craig, Craig’s Soil Mechanics, 2019
Jonathan Knappett, R. F. Craig
To the civil engineer, soil is any uncemented or weakly cemented accumulation of mineral particles formed by the weathering of rocks as part of the rock cycle (Figure 1.1), the void space between the particles containing water and/or air. Weak cementation can be due to carbonates or oxides precipitated between the particles, or due to organic matter. Subsequent deposition and compression of soils, combined with cementation between particles, transforms soils into sedimentary rocks (a process known as lithification). If the products of weathering remain at their original location they constitute a residual soil. If the products are transported and deposited in a different location they constitute a transported soil, the agents of transportation being gravity, wind, water and glaciers. During transportation, the size and shape of particles can undergo change and the particles can be sorted into specific size ranges. Particle sizes in soils can vary from over 100 mm to less than 0.001 mm. In the UK, the size ranges are described as shown in Figure 1.2. In Figure 1.2, the terms ‘clay’, ‘silt’ etc. are used to describe only the sizes of particles between specified limits. However, the same terms are also used to describe particular types of soil, classified according to their mechanical behaviour (see Section 1.5).
Sediments and Sedimentary Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
After sediment is deposited and buried, it doesn’t remain unchanged. It is altered—sometimes quickly and sometimes slowly—by a combination of chemical, physical, and biological processes, collectively termed diagenesis. Diagenesis begins as soon as sediment is deposited, but it occurs more rapidly in some sediments than in others. The process involves physical, mineralogical, and chemical changes. For example, sediment texture can change and new (secondary) minerals can form. Some iron minerals may oxidize, and thus sediment can take on a red or brown color. At the same time, organic material may completely disappear or, alternatively, if oxygen content is low, organic matter may be converted to coal or to kerogen (the first step toward petroleum). Eventually, continued diagenesis may eventually cause loose unconsolidated material to harden and become a rock. This overall process that turns loose material into a hardened rock, generally termed lithification, is gradational and has many manifestations. Consequently, distinguishing between other diagenetic processes and lithification is often arbitrary or impossible. Adding more complications, diagenesis commonly continues after lithification has occurred.
Israelite Plain, southwestern Australia, a siliciclastic, late Quaternary Coorong analogue, without dolomite
Published in Australian Journal of Earth Sciences, 2023
The Pleistocene and Holocene deposits are both calcareous quartz sand, and many of these are covered with microbial mats or microbially influenced sedimentary structures (MISSs). All are orthoquartzite in composition with carbonate in the form of molluscs, other biofragments, mud, calcrete and cement. They are universally composed of fine to medium-size quartz grains that are rounded to subangular and medium to well sorted. Finer grains are mostly angular, whereas coarser grains are generally rounded. Lithification is related to calcrete formation and, on the basis of petrography, interpreted as synsedimentary or meteoric cementation. Pleistocene sediment below calcrete is poorly cemented to unlithified, whereas Holocene loose sediment is a mixture of modern and older particles.
A mid-Permian mafic intrusion into wet marine sediments of the lower Shoalhaven Group and its significance in the volcanic history of the southern Sydney Basin
Published in Australian Journal of Earth Sciences, 2022
G. R. Bann, B. G. Jones, I. T. Graham
The small-scale soft-sediment faulting, which occurs beneath the lava sill, indicates that some of the fine-grained sedimentary layers were sufficiently lithified, or perhaps partly frozen, to fracture, while sediments elsewhere were completely unconsolidated and flowed (e.g.Figure 8d; cf. Kokelaar, 1982). Mud, clay and carbonate cement in the finer-grained silt would provide a cohesive component, possibly allowing fracturing to occur (Figure 6a). The squeezing out of the sediment beneath the tube-like structure of the sill (Figure 7e) indicates lithification was minimal. The loading from the magma has displaced incompetent sediments beneath it, as the magma flowed out and laterally away from the dyke. As the magma formed flow tubes with seemingly unmixed sides (i.e. has not bulldozed through and mixed these sediments within the magma), suggests extrusion either onto or just below the sea bed surface. The faulting and squeezing out effects are due to loading from the lava tube being emplaced at a shallow depth (Figure 7d, e). Delaney and Pollard (1982) suggested that conduits that transport lava in the later phases of most basaltic igneous activity are commonly cylindrical (tubular) rather than tabular in form. Augustithus (1978) suggested that tubes form when lava flows onto unconsolidated sediments. Campbell et al. (2001) and Carr and Jones (2001) have documented the existence of lava tubes within the Gerringong Volcanics.
Paleoshorelines and lowstand sedimentation on subtropical shelves: a case study from the Fraser Shelf, Australia
Published in Australian Journal of Earth Sciences, 2019
T. U. Passos, J. M. Webster, J. C. Braga, D. Voelker, W. Renema, R. J. Beaman, L. D. Nothdurft, G. Hinestrosa, S. Clarke, Y. Yokoyama, R. L. Barcellos, M. A. Kinsela, L. N. Nothdurft, T. Hubble
The paleobarrier (linear feature B) samples yield radiocarbon dates of 22.8 ± 0.26 ka and 20.4 ± 0.22 ka. The 22.8 ± 0.26 ka age is consistent with the sea level curve proposed by Yokoyama et al. (2018). We suggest that the paleobarrier was formed some time following the sea level fall to LGM-a, when sea level varied from ∼80 m to 115 m, including the vertical error in the sea level envelope. However, we acknowledge that the second date (20.4 ± 0.22 ka) for the paleobarrier is not consistent with the sea level curve at the time as this would imply that the carbonate sediment was produced during the full LGM (drop to LGM-b ∼118 m), when the barrier location was exposed subaerially and the coastline migrated ∼10 km to the east. The inconsistent 20.4 ka age may be associated with either sediment reworking or diagenetic alteration during lithification. Therefore, we propose that the sediments composing the paleobarrier were most likely formed during LGM-a (30–22 ka)—a long period of relative sea level stability.