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The Earth: Surface, Structure and Age
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
Isostasy requires that below the relatively strong outer shell of the Earth, the lithosphere, there is a weak layer (or Earth-shell) which has the capacity to yield to stresses which persist for a long time. This weak zone is called the asthenosphere (Greek: a, not, and sthene, strength). It lies in the uppermost part of the mantle (Fig. 1.3) and its distinctive feature is its comparative weakness. Isostasy implies that for a land area undergoing denudation, there is a slow rise of the surface as it is lightened, with an inflow of denser material below the area. Because of the different densities (2.8 and 3.4) the removal of, say, 300 m of granitic crust will be balaced by the inflow of about 247 m of the denser material; the final ground level when isostatic adjustment is complete will thus be only 53 m lower than at first. It is thought that the height of the Himalayas, for example, has been maintained by this mechanism during the erosion of their many deep gorges, which involved the removal of great quantities of rock.
The Geoid and Oceanic Lithosphere
Published in Petr Vaníček, Nikolaos T. Christou, GEOID and Its GEOPHYSICAL INTERPRETATIONS, 2020
The Earth’s geoid represents the integrated mass distribution over the volume of the Earth. For this reason, geoid data alone cannot inform on the lateral density structure. In most instances, however, geoid anomalies are associated with other geophysical anomalies, in particular topography. Used together, these anomalies are able to constrain plausible models of the Earth internal structure. In the framework of the classical concept of isostasy, geophysical applications of satellite altimetry simultaneously use geoid and topography information. Isostasy assumes that loads at the Earth surface (or inside) are compensated by internal density variations such that, at depth, pressure is hydrostatic (cf., Chapter 7). The depth above which density variations are confined is sometimes called the compensation depth although its exact meaning is not always clear. Isostasy may be understood in terms of mass conservation, minimization of strain energy, and mechanical equilibrium. Various mechanisms are able to insure isostatic equilibrium: crustal thickening, thermal expansion or contraction of mantle rocks, thermal thinning, plate flexure, etc. Dynamic compensation is often opposed to static compensation and assigned to convection. Convective stresses produce deformation of mantle interfaces in particular of the Earth surface giving rise to geoid anomalies. Internal density variations associated with thermal anomalies produce geoid anomalies of opposite sign and of magnitude which depends in a complex manner on the mantle stratification and viscosity structure. As for static compensation, observed geoid is the net effect between these opposing anomalies.
Coastal Erosion and Shoreline Change
Published in Yeqiao Wang, Coastal and Marine Environments, 2020
Relative sea level rise results from localized vertical land movement. Tectonic land movement is a significant source of relative sea level rise. The March 2011 Japanese earthquake caused some coastal areas to instantaneously sink several feet, flooding some coastal villages now during normal daily high tides.[3] Relative sea level rise also affects deltas because as their thick sediment deposits compact, the delta surface gradually subsides. Compaction is naturally offset by continuous new sediment deposition. However, upstream dams and levees starve many deltas today, such as the Nile Delta, of essential new sediment buildup. Wetland drainage and petroleum extraction—often accompanying urban or agricultural development of deltas—accelerates compaction and is a significant cause of subsidence on the Mississippi Delta. The Mississippi Delta is sinking as much as 9 mm each year and loses about one football field of area every hour to erosion.[4] The city of New Orleans is as much as 3 m (10 ft) below sea level due to this long-term subsidence. The impact of Hurricane Katrina was more extreme as a result of the long-term subsidence. One last cause of relative sea level rise is isostasy, the uplift or subsidence of the earth’s crust in response to the addition or removal of overlying weight. The massive Pleistocene ice sheets covering North America and Eurasia depressed the crust underneath and pushed up a bulge in the crust around the edges. When the ice sheets melted, the isostatic effect reversed and is still ongoing. The land around Hudson Bay, Canada, and Scandinavia is actually rising, causing relative sea level to fall. Coasts throughout much of the United States and western Europe, located on the former ice age bulge, are now gradually sinking.[1,5] The addition of more than 100 m of sea water over the continental shelf also contributes to subsidence through hyrdroisostasy.[1]
Structure of an accreted intra-oceanic arc: potential-field model of a crustal cross-section through the Macquarie Arc, Lachlan Orogen, southeastern Australia
Published in Australian Journal of Earth Sciences, 2022
Subsequent iterative modelling showed this value to be too low because it was not possible to match the data over the negative flanks of the anomalies produced by the magnetic sources of the Macquarie Arc volcanic rocks. To allow for this, the regional magnetic field was raised to 56 117 nT for model version number 2 and maintained at this value for all subsequent versions. The regional gravity field was set to −119 µm/s2, again to allow a close fit over the Macquarie Arc volcanic rocks. The negative value, relative to the zero-isostatic residual for a uniform 2.67 g/cm3 crust, is needed to accommodate the broad extent of deep igneous bodies in the model, and presumably reflects the deviation from simple Airy isostasy resulting from Moho compensation for the anomalously dense middle crust over the Macquarie Arc (Spencer & Musgrave, 2006).
An ‘experimental’ instrument: testing the torsion balance in Britain, Canada and Australia
Published in Annals of Science, 2019
When the Dominion Observatory of Canada sent Andrew Howard (familiarly known as Joe) Miller in search of a torsion balance, then, it was because this ‘elegant and amazingly sensitive’ instrument was at the leading edge of geophysical work.19 Miller and the Dominion Observatory had become interested in the instrument for several reasons. First, observations of gravity at high latitudes in Canada’s north promised to provide improved values for the flattening of the earth at the poles. Secondly, observations to measure gravitational variation in North America were particularly desirable in order to investigate debated features of the earth’s crust. The theory of isostasy proposed that the continents were floating on the earth’s outer crust, with this outer crust reaching a stable equilibrium with the inner layers of the earth’s core at a depth of about sixty miles. Accordingly, different densities of material must lie under the earth’s land masses in order to maintain the balance. That is, the topographic features of the earth, considered in any column of above sea level, must be compensated by the densities of matter below sea level. Theoretical calculation of these variations, compared to calculations based on the opposing theory of a rigid and uniform crust supporting both continents and oceans, could be tested by a geophysical observer taking actual gravity measurements at different points on the earth’s surface and so building a picture of changes in density over large regions.
Tectonic subsidence and uplift within Canterbury Basin, South Island, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2023
Katherine Dvorak, Michelle Kominz, Martin Crundwell
Backstripping is an inverse approach designed to determine a basin's driving or tectonic subsidence (e.g. Steckler and Watts 1978; Bond et al. 1989; Sircombe and Kamp 1998). The basic assumptions of the method are that sediments and eustatic sea-level changes load the lithosphere resulting in isostatic adjustment. The isostatic response may be flexural, accounting for the rigidity of the lithosphere (e.g. Steckler et al. 1999). Alternatively, it may be Airy isostasy if the lithosphere has no lateral strength (e.g. Watts and Ryan 1976). Our data consists of 4 relatively close-proximity boreholes. Thus, we do not have the data set required for a two or three-dimensional unloading model. Application of the Airy, local isostatic model may be appropriate in this case because the overall rigidity of this portion of New Zealand has been estimated using gravity, sediment thickness, and topography data by Ji et al. (2020) at an effective elastic thickness of about 15 km, which is relatively low. We make the Airy assumption (equation 1; e.g. Kominz et al. 2008) to estimate tectonic subsidence (Y) based on observed lithology, porosity, decompacted sediment thickness (S*), sediment bulk density (ρs*) paleoenvironment (WD) eustatic sea-level change (ΔSL) and age. We use observed, lithology-dependent porosity vs. depth relations to decompact sediments as they are unloaded (elaborated below). The bulk density of sediment (ρs*) includes both the lithology-dependent grain density and the porosity of the deposits as they compact through time. The density of seawater (ρw) is taken as 1.03 g/cm3, and the density of the asthenosphere (ρa) is taken as 3.18 g/cm3 (at an asthenospheric temperature of 1300°C, which is 3.3 g/cm3 at 0°C, e.g. Mckenzie, 1978; Kominz et al. 2008).