Tectonic cracks

Both tectonic cracks and thermally produced Chaotic terrain are caused by tides, and both involve linkage of the underlying ocean to the surface. The darkening associated with each of these formation processes probably involves substances from the ocean, which makes the features visible even at the low resolution of global images like Figure 5. The cracks are caused by tidal stress and the chaos is probably caused by melt-through of patches of crust by tidal heat. 

Some trace metals are transported into the ocean as a component of hydrothermal fluids. This process is discussed further in Chapter 19- To briefly summarize, hydrothermal fluids are produced when seawater penetrates into cracks in the crust near tectonic spreading centers. The seawater is heated as it comes into contact with magma. The hot seawater leaches a number of trace metals from the magma. The resulting hydrothermal fluids are acidic and do not contain O2, so most of the metals are present in reduced form. Because of their high temperatures, the hydrothermal fluids have a lower density than cold seawater. Their increased buoyancy causes them to rise until they are emitted into the deep sea. Admixture with cold, oxic, alkaline seawater causes the hydrothermal metals to undergo various redox and precipitation reactions. 

As shown in Table 11.1, hydrothermal emissions are a major source of soluble iron, manganese, and zinc and a minor source of aluminum, cobalt, copper, and lead. Other elements with significant hydrothermal inputs include lithium, rubidium, cesium, and potassium. Considerable uncertainty also surroimds these flux estimates because they are the result of extrapolations from measurements made at a small number of hydrothermal systems at single points in time. These fluxes appear to vary significantly over short time scales as tectonic activity abruptly opens and closes cracks in the oceanic crust. 

Sometimes tectonic stresses pulled the forming continents apart, creating cracks hundreds of miles or kilometers long in the crust. These cracks quickly filled with upwellingmagma to form dikes of solid rock. There are so many of these dikes of black rock that they are collectively called dike swarms. 

Tidal theory applied to observations of tectonic and thermal features suggests that the ice is significantly thinner, so that the ocean can connect with the surface through cracks and melt sites. Tidal stress can probably open cracks no deeper than a few km, so we infer that the ice is probably less than 10 km thick. 

This tidal tectonic process was described by Greenberg et al. (1998). A key point is that the magnitude of the tidal tensile stress is probably not adequate to open cracks to a depth greater than a few km, because the overburden compression would be too great at depth. Therefore for this ridge-building process to work, the ice must be fairly thin. 

Strike-slip displacement, in which one crustal plate shears past another, is relatively rare in the solar system except in a few places. On Earth, with its global tectonics, strike-slip between plates is well known. The San Andreas fault in California is a famous example. Similar strike-slip is ubiquitous on Europa. Faults there include some longer than the San Andreas, with Strike-slip displacement of tens of km. Strike-slip displacement on Europa is probably driven by diurnal tides, and thus by the Laplace resonance. Over the course of a day, the tidal stress follows a sequence that can open a crack, shear it, then close it. Then, stress that would reverse the shear is resisted while the crack is closed. This process repeats on a daily basis. In this process, which is analogous to walking, one plate of crust can shear past another, with Strike-slip displacement visible along the boundary. This process has been described in detail by Hoppa et al. (1999b). 

Garagash, l.A. 1983. On brittle failure of elastic bodies with large number of cracks. In Mechanics of Tectonic Processes. Alma-Ata Nauka. 

In a nutshell, plate tectonics is based on recent evidence which suggests that the crust of the Earth is cracked into six large plates with a number of smaller plates of different sizes. These plates compose the outer solid rigid layer of the Earth (the lithosphere) and they move across the Earth s surface as giant slabs of rock floating on the semi-fluid layer of the mande. They are probably carried by giant convection currents in the mantle. 

Earthquakes mostly occur along fault lines, which are cracks crust, formed from the movement of two tectonic plates. 

Unlike dislocation models for tsunami generation in which slip is prescribed, the slip distribution associated with crack models is calculated and physically consistent with the applied tectonic stresses and frictional conditions on the fault. For dip-slip faults of geometry shown in Fig. 5, the slip gradient in the dip direction 5 ( ) is given... 

Alternatively Bowers and Hudson (1999) define the total scalar moment Mq) as the sum of the isotropic moment and the largest deviatoric eigenvalue, which provides an estimate equivalent to Mo for double couples, but provides better estimates for sources that deviate from a simple double couple for tectonic earthquakes such as the CLVD and tensile crack. 

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