Читать книгу Geology and Mineralogy of Gemstones - Karl Wallace, David Turner R. - Страница 27

Our solid planet is not homogeneous but is made up of a number of very distinct layers (Figure 2.2). These layers, from exterior to interior, are:

 Crust. The Earth’s crust is the uppermost layer. It represents ~1% of the total volume and generally consists of continental and oceanic crust. This uppermost layer is separated into a number of rigid sections, known as tectonic plates. Continental crust and oceanic crust have different overall compositions; continental crust has a higher silicon (Si) content but is more heterogeneous while oceanic crust has higher iron (Fe) content and is more homogeneous. Continental crust also tends to be much thicker than oceanic crust. The thickness of the continental crust is generally ~40 km, but reaches up to 60 km in mountainous areas and near 90 km is select locations. In contrast, oceanic crust is generally only ~10 km in thickness.

 Mantle. The mantle comprises ~85% of the Earth’s volume and is hot and relatively viscous. The mantle is in continual motion with hot mantle material rising from depth and cooler upper mantle material sinking to the lower areas. These motions are called convection currents and may in part help drive the motion of the lithospheric plates.Figure 2.1 The Earth System. A schematic model of the Earth as a series of integrated systems. Drawn by G. Lascu.Figure 2.2 Simplified schematic of the Earth’s principal internal geological structure.The mantle is often divided into Upper Mantle, a Transition Zone, and the Lower Mantle. The Upper Mantle is distinct from the overlying crust and this boundary between layers is marked by a zone termed the Mohorovicic Discontinuity, defined by a distinct change in physical properties and geochemical composition, that occurs at a depth of ~7 km depending on local and regional conditions. The upper mantle material acts as a relatively soft, lubricating layer over which the crustal plates move.Greater depths and higher temperatures lead to other structural and mineralogical changes in the heterogeneous mantle, which give rise to a broad Transition Zone from ~410 to ~660 km depth, and the Lower Mantle from ~660 to ~2900 km depth.

 Core. The core sits interior to the mantle and is divided into two parts. The outer core from ~2900 to 5150 km is molten metal while the inner core from ~5150 to 6370 km is solid and also of metallic composition. Both the inner and outer core regions have compositions dominated by iron and nickel.

The upper part of Earth’s structure can also divided based on rheological properties and how the material responds under tectonic forces. The lithosphere comprises the more rigid portion and consists of the crust and parts of the upper mantle that respond to tectonic forces in a predominantly cohesive and brittle manner. The asthenosphere exists within the mantle only and behaves in a ductile manner. The transition between the lithosphere and asthenosphere is dependent on local conditions; it can be as shallow as 50 km near spreading oceanic ridges or as deep as 250 km under old and stable continental plates, often termed cratons. At deeper regions, the asthenosphere transitions to the mesosphere, a more rigid zone within the lower mantle.

The theory of plate tectonics is sometimes called the Grand Unifying Theory of geology and began its formal development in the early twentieth century (Wegener 1912). It explains many of the geological phenomena that had puzzled scientists for so many years, such as the processes that build mountains and the patterns of distribution of earthquakes and volcanoes. The theory describes how the lithospheric plates and the continents they contain are pushed and pulled around the surface of the Earth. The surface of the Earth resembles a fractured eggshell with each fragment termed a plate (Figure 2.3). Continental plates and oceanic plates are the two basic plate types. Continental plates are generally composed of many different rock types of diverse ages. Oceanic plates form at spreading ridges within an ocean basin and are overall higher in density than continental plates.

In general, most geological activity (such as earthquakes, volcanic activity, and mountain building) that affects the surface of the Earth occurs at the plate boundaries while the central portions of the plates tend to be quite “stable” and experience little larger scale geological activity. The three main types of plate boundaries are convergent, divergent, and transform (Figure 2.4).

Divergent plate boundaries occur where tectonic plates move away from each other and new crust is produced. An example of a constructive divergent plate boundary is the Mid‐Atlantic Ridge. This geological feature has been widening the Atlantic Ocean at an average rate of about 2.5 cm per year (this rate varies along its length). It is notable in that it is also one of the few ocean ridges that can be observed on land in Iceland. Divergent boundaries can also form within a continental plate (such as the East Africa Rift) and may ultimately form a new ocean basin.

Convergent boundaries occur where two plates move toward each other and collide (Figure 2.5). The Himalayan mountain range was formed when two continental plates, the Indian and Eurasian plates, collided (continental–continental collision). Intense pressures and temperatures are produced during these collisions and the rocks within the plates are affected accordingly. When two oceanic plates or an oceanic plate and a continental plate collide, one plate is pushed under or subducted below the other. Chains of inland volcanoes or volcanic islands often develop above and parallel to these zones of subduction, such as what is seen today along the Japanese island arc system and Cascade volcanic arc. In the case of oceanic–continental plate collision, the oceanic plate is always subducted below the continental plate because oceanic crust is denser than continental material.

Figure 2.3 The major tectonic plates of the Earth, their boundaries, and relative motions (red arrows). U.S. Geological Survey / Public domain.

Figure 2.4 This cross‐section illustrates the main types of plate boundaries: convergent, divergent, and transform. U.S. Geological Survey / Public domain.

Figure 2.5 Schematic diagrams of (a) continental–continental convergent plate boundary, (b) oceanic–oceanic convergence, and (c) oceanic–continental plate convergence. U.S. Geological Survey / Public domain.

Transform boundaries are characterized by the plates moving past each other without the creation or significant destruction of crustal material (Figure 2.6). The most famous transform plate boundary is coincident with the feature known as the San Andreas Fault where the North American Plate is moving past the Pacific Plate.

The thickness of the world’s crust varies in time and space as geological processes incessantly march forward. Figure 2.7, from the U.S. Geological Survey, is a map of the world with the thickness of the crust mapped out – each line traces areas of equal thickness (measured in kilometers) with the colors corresponding to altitude of the Earth's surface. Roughly, the continents and their margins are outlined by the 30 km contour. Continental crust with a thickness greater than 50 km is rare and accounts for less than 10% of the continental crust. Total continental crust thickness is important for understanding the distribution of certain gem deposits, such as diamondiferous kimberlites or high‐grade metamorphic terranes formed via continental–continental collisions.

Figure 2.6 Map and aerial view of the San Andreas Fault that cuts across the Carrizo Plain. The San Andreas Fault is well known for being an active geological structure that is easily observed at the surface. The many fracture zones provide relief from tectonic stresses applied to the Pacific Plate in this complex area. The thicker purple lines delineate extensional environments and volcanism at the sea floor (e.g., Juan de Fuca Ridge). U.S. Geological Survey / Public domain.

Figure 2.7 Thickness contour map of the of the Earth's crust, developed from the CRUST 5.1 model with a contour interval of 10 km with greater detail on the continents above 45 km thickness. Colors indicate surface elevation above average sea level (blue = below sea level, green = low lying, yellow = mid elevation, brown = high elevation). U.S. Geological Survey / Public domain.