Interior of the Earth

Seismic Discontinuities

Over the past 200 years, geologists have accumulated an impressive set of data describing the structure and morphology of the continents. In this century, we have learned much more about the features of the sea floor, and in the last half of this century, we have developed a unifying theory describing the tectonics that have produced these features. The natural sequence is to describe the tectonic processes in the ocean basins that form the features, the modification by erosion and deposition and finally the description of the geomorphic forms moving from ocean to land.

The surface of the Earth is a complex result of internal forces, the materials of construction and the physical processes acting on the surface. The basic division into continents and ocean basins results from the difference in composition of the oceanic crust and the continental crust - the rocks of the upper 5 to 70 km of the earth. The theory of plate tectonics provides a framework for understanding the forces acting on the Earth's crust, and the resultant surface features. The ocean floor has been mapped in detail that would have seemed impossible only a few decades ago. Satellite measurement of the sea surface processed with computers and side scan sonar has joined fathometer profiles in measuring ocean bathymetry.

The solid earth is divided into three principal units:

The composition and structure of the Earth has been deduced from seismic data and study of the solar system. The basic structure of the earth is a layered globe with increasing density of material inward. The core of the earth is composed of an inner solid core, and an outer liquid core. The next layer is the mantle, which forms most of the earth.

The crust is that part of the Earth from the surface to less than 70 km inward. The base of the crust is marked by a seismic event called the Mohorovicic Discontinuity (Moho). Seismic data show a number of discontinuities in seismic wave propagation in the earth's interior which occur when abrupt changes in rock density are encountered and the seismic waves are reflected and refracted. Studies of the topography, crustal thickness, and seismic wave transmission indicate that the crust is part of a rigid plate about 100 km thick (lithosphere) that includes the upper part of the mantle. This lies above a plastic or more fluid zone of the mantle called the asthenosphere. This is a low velocity zone to seismic wave transmission.

Although the crust has igneous, sedimentary, and metamorphic rocks, more than 95 percent of the rock is igneous. The crust is not a layer of uniform thickness, but is characterized by irregularity. Basically, it is thinnest beneath the oceans and thickest under continents.

Crustal Units

Crustal rocks are divided into granitic type continental crust (average density 2.8 gm/cm3 and basaltic type oceanic crust (average 3.0 gm/cm3). The continental granitic crust is 50 to 70 km thick, compared to about five km for the oceanic crust. The oceanic crust has remarkably similarity of composition and thickness in all oceans. The difference in density between the two types of crust results in a physiographic difference in elevation of the ocean basins and the continents. If we average the elevations of individual kilometer squares of the earth's surface and construct a curve of percentage of elevation, two dominant levels emerge. These correspond to the average continental platform and the ocean floor, with the boundary between them being the continental slope; extreme highs and lows are the mountains and ocean trenches.

Oceanic crust is remarkably similar in all oceans. Three layers have been measured in the ocean crust: Layer 1 is 0.1 to 1.0 kilometers of unconsolidated sediments. The second layer varies in thickness and is difficult to detect if it is thin. It averages 1.7 km in thickness and is generally assumed to be basaltic material. This layer can be traced into the abyssal hills. Layer 3 is called the is called the oceanic layer, and it typifies the oceanic crust. The average thickness is 4.9 kilometers. The widespread occurrence of layer 3 and the remarkable velocity stability in widely different parts of the ocean show that it is a characteristic feature of oceanic crust. The velocity measurements put it in a composition range between acidic granite and ultrabasic dunite, gabbro, basalts and similar materials. Whatever the composition, the uniformity throughout the world indicates similar material underlying all oceans. The variability of measured velocities is larger than experimental error and do indicate some small scale variability in this layer, but averages over large regions show little systematic difference.

On the assumption of spreading seafloor and renewal of the oceans, a model of composition of the layers can be made. At the mid-ocean ridge, splitting and spreading of the central rift valley occurs in response to rising convection in the mantle. Molten rock wells up through the fissures and forms rounded masses of pillow lava on the valley floor. The molten lava cools and solidifies to form a linear wall of volcanic basalt. The next split forms on the same line and forces the dike apart so that another feeder of magma is injected along the axis of the previous mass. These volcanic basalts form layer 2 of the oceanic crust. In time, layer 1 sediments are deposited over this material. The immediate supply of magma does not come directly from the asthenosphere but is from an intermediate chamber within the oceanic crust itself, which is refueled from the inner mantle. As plates move apart, the chamber walls are carried sideways and molten rock solidifies against the walls as they cool. Since these cool slowly, they form the coarsely crystalline rocks such as gabbros and peridotites of layer 3.

The crust probably derived from the denser mantle of the earth. As the mantle differentiated, relatively light silicon, oxygen, aluminum, potassium, sodium, calcium, carbon, nitrogen, hydrogen, helium, and lesser amounts of other elements rose to the surface to form the crust, seawater, and atmosphere. For oceanic crust, the melting of upper mantle material and extrusion at the surface of the resulting magma seems straightforward. The continental crust origin has been more complex. Continental crust did not evolve in a stable form until about 3.9 to 4.1 billion years before the present. The oldest rocks dated from northwestern Canada are about 3.8 to 3.96 billion years old. However detrital zircons found in younger rocks in Australia have been dated at 4.1 to 4.2 billion years. Only when the heat from radioactivity and meteorite impacts declined to the point where large chunks of crustal rock could be preserved did we develop a stable continental crust.

Mantle Convection

Convection in the mantle is believed to be the fundamental process responsible for tectonic motion, but hot spot plumes discussed next, are also postulated as having a role. The nature of convection and where it occurs may be within the mantle and carry the plates on a conveyor belt, or the plates themselves may be an active part of the convecting process, not passive passengers. In the conveyor belt model, the rising limbs of the convecting cells in the mantle determine the positions of oceanic ridges. The convecting mantle would cause the lithosphere to split, and the moving mantle would carry the lithosphere laterally toward the subduction zone. The descending cell would mark the location of the trench and would drag the lithosphere down into the mantle. Movements in the asthenosphere were thought to be strongly coupled to the lithosphere.

Another model considers the plates themselves to be active participants in the convection process. The lithosphere is considered to be the cold upper layer of the convection cell. Because of its greater density, the lithosphere tends to sink. Subduction occurs not because the plate is pulled down by descending mantle, but because the plate is the dense sinking limb of the cell, driven by slab-pull.

The size and shape of the convection cells within the mantle are also a matter of considerable debate. The principle models are:

In the first model, two separate convecting layers of mantle are envisioned. The upper layer is confined largely to the asthenosphere and lithosphere. Slabs of lithosphere are known to penetrate to depths of 700 km before becoming undetectable by their seismic activity, so a significant part of the upper mantle must be involved in convection. Below 700 km, the mantle is thought to convect independently of the upper mantle and probably at a very slow rate.

The second model considers convection to involve the entire mantle. Heat for whole mantle convection is supplied from the outer core. The major difference in these models is the size of the convection cells. Another variety of mantle convection involves the rise of jet-like plumes of low-density material from the core-mantle boundary region.