Structure of the Oceanic Lithosphere, Plate Tectonics and Seafloor Spreading, Seafloor Topography

The Earth accreted from the solar nebula ~4.5 By ago, forming a more or less homogeneous body with two sources of heat: (1) energy associated with impacts during the early history of the Earth and (2) continuing decay of radioactive elements (U, Th, K). Strong convection coupled with cooling at surface gave rise to a chemically stratified body with a layered structure.

Seismic layering, marked by sharp transitions in the velocity at which seismic waves propagate (the Mohorovicic discontinuity at the crust/mantle boundary and the Gutenberg discontinuity at the mantle/core boundary), can be used to recognize the crust (thin beneath oceans (~6 km), thick beneath continents (~30-45 km), mantle, and core:

Figure 1-1. (from (41))

This layering is inferred to reflect a chemical layering:

• crust: average composition grandiorite on continents, basalt in ocean
• mantle: Fe-Mg-Si
• core: Fe-Ni-S

Nature of layering: seismic waves, either natural (earthquake generated) or man-made (explosions), propagate through the earth. These waves are of two kinds: compressional (P waves) and shear (S waves). P wave velocity and density within the Earth is shown in this figure:

Figure 1-2. The Preliminary Reference Earth Model (PREM2), calculated from the prem2.m MATLAB script in the Coral software package.

P waves propogate at higher speed than S waves, thus the difference in arrival time is a measure of distance from the source and the basis for earthquake location. As seimic waves propagate they are refracted by density contrasts; in addition S waves cannot propagate through the liquid outer core. Refraction by major layers gives rise to a shadow for P waves; Refraction and inability to propagate through liquid gives rise to a larger shadow for S waves:

Figure 1-3. Selected ray paths for P-waves passing through the earth. The P and S shadow zones are marked on the right. The dashed ray paths represent weak P waves detected within the shadow zone which provide the principal evidence for a narrow zone of high P velocity in the core.
From (2), Figure 2-1, pg 14.

Within the crust and upper mantle there is also an inferred mechanical layering distinguishing lithosphere, asthenosphere and mesosphere. Lithosphere (from Greek, for stone): cool (<1200°C) and mechanically rigid. Asthenosphere (from Greek, for weak or soft): ~600 km thick, capable of plastic deformation, containing perhaps a few % melt.

The term lithosphere is used in a variety of ways: one can distinguish a mechanical lithosphere, the asthenospheric boundary representing the transition from brittle to ductile behavior, and a thermal lithosphere whose asthenospheric boundary represents a particular isotherm, usually equated with the solidus or melting temperature of the rock making up the mantle. The mechanical lithosphere is ~60 km thick, while the thermal lithosphere is ~120 km thick. There is an extended discussion of this and related semantics in reference (29). We will generally use the term lithosphere to mean the thermal lithosphere, reflecting our particular emphasis on heat transfer.

II. Gravity, Isostasy

A second tool providing information on subsurface structure is the gravity field. Gravitational acceleration near the surface of the earth is order 9.8 m s. Of interest are small deviations from this average. The unit for these deviations is the gal (honoring Galileo); 1 gal = 1 cm s, approximately 1 part in 1000. The practical unit is the mgal; 0.01 mgal deviations (1 part in 10⁸ can be measured with modern instrumentation). Gravity varies with:

1. latitude for 2 reasons: a) Earth is oblate sphere, bulges at equator about 20 km, so differences to center of mass; b) but more important, component of centrifugal force opposing gravity, strongest at equator. The net effect is that gravity is smaller at equator than pole. The variation is ~0.81 mgal/km of N-S travel. By convention apply latitude correction to move measurement onto standard geoid, level of constant gravity.
   
2. elevation of measurement station. This also changes the distance from the center of mass. Applying a free air correction to sea level yields the free air anomaly. The magnitude of the correction is ~0.31 mgal/m.
   
3. density of materials affecting measurement. Air does not fill the space between the measurement station and sea level. To compensate one applies the Bouguer slab correction replacing air with inferred material filling space between measurement station and standard geoid. For density 2.67 g/cc the correction is ~0.11 mgal/m.

The remaining variations are termed the Bouguer anomaly:

• when negative, indicates an excess of less dense material beneath measurement station.
• when positive, indicates an excess of more dense material.

Figure 1-4. from (30)

The explanation lies in concept of isotasy or isostatic compensation. Crustal blocks are floating on more viscous material beneath. Continental rocks are less dense, float higher and have deeper roots (the minimum in the Bouguer anomaly reflecting this excess of light material). In contrast the oceanic crust is more dense, thinner, and floats at lower elevation (and so is covered with water!).

The response time for isostatic compensation is slow (for example once load of an ice cap is removed, underlying crust rebounds upwards); these response times help characterize subsurface properties. See for example see W.R. Peltier, Postglacial variations in the level of the sea: Implications for climate dynamics and solid-earth geophysics, Reviews of Geophysics. 1998; 36(4): 603-689.

III. Plate Tectonics

Movement of earth's surface reflects large scale convection of upper mantle. Compare and contrast these two views, the first very schematic, the second showing a cross section illustrating active and passive continental margins and the arc environment behind the zone of subduction:

Figure 1-5. Motion of the lithospheric plates in the context of convection of the upper mantle. Note boundaries: constructive with upwelling mantle and generation of the lithospheric plate; destructive with downwelling plate closing the convection cell; transform offsets.

From (2), Figure 5-3, pg 135.

Figure 1-6. From the right hand side of the figure: Genesis of the lithosphere at a mid-ocean ridge. Spreading causes the ocean basin to grow. A passive margin between the ocean crust and continental margin is shown, i.e., the continent is also moving away from the ridge (the eastern seaboard of the United States is an example). In the ocean basin to the left, the plate is subducted back into the asthenosphere. This is recognized as a trench on the seafloor. Frictional heating along the plate boundary gives rise to arc volcanism (for example the islands of Japan), and a secondary mantle circulation causing spreading in the back arc basin. From (3), Figure 19-3, pg 461.

Plate tectonics is a modern idea, taking hold as the central paradigm of earth sciences in the late 1960s. Lines of evidence developed at that time included:

Earthquake Distribution. Relative motion at plate boundaries is origin of major earthquakes, thus expect the locations of these earthquakes to mark plate boundaries:

Figure 1-7. (slide 17 of the NGDC Relief Globe Slides). Red dots are earthquakes of magnitude >5 since 1980; yellow lines are plate boundaries.

At zones of subduction, the depth to earthquake epicenters marks the dip of the plate returning to the asthenosphere:

Figure 1-8. From (5).

Magnetic lineations. The polarity of the Earth's magnetic field undergoes reversals on time scales of 10 to 10 years. (For more information on the origin of the Earth's magnetic field and the present state of understanding of the origin of reversals can be found in Physics Today, 49(1), 17-19.) The polarity of Earth's magnetic field is recorded in volcanic rocks as cool beneath Curie temperature:

Figure 1-9a,b. Production of magnetic anomaly patterns. From (41)

For example these magnetic lineations off our coast result from spreading away from the Juan de Fuca and Gorda Ridges during alternating periods of normal and reversed polarity:

Figure 1-10. From (41).

The present configuration of the Earth's surface involves seven major plates and about 20 in all:

Figure 1-11. From (41)

Variable rates of spreading are evident in the map of crustal age below. Spreading rate ranges from 2 cm/y on Mid-Atlantic Ridge to ~20 cm/y on fastest parts of East Pacific Rise.

Figure 1-12. This rendering and the data set available from Muller

Topography of the Ocean Basins

Hypsometric Curve

As the lithosphere ages it cools and contracts. On ocean basin scales this cooling is principally by conduction and oceanic topography can be modelled, as we shall see, by conductive cooling of the lithosphere such that:

In this model, seafloor depth depends mainly on seafloor age, due to thermal cooling and subsidence. However considerable additional topography is superimposed. Consider these three examples from a range of spreading rates, which help illustrate the role of subsidence, magmatic supply and tectonics.

• Fast Spreading--Southern East Pacific Rise from 13 to 23°S

  

  

• Medium Spreading--Juan de Fuca Ridge near 48°N

  

  

• Slow Spreading--Mid Atlantic Ridge near 22°N

  

  

The Oceanic Crust

• pillow and sheet lavas are volcanic products (rapidly cooled)
• gabbros are plutonic products (slowly cooled)
• dikes feeder zones for magma to rise to surface.

Figure 1-14. From (2), Figure 7-4, pg 211.