Meridian Magazine article

09 May 2000

Just as a child may shake an unopened present in an attempt to discover the contents of a gift, so man must listen to the ring and vibration of our Earth in an attempt to discover its content. This is accomplished through seismology, which has become the principle method used in studying Earth's interior. Seismos is a Greek word meaning shock; akin to earthquake, shake, or violently moved. Seismology on Earth deals with the study of vibrations that are produced by earthquakes, the impact of meteorites, or artificial means such as an explosion. On these occasions, a seismograph is used to measure and record the actual movements and vibrations within the Earth and of the ground.

Types of seismic waves
(Adapted from, Beatty, 1990.)

Scientists categorize seismic movements into four types of diagnostic waves that travel at speeds ranging from 3 to 15 kilometers (1.9 to 9.4 miles) per second. Two of the waves travel around the surface of the Earth in rolling swells. The other two, Primary (P) or compression waves and Secondary (S) or shear waves, penetrate the interior of the Earth. Primary waves compress and dilate the matter they travel through (either rock or liquid) similar to sound waves. They also have the ability to move twice as fast as S waves. Secondary waves propagate through rock but are not able to travel through liquid. Both P and S waves refract or reflect at points where layers of differing physical properties meet. They also reduce speed when moving through hotter material. These changes in direction and velocity are the means of locating discontinuities.

Divisions in the Earth's Interior
(Adapted from, Beatty, 1990.)

Seismic discontinuities aid in distinguishing divisions of the Earth into inner core, outer core, D", lower mantle, transition region, upper mantle, and crust (oceanic and continental). Lateral discontinuities also have been distinguished and mapped through seismic tomography but shall not be discussed here.

• Inner core: 1.7% of the Earth's mass; depth of 5,150-6,370 kilometers (3,219 - 3,981 miles)
  The inner core is solid and unattached to the mantle, suspended in the molten outer core. It is believed to have solidified as a result of pressure-freezing which occurs to most liquids when temperature decreases or pressure increases.
• Outer core: 30.8% of Earth's mass; depth of 2,890-5,150 kilometers (1,806 - 3,219 miles)
  The outer core is a hot, electrically conducting liquid within which convective motion occurs. This conductive layer combines with Earth's rotation to create a dynamo effect that maintains a system of electrical currents known as the Earth's magnetic field. It is also responsible for the subtle jerking of Earth's rotation. This layer is not as dense as pure molten iron, which indicates the presence of lighter elements. Scientists suspect that about 10% of the layer is composed of sulfur and/or oxygen because these elements are abundant in the cosmos and dissolve readily in molten iron.
• D": 3% of Earth's mass; depth of 2,700-2,890 kilometers (1,688 - 1,806 miles)
  This layer is 200 to 300 kilometers (125 to 188 miles) thick and represents about 4% of the mantle-crust mass. Although it is often identified as part of the lower mantle, seismic discontinuities suggest the D" layer might differ chemically from the lower mantle lying above it. Scientists theorize that the material either dissolved in the core, or was able to sink through the mantle but not into the core because of its density.

• Lower mantle: 49.2% of Earth's mass; depth of 650-2,890 kilometers (406 -1,806 miles)
  The lower mantle contains 72.9% of the mantle-crust mass and is probably composed mainly of silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminum. Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites.
• Transition region: 7.5% of Earth's mass; depth of 400-650 kilometers (250-406 miles)
  The transition region or mesosphere (for middle mantle), sometimes called the fertile layer, contains 11.1% of the mantle-crust mass and is the source of basaltic magmas. It also contains calcium, aluminum, and garnet, which is a complex aluminum-bearing silicate mineral. This layer is dense when cold because of the garnet. It is buoyant when hot because these minerals melt easily to form basalt which can then rise through the upper layers as magma.
• Upper mantle: 10.3% of Earth's mass; depth of 10-400 kilometers (6 - 250 miles)
  The upper mantle contains 15.3% of the mantle-crust mass. Fragments have been excavated for our observation by eroded mountain belts and volcanic eruptions. Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 have been the primary minerals found in this way. These and other minerals are refractory and crystalline at high temperatures; therefore, most settle out of rising magma, either forming new crustal material or never leaving the mantle. Part of the upper mantle called the asthenosphere might be partially molten.
• Oceanic crust: 0.099% of Earth's mass; depth of 0-10 kilometers (0 - 6 miles)
  The oceanic crust contains 0.147% of the mantle-crust mass. The majority of the Earth's crust was made through volcanic activity. The oceanic ridge system, a 40,000-kilometer (25,000 mile) network of volcanoes, generates new oceanic crust at the rate of 17 km³ per year, covering the ocean floor with basalt. Hawaii and Iceland are two examples of the accumulation of basalt piles.
• Continental crust: 0.374% of Earth's mass; depth of 0-50 kilometers (0 - 31 miles).
  The continental crust contains 0.554% of the mantle-crust mass. This is the outer part of the Earth composed essentially of crystalline rocks. These are low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates). The crust (both oceanic and continental) is the surface of the Earth; as such, it is the coldest part of our planet. Because cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or strong layer).

Oceanic Lithosphere

The rigid, outermost layer of the Earth comprising the crust and upper mantle is called the lithosphere. New oceanic lithosphere forms through volcanism in the form of fissures at mid-ocean ridges which are cracks that encircle the globe. Heat escapes the interior as this new lithosphere emerges from below. It gradually cools, contracts and moves away from the ridge, traveling across the seafloor to subduction zones in a process called seafloor spreading. In time, older lithosphere will thicken and eventually become more dense than the mantle below, causing it to descend (subduct) back into the Earth at a steep angle, cooling the interior. Subduction is the main method of cooling the mantle below 100 kilometers (62.5 miles). If the lithosphere is young and thus hotter at a subduction zone, it will be forced back into the interior at a lesser angle.

The continental lithosphere is about 150 kilometers (93 miles) thick with a low-density crust and upper-mantle that are permanently buoyant. Continents drift laterally along the convecting system of the mantle away from hot mantle zones toward cooler ones, a process known as continental drift. Most of the continents are now sitting on or moving toward cooler parts of the mantle, with the exception of Africa. Africa was once the core of Pangaea, a supercontinent that eventually broke into today’s continents. Several hundred million years prior to the formation of Pangaea, the southern continents - Africa, South America, Australia, Antarctica, and India - were assembled together in what is called Gondwana.

Plate tectonics involves the formation, lateral movement, interaction, and destruction of the lithospheric plates. Much of Earth's internal heat is relieved through this process and many of Earth's large structural and topographic features are consequently formed. Continental rift valleys and vast plateaus of basalt are created at plate break up when magma ascends from the mantle to the ocean floor, forming new crust and separating midocean ridges. Plates collide and are destroyed as they descend at subduction zones to produce deep ocean trenches, strings of volcanoes, extensive transform faults, broad linear rises, and folded mountain belts. Earth's lithosphere presently is divided into eight large plates with about two dozen smaller ones that are drifting above the mantle at the rate of 5 to 10 centimeters (2 to 4 inches) per year. The eight large plates are the African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific, and South American plates. A few of the smaller plates are the Anatolian, Arabian, Caribbean, Cocos, Philippine, and Somali plates.

The explanation of the Channeled Scablands as the result of catastrophic glacial floods was difficult to accept because it was an exception to the general ideas of Uniformitarianism. Since the processes of glaciation and rain/snow fall fit well in the Uniformitarian framework, it is now seen as a catastrophe brought about by an unusual combination of Uniformitarian processes.

Our next topic is of much broader scope and involves processes which are thoroughly Uniformitarian, even though unsuspected as the story begins. A computer program "Plate Tectonics" is available on Macintoshes C1, C2, C3, C5, C7 and C8 at SLIC for your use in reviewing and studying this material.

Almost everyone who has looked at a world map or globe has noticed that the continents of South America and Africa seem to fit together fairly well. An even better fit is obtained if continental margins (the underwater edge of the continent) are used. Other "fits" of this sort can be assembled as well. It is difficult to believe that this is purely accidental, yet it is equally difficult to imagine and support a theory that explains it.

It is one thing to notice a possibility and another to take it seriously enough to seek evidence to support it. Alfred Wegener, a German meteorologist, decided to look for such evidence. He theorized that if South America and Africa had been joined at some earlier time, there would be similar geological formations such as mountain ranges which would extend from one continent to across the boundary. Similarly, there would be fossils of extinct animals and plants distributed across the boundary. Marks left in rocks by glaciers would indicate similar glacial flow patterns.

Wegener carried out several successful expeditions in search of such evidence. Mountain chains extending from one continent to the other were found. Fossil distribution patterns supported the idea, as did glaciation patterns. It seemed that Africa and South America had been joined. Since they are not now joined, they have drifted apart, which leads to the term "Continental Drift" as a name for the process. Wegener proposed (1912) a supercontinent named "Pangaea" which broke up to form the continents as we now see them. Wegener died in 1930 in Greenland.

While the evidence for Continental Drift is substantial, resistance to the idea was strong. This resistance is quite understandable, since it is difficult to imagine a continent moving. Not only is it a very large object, but it would have to plow through miles of solid rock at the floor of an ocean to change its position. Continental Drift was difficult to accept because it lacked a mechanism. Wegener's idea was regarded as interesting, but not accepted as a satisfactory theory.

The first evidence which eventually led to further development and acceptance of Wegener's ideas came not from research directed towards settling the issue, but rather from measurements of the magnetism of rocks. When molten rock cools near a magnet (the Earth's magnetic field), magnetic particles in the rock are trapped in the orientation imposed upon them by the magnetic field. They preserve a record of the direction of the Earth's magnetic field and thus the location of the magnetic poles at the time of solidification. When such measurements were made, it was found that either the poles were wandering erratically (and differently if data from different continents were compared) or the continents were moving. This gave support to the continental drift idea, but still a mechanism was lacking.

A glimpse of this mechanism came from studies of the magnetic characteristics of the ocean floor. During the 1950's, at least partly driven by the military need to understand the ocean as the arena of submarine warfare, intensive studies of the ocean floor were carried out. It was found that the center of each major ocean was occupied by a ridge at whose center was a valley. On each side of this, parallel stripes of rocks magnetized in opposite directions were found. The pattern of stripes on one side was the mirror image of the pattern on the other side. The reversal of the earth's magnetic field recorded in the rocks was repeated on each side of the ridge.

This led H. H. Hess (1960) to propose the idea of sea-floor spreading. Molten rock is continuously extruded and cools to form the ridge. As it solidifies, it records the magnetic field at that time. Since it spreads to each side of the ridge, each side has the same record magnetic field record (one is the mirror image of the other). Since new crust is being formed at the ridges, it must be consumed somewhere. Hess proposed that this happens at deep sea trenches, where oceanic crust "dives" under a continent.

There are three types of plate boundaries on the surface of the earth: divergent, convergent and transform. Divergent boundaries (A) are two plates which are moving away from each other leaving room for material from the mantle to seep into the space and form new sea floor. Convergent boundaries (B) are plates which are moving towards each other causing one plate to submerge beneath the other. And transform boundaries (C) are two plates which are sliding past each other without much disturbance.

The tectonic plates do not randomly drift or wander about the Earth's surface; they are driven by definite yet unseen forces. Although scientists can neither precisely describe nor fully understand the forces, most believe that the relatively shallow forces driving the lithospheric plates are coupled with forces originating much deeper in the Earth.

From seismic and other geophysical evidence and laboratory experiments, scientists generally agree with Harry Hess' theory that the plate-driving force is the slow movement of hot, softened mantle that lies below the rigid plates. This idea was first considered in the 1930s by Arthur Holmes, the English geologist who later influenced Harry Hess' thinking about seafloor spreading. Holmes speculated that the circular motion of the mantle carried the continents along in much the same way as a conveyor belt. However, at the time that Wegener proposed his theory of continental drift, most scientists still believed the Earth was a solid, motionless body. We now know better. As J. Tuzo Wilson eloquently stated in 1968, "The earth, instead of appearing as an inert statue, is a living, mobile thing." Both the Earth's surface and its interior are in motion. Below the lithospheric plates, at some depth the mantle is partially molten and can flow, albeit slowly, in response to steady forces applied for long periods of time. Just as a solid metal like steel, when exposed to heat and pressure, can be softened and take different shapes, so too can solid rock in the mantle when subjected to heat and pressure in the Earth's interior over millions of years.

Above: Conceptual drawing of assumed convection cells in the mantle (see text). Below a depth of about 700 km, the descending slab begins to soften and flow, losing its form. Below: Sketch showing convection cells commonly seen in boiling water or soup. This analogy, however, does not take into account the huge differences in the size and the flow rates of these cells.

The mobile rock beneath the rigid plates is believed to be moving in a circular manner somewhat like a pot of thick soup when heated to boiling. The heated soup rises to the surface, spreads and begins to cool, and then sinks back to the bottom of the pot where it is reheated and rises again. This cycle is repeated over and over to generate what scientists call a convection cell or convective flow. While convective flow can be observed easily in a pot of boiling soup, the idea of such a process stirring up the Earth's interior is much more difficult to grasp. While we know that convective motion in the Earth is much, much slower than that of boiling soup, many unanswered questions remain: How many convection cells exist? Where and how do they originate? What is their structure?

Convection cannot take place without a source of heat. Heat within the Earth comes from two main sources: radioactive decay and residual heat. Radioactive decay, a spontaneous process that is the basis of "isotopic clocks" used to date rocks, involves the loss of particles from the nucleus of an isotope (the parent) to form an isotope of a new element (the daughter). The radioactive decay of naturally occurring chemical elements -- most notably uranium, thorium, and potassium -- releases energy in the form of heat, which slowly migrates toward the Earth's surface. Residual heat is gravitational energy left over from the formation of the Earth -- 4.6 billion years ago -- by the "falling together" and compression of cosmic debris. How and why the escape of interior heat becomes concentrated in certain regions to form convection cells remains a mystery.

Until the 1990s, prevailing explanations about what drives plate tectonics have emphasized mantle convection, and most earth scientists believed that seafloor spreading was the primary mechanism. Cold, denser material convects downward and hotter, lighter material rises because of gravity; this movement of material is an essential part of convection. In addition to the convective forces, some geologists argue that the intrusion of magma into the spreading ridge provides an additional force (called "ridge push") to propel and maintain plate movement. Thus, subduction processes are considered to be secondary, a logical but largely passive consequence of seafloor spreading. In recent years however, the tide has turned. Most scientists now favor the notion that forces associated with subduction are more important than seafloor spreading. Professor Seiya Uyeda (Tokai University, Japan), a world-renowned expert in plate tectonics, concluded in his keynote address at a major scientific conference on subduction processes in June 1994 that "subduction . . . plays a more fundamental role than seafloor spreading in shaping the earth's surface features" and "running the plate tectonic machinery." The gravity-controlled sinking of a cold, denser oceanic slab into the subduction zone (called "slab pull") -- dragging the rest of the plate along with it -- is now considered to be the driving force of plate tectonics.

We know that forces at work deep within the Earth's interior drive plate motion, but we may never fully understand the details. At present, none of the proposed mechanisms can explain all the facets of plate movement; because these forces are buried so deeply, no mechanism can be tested directly and proven beyond reasonable doubt. The fact that the tectonic plates have moved in the past and are still moving today is beyond dispute, but the details of why and how they move will continue to challenge scientists far into the future.

We continue to hear from many people throughout the world that earthquakes are on the increase. Although it may seem that we are having more earthquakes, earthquakes of magnitude 7.0 or greater have remained fairly constant throughout this century and, according to our records, have actually seemed to decrease in recent years.

A partial explanation may lie in the fact that in the last twenty years, we have definitely had an increase in the number of earthquakes we have been able to locate each year. This is because of the tremendous increase in the number of seismograph stations in the world and the many improvements in global communications. In 1931, there were about 350 stations operating in the world; today, there are more that 4,000 stations and the data now comes in rapidly from these stations by telex, computer and satellite. This increase in the number of stations and the more timely receipt of data has allowed us and other seismological centers to locate many small earthquakes which were undetected in earlier years, and we are able to locate earthquakes more rapidly. The NEIC now locates about 12,000 to 14,000 earthquakes each year or approximately 35 per day. Also, because of the improvements in communications and the increased interest in natural disasters, the public now learns about more earthquakes.

According to long-term records (since about 1900), we expect about 18 major earthquakes (7.0 - 7.9) and one great earthquake (8.0 or above) in any given year. However, let's take a look at what has happened in the past 28 years, from 1969 through 1996.

Our records show that 1992 is the first time that we have reached or exceeded the long-term average number of major earthquakes since 1971. In 1970 and in 1971 we had 20 and 19 major earthquakes, respectively, but in other years the total was in many cases well below the 18 per year which we may expect based on the long-term average. The following is a list of major earthquakes during this period

Conclusion
Eventually the Earth will lose so much heat that its interior will stop convecting. Earthquake and volcanic activity will then cease. No new mountains will form, and the geologic cycle of mountain building, erosion, sedimentation, and soil formation will be disrupted and also will cease. Exactly how a cooled-down Earth will change surface conditions -- and whether our planet will still be habitable -- nobody knows. Fortunately, these changes will not happen for many billions of years!