Much has been written and spoken about the Mohole Project during the last several months. With the exception of our space probes few scientific endeavors have received the favorable publicity which has been accorded this project. Numerous articles have appeared and are continuing to appear in the various technical magazines as well as a great many of the news magazines and papers of the country. Most of these articles have approached the Mohole Project from the standpoint of news worthiness, but, in varying degrees, have failed to make clear the aims and objectives of the project. Most of these articles do not give much of the background nor go into the history of the project. There is far more to the story than just an effort to drill a very deep hole or to beat the Russians in the conquest of inner space. This paper will attempt to give a little history and background of the project including a summary of the experimental drilling phase which was accomplished last year. It will also attempt to outline the current status of the project as Phase II is entered, as well as give a brief summary of the Soviet efforts in this direction. In order to understand this project in proper perspective it is necessary that there be general understanding of the current concept of the earth and its crust. I am sure that most of you know that the Mohole Project is the name which has been given to the effort to drill a hole to the Moho or Mohorovicic discontinuity.
A great variety of scientific measurements indicate that the interior of the earth is divided into two main zones, the core and the mantle. The core is believed to be composed of a nickle-iron mixture and the mantle of a material similar to the mineral peridotite. Actually, no one knows. Covering the mantle is a thin outer crust having the same relation to the earth that the cover does to a golf ball. This outer crust is the only part normally seen by man. The Moho is the boundary between this crust and the mantle, so called in honor of Professor Andrija Mohorovicic, the Yugoslav scientist who defined it as the depth at which seismic waves abruptly increased in velocity. The Mohole then, will hopefully pass completely through the earth's crust and sample the mantle underneath.
Until relatively recent time the crust of the earth was believed to be the only solid part - a thin layer of slag floating precariously upon a worldwide sea of molten lava. Today this idea is largely discredited. Modern techniques of sounding the earth's depths by observation of earthquake waves give a quite different picture. The present view is that the crust rests upon relatively solid rock several hundred miles deep, called the mantle. Underneath the mantle comes the molten outer core, probably of iron and nickle, and then a solid thinner core of the same material unable to melt at the extreme pressure near the earth's center.
The crust or cover is composed of two distinct kinds of material continental rocks, which have an average thickness of about 20.5 miles; and oceanic rocks, which have an average thickness of about 5 miles. These great masses of rock appear to float in isostatic equilibrium on the surface of the mantle which behaves like a very viscous liquid. The waters of the earth naturally occupy the lowest surfaces and form the oceans, the average depth of which is approximately 2.5 miles. Fig. 1 shows a stylized section through the crust of the earth, and demonstrates the very great difference in drilling to the Moho on land as opposed to drilling to the Moho at sea. As will be noted from Fig. 1, the undersurface of the crust is believed to approximate a smoothed-out mirror image of the visible surface. This means that beneath the continental land masses and oceanic islands the depth to the Moho is large. Even beneath the ocean, this depth is in most places quite beyond the reach of present drilling techniques. However, it is possible by means of seismic surveying techniques to locate thin places in the ocean basins where the total depth to the Moho is less than 6 miles, or about 31,000 ft. Until recently cores obtained by oceanographers had penetrated less than 100 ft into the materials of the deep ocean floor. These cores reveal that sediments to that depth are composed of red clays and calcareous oozes formed since early Cretaceous times. Information about deeper layers comes almost entirely from seismic surveys. If this evidence has been correctly interpreted, there is remarkable uniformity of the layers of rock underneath the ocean. The concensus is that these layers are as shown in Fig. 2. Where the oceanic crust is thin, the soft sediments seem to average about 1,300 ft in thickness, the second layer from 3,000 to 6,000 ft in thickness, and the deep crust approximately 13,000 ft in thickness. How these layers are formed, and why their thicknesses are as they are, is a matter of considerable speculation. As a matter of fact, the nature of the Moho itself is also a subject of much scientific debate. It may be an abrupt change, or it may be a gradual one. Present seismic methods are unable to determine its depth closer than about 1,500 ft. Of course the composition of the underlying mantle itself is unknown.
Sooner or later we are always asked, "Why do you want to drill to the Moho?" I think this can best be answered by paraphrasing Sir Edmond Hillary who said, upon being asked why he desired to climb Mt. Everest, "Simply because it is there." It may seem strange and come as a surprise to many to learn that much of what we know about the earth upon which we live we have learned from astronomical observations of other planets and heavenly bodies in our solar system. It seems paradoxical that today we know more about the composition of outer space 100 miles over our head than we do the earth upon which we live, 5 miles below our feet. The overall objective of continuing scientific studies of our planet is to produce more and more evidence which will confirm or deny existing ideas and continuously increase man's confidence in his knowledge of the earth.
The scientific objectives of the Moho Project may be stated in several different ways. First, the earth scientists are not really sure what they know about-these rocks. The indirect evidence that has been so carefully gathered to support the present theories needs to be confirmed by direct measurements and examination of an actual specimen of the rock. When it is finally determined of what each layer is actually composed, the great mass of indirect evidence that has been so painstakingly and expensively amassed over a period of years will be tremendously enhanced in value, and differences in geological concepts may be resolved.
