Marine Geology: Research Beneath the Sea Marine Geology: Research Beneath the Sea

U.S. Department of the Interior/Geological Survey Marine Geology: Research Beneath the Sea Marine Geology: Research Beneath the Sea

by Sarah B. Griscom

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Applire • • - R R • " . 31214." • .1.;41111, The USGS research vessel Neecho is used for studies of nearshore and inland water (USGS photo).

Offshore, deep-water research is conducted from the USGS' RN S. P. Lee (front cover, ohoto by George W. Moore. USGS). "There is, one knows not what sweet mystery about the sea, whose gentle awful stirrings seem to speak of some hidden soul beneath." —Herman Melville '

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Imagine the ocean basins drained Scope of Marine of all their water. What would the bottom topography look like? Were Geology mountain ranges and carved canyons hidden beneath the dark The Marine Geology program of waters? How old are the rocks and the U.S. Geological Survey (USGS) sediments on the ocean floors? strives to increase our understand- Geologists in the early nineteenth ing of the geology of the lands century speculated that the ocean covered by water, just as the Sur- floors were dull expanses of mud— vey's program on land has, since featureless and flat. For centuries, 1879, worked to understand the naturalists also thought that the geology of U.S. lands ashore. oldest rocks on Earth were on the Marine geologists compile data ocean floors. They believed that about the topography or shape of the present-day ocean basins the ocean floors, the distribution formed at the very beginning of the and type of bottom sediments, the Earth's history and throughout time composition and structure of the they had slowly been filling by a underlying rocks, and the geologic constant rain of sediment from the processes that have been at work lands. Data gathered since the throughout the seafloor's history. 1930's have enabled scientists to Using this information, marine view the seafloor as relatively geologists assess the mineral re- youthful and geologically dynamic, sources of the seafloor, predict the with mountains, canyons, and other location of certain hazards, investi- topographic forms similar to those gate marine geologic processes, found on land. The seafloor is no and, in a more aesthetic sense, add more than 200 million years old—a to our overall scientific understand- "young" part of the globe's crust ing of the Earth. compared to the continents which may contain rocks nearly 20 times that age.

2 Methods and Equipment search requires vessels and so- phisticated oceanographic Used by Marine Geologists instrumentation. The USGS now uses from 12 to 15 ships of various Most of the tools and methods sizes, and from these floating used to study the ocean floors have laboratories marine geologists been invented and developed with- deploy their instruments. in the last half-century. Marine The quickest and easiest method geology, in this regard, is still a of sampling the seafloor from a ship relatively young science with many is by coring using a long metal unexplored frontiers. pipe weighted at the top. The pipe Because direct observation of is attached to a long multipurpose the seafloor is difficult and time cable and is allowed to "free-fall" consuming, virtually all marine re- into unconsolidated sediments on

Gravity corers (inset) collect samples of seafloor sediments; the cores give valuable information on the Earth's recent climatic history (photo by H. Gary Greene, USGS).

3 the bottom. Data from core samples descend beneath the water to ob- obtained by this method provide serve and sample the seafloor. A much information about the Earth's small-sized deep submersible car- recent geologic history—for ex- ries one navigator and one ample, comparisons of the relative scientist, but some larger ones dates of volcanic eruptions and operate as much as 6,000 feet periods of glaciation. Highly dis- below the ocean's surface and turbed or specially sorted layers carry a five-person crew. The deep- can document catastrophic sub- est oceanic trenches at depths ex- marine landslides initiated by ceeding 7 miles have been visited earthquakes. Hard bedrock is by these vessels. Most submer- sampled by dragging dredges sibles are equipped with cameras, along the bottom, or up submarine lights, mechanical arms for collect- cliffs or canyons to dislodge and ing samples, and can accommodate recover pieces of rock. specialized instruments for measur- Several new types of marine re- ing shear strength, inclination of search vehicles have been de- the sediment surface, and tempera- veloped that are proving to be ture of sediments. These research valuable tools for marine geologic vehicles are also used to investi- studies. They allow scientists to gate the nature of bottom

The deep submersible Alvin carries two scientists and one navigator to the seafloor. Mechanical arms dislodge samples of rock. Cameras, water samplers, and many other instruments collect an array of data (photo courtesy of Woods Hole Oceanographic Institu- tion).

4 sediments, submarine canyons and million years. These rocks are seamounts, mineral deposits on the mostly the skeletons of countless ocean floor, and submarine microscopic plankton and clay- volcanoes. sized particles. If the history of the In 1963, the National Science Earth is to be better known, it is Foundation initiated a worldwide essential to look to the sediments investigation of the seafloors called and sedimentary rocks in the the Deep Sea Drilling Project oceans where the record is more (DSDP). Using technology de- complete. Information about the veloped by the petroleum industry, ancient climates of the Earth, DSDP ships drilled and recovered oceanic current patterns, and var- extensive core sections, some over iations in volcanic activity in the a mile long, from the ocean floor. past can be found within these In certain areas of the world, some rocks. of the oldest sedimentary rocks on Geophysical surveys provide a the ocean floor record an uninter- more sophisticated means of gath- rupted rain of sediment for over 180 ering data about the ocean floor.

Long coring pipes of the Deep Sea Drilling Project could recover a complete column of sediment and rock over a mile long ( photo by H. Gary Greene, USGS). The DSDP has been succeeded by the Ocean Drilling Program.

5 Information about the nature of create a spectrum of sonar fre- sediment-covered bedrock can be quencies. The returning signals, or obtained by shipboard gravimeters, echoes, are printed on moving chart which measure the rocks' density, paper to create a graphic profile, or and by magnetometers, which cross section, revealing the measure their magnetic properties. sediment rock layers, The profiles Seismic surveys, using reflected are recorded with great clarity and sound waves, give valuable infor- in many cases show structures as mation about submarine topog- deep as 6 miles beneath the raphy and the thickness and fold- seafloor. ing and faulting of rocks that are Side-scan sonar, the latest covered with sediment. Seismic acoustic system, sends out beams surveys are particularly useful for of sound waves sideways from the locating oil and gas deposits com- ship's course to map the seabed monly found trapped in deep accu- topography in broad swaths. mulations of sedimentary rocks. Irregularities in the seafloor topog- Seismic sound waves can be raphy alter the energy in the sig- made by releasing compressed air, nal bounced back to the receiver high voltage spark, mechanical and these irregularities are used to clappers, or electronic pulse to

Seismic recorders are used to identify the depth to the ocean floor and the configuration of its sediment and rock layers. In this graphic record, the ship has just transected a submarine canyon (photo by Jeep Johnson, New York City).

6 produce an acoustic picture of the search vessels carry banks of ocean floor. This system is es- multipurpose computers. Magnetic pecially useful for mapping large and gravity data are recorded con- frontier regions, but it also can be tinuously on computer tape used to map features as small as 20 throughout the day and night. An- feet across. Intricate patterns of other type of onboard computer meandering gullies and channels system receives signals from navi- of oceanic canyon systems are gation satellites and radio beacons transformed by side-scan sonar and can locate a ship's position to into a two-dimensional format within 300 feet on the often feature- much like an aerial photograph. less expanse of open ocean. On- As in all scientific fields, corn shore computers perform statistical puters are important tools for analyses, plot maps, and transform marine geologists. All large re- seismic data into a clearer form.

SATELLITE AND RADIO POSITIONING

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TOW CABLE

15 The side-scan sonar fish sends out acoustic signals in a broad swath. Inset is a sonograph of a submarine canyon system (photo courtesy of Bonnie A. McGregor, USGS).

7 Plate Tectonics relative to each other. Over hun- dreds of millions of years, these In the 1960's the unifying theory plates have collided with each of plate tectonics was proposed to other to form deep trenches and explain many regional and global they are periodically broken along geologic phenomena, including the rift zones by processes acting drifting continents, spreading sea- deep within the Earth's mantle so floors, and the worldwide distribu that the huge fragments then tion of mountains, earthquakes, spread away from each other. and volcanoes. According to the Marine geologists are making plate tectonic model, the Earth's major contributions to this new outer crust is a mosaic of gigantic explanation of the Earth's history continental and oceanic crustal by studying the trenches and plates, all of which are in motion spreading zones, most of which lie beneath the oceans.

late Eurasian Plate

Juan de Fuca Plate

Caribbean Philippine Plate Plate

Cocos Plate Pacific Plate

Australian Plate

Antarctic Plate Antarctic Plate

Tectonic plates of the Earth (below) and idealized cross section (AB) across the continents of South America and Africa (above).

8 Resources in the Marine limit. The newly designated Realm Exclusive Economic Zone (EEZ) extends from the shoreline and reaches across broad zones of Much of the research in marine flooded continental rocks known as geology relates directly or indirect- the continental shelf, as well as ly to assessing the seabed re- deeper oceanic crust. Surprisingly, source potential. With greater de- the area of the EEZ covers 3.9 bil- mands for certain mineral re- lion acres, one-and-two-thirds sources and depleting onland times larger than the onshore area supply, the United States is looking of the United States. In this vast toward the ocean as a new frontier domain lie resources of great im- and source of these vital resources. portance to the Nation: an esti- In 1983, the United States ex- mated 35 percent of the economic- tended its exclusive mineral rights ally recoverable oil and gas yet to outward to a 200 nautical mile be found in the United States;

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The United States Exclusive Economic Zone.

9 major resources of metals like co- where thick piles of organic and balt, manganese, and nickel in sea- terrigenous sediments have accu- floor crusts, pavements, and mulated. Marine geologists have nodules; and major concentrations just learned, however, of the poten- of heavy minerals such as gold and tial for natural gas in deeper water platinum in nearshore sand bodies. basins such as the Bering Sea in Along continental margins Alaska. around the world, USGS scientists The new mineral deposits in the are discovering areas which may ocean are among the most exciting contain large amounts of petro- geologic discoveries of the past leum. Petroleum and natural gas decade. Geologists now believe are generally restricted to ancient that most of the valuable mineral basins within the continental crust deposits mined on land originate at ocean-spreading ridges. Along

Scientists inspect a just-re- covered dredge haul (below), which contains samples of volcanic basalt (above left) and a seafloor crust rich in manganese (above right) (photos courtesy of William R. Normark, USGS).

10 fractures and faults caused by spectacular, chimney-like columnal crustal spreading, molten rock vents. These dynamic geysers or rises from beneath the Earth's sur- "smokers" were photographed from face, is injected in a linear zone the submersible Alvin along the hy- along the axis of the ridge, and drothermally active Galapagos cools to create new seafloor. As the spreading ridge in 1979. Recently, sea-floor spreading continues, the USGS scientists have discovered faults also provide conduits so that these vents along other Pacific cold seawater can circulate down- spreading ridges, including the Juan ward into the hot crust. The water de Fuca Ridge off Oregon and reacts with the hot rock, leaching Washington. Analyses of photo- from it elements such as manga- graphs, samples, and other data, nese, zinc, iron, silver, copper, and some obtained by the Alvin, from these vents are giving onland cadmium in the form of metallic geologists, who study ancient "fos- sulfides. When this hot mineral- silized" spreading zones, a greater laden water reaches the seafloor, it ability to predict the location and shoots upward in a plume of extent of important mineral deposits. precipitating minerals and forms

Scattered along spreading ridges are submarine "hot springs" rich in metallic sulfides. The min- eral-laden hot water shoots upward in a plume ("smoker") from vents on the seafloor (photo by William R. Normark, USGS).

11 Predicting Effects of sand. But the construction of a jetty or breakwater interrupts the Marine Processes natural movement of sand along a If people are to live and build shore. Sand will tend to pile up on along coastlines and out into the one side of a structure but will be sea, they must understand and be completely stripped away on the able to predict the behavior of other side, exposing sea cliffs to coastal geologic processes. Natural the full vigor of the waves and pro- erosion along coasts is generally ducing a disastrous increase in the slow. Sea cliffs tend to retreat at rate of erosion. These undesirable moderate rates because they are effects, however, can be minimized protected against direct wave if the designs of such structures attack by natural bulwarks of beach are guided by detailed studies of

Erosion often drastically increases along coastlines where jetties or breakwaters have been con- structed (photo by Gary B. Griggs, University of California, Santa Cruz).

12 coastal erosion processes or if the coasts. Buildings that would stand structures are not built. well on bedrock may be shaken to Major earthquakes, devastating the ground if they are anchored in as they may be in inland areas, are artificial fills or mud. Seismic sea- even more destructive along waves (tsunamis) set off by an

Effectively predicting the effects of storms along heavily populated shores is an important part of coastal planning (photo by Asbury H. Sallenger, Jr., USGS).

13 earthquake can sweep thousands of sweeping onto the shore hun- of miles across the ocean to ex- dreds of feet above sea level. pend their destructive energies on Surveys by marine geologists at the distant shores. These tsunamis now USGS show the location of faults can be forecasted in time for pub- near the coast and nearby unstable lic warning, but local waves gen- seafloors. Information of this kind erated by submarine landslides may help identify, in advance, are a more insidious hazard . coastal regions where tsunamis are When submarine landslides are set likely to originate and can aid in motion by seismic vibrations, the coastal planners by discouraging water above them may be thrown development in areas with potential into sudden violent waves capable geologic hazards.

Nearshore submarine landslides pose a threat to coastal developers (from a drawing by Tau Rho Alpha, USGS).

14 New Frontiers that time, the need for regional and countrywide geologic studies led The U.S. Geological Survey's to the establishment of the U.S. marine program is global in its Geological Survey. Today, with scope; its research began in the growing interest from other coun- North Pacific and North Atlantic tries, cooperative marine geologic Oceans and now extends into both programs are being established polar regions, the South and South- with the United States. By combin- west Pacific Islands, off New ing the expertise of marine scien- Zealand, and the Caribbean. In tists around the world, we can many respects, the present state of expand our understanding of the knowledge of offshore regions is oceans and ultimately appraise the comparable to what was known of extent of all the resources which the Western States and territories lie beneath their surface. immediately after the Civil War. At

The USGS research vessel Kar/uk carefully navigates around an iceberg in the Arctic Ocean. Harsh conditions permit less than 2 months of research in Arctic seas each year (photo by Peter W. Barnes, USGS). 15 The USGS research vessel J. W. Powell investigating the U.S. continental margin off Florida (photo by Andrew J. Stevenson, USGS).

Selected Readings

Edgar, T. N., 1983, The marine geology Rowland, R. W., Goud, M. R.. and program of the U.S. Geological Survey: McGregor, B. A., 1983. The U.S. U.S. Geological Survey Circular 906, Exclusive Economic Zone—a summary 23 p. of its geology. exploration, and resource McGregor, B. A.. and Offield, T. W., 1983, potential: U.S. Geological Survey The Exclusive Economic Zone: An Circular 912, 29 p. exciting new frontier: U.S. Geological Survey General Interest Publication, 24 p.

This publication is one of a series of general interest publications prepared by the U.S. Geological Survey to provide information about the earth sciences, natural resources, and the environment. To obtain a catalog of additional titles in the series "Popular Publications of the U.S. Geological Survey," write: Eastern Distribution Branch or Western Distribution Branch U.S. Geological Survey U.S. Geological Survey 604 South Pickett Street Box 25286, Federal Center Alexandria, VA 22304 Denver, CO 80225

16 U.S.GOVERNMENT PRINTING OFFICE 1985.461-428/10016 Deploying a Sediment Transport System on the Outer Continental Shelf (USGS photo).

17 As the Nation's principal conservation agency, the Department of the Interior has responsibility for most of our nationally owned public lands and natural resources. This includes fostering the wisest use of our land and water resources, protecting our fish and wildlife. preserving the environmental and cultural values of our national parks and historical places, and providing for the enjoyment of life through outdoor recreation. The Department assesses our energy and mineral resources and works to assure that their development is in the best interests of all our people. The Department also has a major responsibility for American Indian reservation communities and for people who live in Island Territories under U.S. administration.