Marine Geology: Research Beneath the Sea Marine Geology: Research Beneath the Sea
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U.S. Department of the Interior/Geological Survey Marine Geology: Research Beneath the Sea Marine Geology: Research Beneath the Sea by Sarah B. Griscom a -No i;i4 9 1 66 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 ' A 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 •• • 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.