Imaging the Sea Floor 

U.S. GEOLOGICAL SURVEY BULLETIN 2079 
Area of sea floor imaged by a single pulse from the GLORIA towed vehicle. 

FRONT COVER: GLORIA sidescan-sonar mosaic of the sea floor off Oregon and 

California. The two conspicuous linear features are the Blanco Fracture Zone and 

the Mendocino Fracture Zone, labeled on the map at right. The blank area just off 

the coast was not imaged because it is on the continental shelf, which is too shallow 

for GLORIA to be operated. The image of the land area is from Landsat 2 multispectral scanner, band 5. One inch on the image represents approximately 36 miles. 

Features shown 

on front cover 
Imaging the Sea Floor 

By John S. Schlee, Herman A. Karl, and M.E. Torresan 

U.S. GEOLOGICAL SURVEY BULLETIN 2079 

A simplified description of two methods of sonic imaging 

of the sea floor-GLORIA and Sea Beam-plus sample 

images of the sea floor off the United States in the 

Atlantic and Pacific Oceans 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1995 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director 

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government. 

Published in the Eastern Region, Reston, Va. 

Manuscript approved for publication November 29, 1993. 

Library of Congress Cataloging in Publication Data 

Schlee, John Stevens, 1928-

lmaging the sea floor I by JohnS. Schlee, Herman A. Karl, and M.E. Torresan. 

p. em. - (U.S. Geological Survey bulletin ; 2079) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2079 

I. Ocean bottom-Remote sensing. I. Karl, Herman A. II. Torresan, Michael. 

III. Title. IV. Series. 

QE75.B9 no. 2079 

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557.3 s-dc20 

[551.46'084'028] 93-50144 

CIP 
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CONTENTS 

Abstract ............................................................................................ . 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Early Sounding Methods........................................................................ 2 

New Techniques for Imaging the Sea Floor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Sea Beam.................................................................................... 2 

Sidescan Sonar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Selected Views of the Deep Sea Floor.. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. 7 

Part of the Atlantic Continental Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Monterey Canyon off California and Zhemchug Canyon off Alaska............ 12 

Cascadia Seachannel off Washington and Oregon.................................. 15 

Plate Tectonic Crustal Features .. .. . .. .. .. .. .. .. .. .. .. .. .. .. .. .... .. .. .. ... .. .. .. .. . .. . 18 

Volcanic and Landslide Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

FIGURES 

Schematic diagram of both the GLORIA and Sea Beam systems showing their towing configurations 

and the width of the swath that each system images .. .. .. .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. . .. .. .. . .. .. .. . .. . .. .. .. .. . .. . 2 

Schematic diagram of a sidescan-sonar vehicle being towed over the sea floor and the resulting 

sonograph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Map showing Exclusive Economic Zone of the United States and U.S. insular areas........................... 4 

Two high-frequency sidescan-sonar images of the shallow sea floor off Oregon showing a sunken ship 

and a sunken aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Mosaic of high-frequency sidescan-sonar images of part of the Massachusetts Bay sea floor.................. 6 

Topographic profile through the submerged part of a typical passive continental margin like that off the 

U.S. Atlantic Coast........................................................................................................... 7 

GLORIA mosaic of the continental slope and upper continental rise off the Middle Atlantic States.. . . . . . . 9 

Sea Beam bathymetric map of the lower continental slope and upper continental rise for part of the 2' ;a 

shown in figure 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

An enlarged part of the GLORIA mosaic shown in figure 7 to provide details for the same area mapped 

in figure 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

GLORIA mosaic of the sea floor off California showing the upper part of Monterey Canyon.. . . . . . . . . . . . . . . 13 

GLORIA mosaic of the continental margin in the Bering Sea off Alaska showing Zhemchug Canyon . . . . . 14 

Map showing major sea-floor features off Washington and Oregon pertaining to figures 13 and 14... ...... 16 

GLORIA mosaics of the sea floor off Oregon showing13. Cascadia Seachannel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

14. Blanco Fracture Zone................................................................................................. 19 

GLORIA mosaic showing a large part of the North Arch lava field on the sea floor off Oahu, Hawaii . ... 21 

Map showing major submarine landslides off the Hawaiian Islands................................................ 22 

GLORIA mosaic showing the coalesced Nuuanu and Wailau debris avalanches off Oahu and Molokai, 

Hawaii......................................................................................................................... 23 

m 

Imaging the Sea Floor 

By JohnS. Schlee, 1 Herman A. Karl, 2 and M.E. Torresan2 

ABSTRACT 

Between the First and Second World Wars, acoustic 

methods (sonar) were developed to provide depth soundings 

of the sea floor. However, only in the last three decades 

have technologies evolved that permit scientists to survey 

large areas of the deep sea floor efficiently and to produce 

accurate maps and images that are analogous to topographic 

maps and aerial photographs of the sea floor. In the last 10 

years, the use of computers to process the data collected by 

these technologies has greatly improved the quality of the 

resultant sea-floor maps and images. Scientists use bathymetric maps and images to help them understand the 

processes that created and shape the sea floor. Examples of 

these latest technological advances are discussed in nontechnical terms and focus on two specific mapping and 

imaging systems-the Sea Beam bathymetric mapping 

system and the GLORIA sidescan-sonar imaging system. 

Examples of the data collected by these systems are shown 

for various deep-sea areas off the United States. 

INTRODUCTION 

More than 70 percent of our planet is covered by 

water, which blocks a clear view of the sea floor. The 

largest, longest mountain range in the world-the midoceanic ridge system approximately 40,000 miles long and 

thousands of feet high-is entirely out of view, submerged 

except for a few islands like Iceland that extend above sea 

level. Very little of the deep sea floor has been seen by 

humans; the only direct observations come from a few 

thousand dives by research submarines to limited parts of 

the sea floor. Some bottom photographs and videotapes 

have been obtained by equipment dangled from surface 

1 U.S. Geological Survey, Woods Hole, MA 02543. 2 U.S. Geological Survey, Menlo Park, CA 94025. 

ships. Even though the submarine dives give a good closeup 

view (the windows are only a few feet from the bottom) of 

a small part of the sea floor, dive participants lack the broad 

panoramic views of the sea floor we take for granted on 

land. 

From inside a small research submersible like Alvin, in 

which the passenger sphere is 6 feet in diameter, two 

scientists look out through 3.5-inch-diameter portholes onto 

a totally darkened sea floor illuminated only by the lights of 

the submarine. The circle of lighted sea floor is only a few 

square yards, and the scientist can look but not touch or feel 

the world on the other side of the porthole. On land, this 

would be like trying to get an impression of a totally dark 

landscape from a car; the observer's view would be limited 

to a 3.5-inch-diameter clear area in the windshield, and only 

the headlights would illuminate the darkness. 

A panoramic view of the sea floor can be obtained by 

using remote-sensing techniques from ships; to penetrate 

the darkness of thousands of feet of water, these techniques 

use sound waves rather than light to image and make maps 

of the face of the deep sea floor. The new surveying tools 

described in the next few pages are the latest techniques to 

image the sea floor, and they offer us the first broad view of 

the deep sea floor through computer-aided technology. 

Two of the techniques described in this bulletin allow 

the scientist to construct mosaics of large areas of the sea 

floor. The mosaics produced by these techniques are similar 

to aerial photographs and topographic maps of the land 

surface. On these mosaics, we can see such features as 

volcanoes, submarine channels, and mountain ranges. The 

mosaics are important because they help the scientist 

understand the forces and processes that shape the deep sea 

floor. 

ACKNOWLEDGMENTS 

This report has been improved by the reviews, interest, 

and suggestions of Bradford Butman, David Folger, Bonnie 

McGregor, and Thomas O'Brien, all of the U.S. Geological 

Survey (USGS). Simon Cargill (USGS) aided in many ways 

to get the paper through the publication process. 
2 IMAGING THE SEA FLOOR 

EARLY SOUNDING METHODS 

Prior to the 1930's, most of the sounding of oceanic 

depths was done with a lead line; a line with a piece of lead 

attached was lowered to the sea floor, and the length of line 

was a measure of the water depth. This method was time 

consuming and laborious and yielded relatively few values. 

Between the two World Wars, scientists started using sound 

waves to make continuous profiles of water depth along the 

ship's track. Sound was put into the water, and it bounced 

back from the sea floor. The time it takes for the echo to 

return from the sea floor is proportional to the oceanic 

depth, and the resulting record gives an echogram or profile 

of the sea floor below the ship. However, a profile gives 

only a two-dimensional view (depth and a profile along the 

ship's track). No information is acquired about what the sea 

floor is like on either side of the ship's track. Whole 

canyons and mountains can be missed unless the ship passes 

directly over them. Echo sounding vastly improved our 

knowledge of the sea floor, but the true shape of many 

features could be determined only with difficulty. 

During the 1950's, Bruce Heezen and Marie Tharp 

(Heezen and others, 1959) compiled echograms covering 

thousands of miles of tracklines for the world's oceans to 

create the first diagram maps of the deep sea floor. Their 

maps clearly delineated the continuity of the enormous 

midoceanic ridge system and showed other features (seamount chains and the midocean channel) poorly known 

before their maps. 

NEW TECHNIQUES FOR IMAGING THE 

SEAFLOOR 

Within the past 20 years, new techniques have been 

created that let us directly image large areas of the sea floor. 

These techniques are illustrated by describing two specific 

systems (fig. 1). The Sea Beam system is an improved way 

to map sea-floor elevations or bathymetry; the sidescansonar system provides an acoustic image that looks like an 

aerial photograph of the sea floor. 

SEABEAM 

Sea Beam was developed by the General Instrument 

Corporation for the Royal Australian Navy; it is a computeraided improvement of the Narrow Beam Echo Sounder. The 

system was first installed aboard the French research vessel 

Jean Charcot in 1977 to provide real-time detailed bathymetric maps (Tyee, 1986). Software and hardware were 

improved during the 1980's. 

The present Sea Beam uses two groups of transducers 

in strips (9. 2 feet long) under the ship's hull parallel to and 

Figure 1. Schematic diagram of both the GLORIA and Sea 

Beam systems showing their towing configurations and the width 

of the swath that each system images. Modified from Davis and 

others (1986, fig. 1). 

at right angles to the keel. From these transducers, a sound 

(acoustic) signal is emitted; return signals are received by 

hull-mounted transducers positioned across the hull of the 

ship. The swath width across the sea floor is approximately 

0. 8 of the water depth, and thus the system is most useful in 

the deep ocean beyond the edge of the continental shelf. 

The system is able to survey an area of approximately 1 ,030 

square miles per day. The data from the computer are 

plotted as contoured swath maps shortly after acquisition. 

When the ship's track is spaced so that swaths overlap, 

depth measurements do not have to be projected between 

profiles. Experienced geologists revise the computerplotted data to construct an accurate bathymetric map of the 

sea floor. · 

These bathymetric maps are important because they 

give us an indication of how sea-floor features formed and 

the processes that have shaped the sea floor. For example, 

a block-faulted mountain range does not have the same 

appearance or topography as a chain of submarine volcanoes. Some submarine volcanoes are conical and commonly 

have a crater or depression at the top of the cone. A slump 

or landslide caused by the collapse of a submarine hillside 

can create a tongue-shaped feature. A sharply defined 

V -shaped canyon and channel system are thought to be cut 

by sediment-laden water flowing intermittently in turbidity 

currents. 
NEW TECHNIQUES FOR IMAGING THE SEA FLOOR 3 

Figure 2. Schematic diagram of a sidescan-sonar vehicle being 

towed over the sea floor (upper) and the resulting sonograph 

(lower) with images of trawl door tracks, tilefish burrow, gravel, 

and a boulder. From Able and others ( 1987). 

SIDESCAN SONAR 

Sidescan sonars look obliquely off to the side of the 

ship's track (fig. 1) and do not collect data from the sea 

floor directly beneath the ship, in contrast to conventional 

echosounders. Rather than being hull mounted like those in 

the Sea Beam system, the transducers are mounted in a 

torpedo-shaped vehicle (called a fish) towed behind the 

ship. In effect, sidescan sonar provides an image of the sea 

floor. By selecting the appropriate sidescan system, it is 

possible to obtain images of features as large as seamounts 

many miles in diameter and as small as sand ripples having 

wavelengths of only a few inches. 

Sidescan sonar provides an acoustic image or sonograph of the sea floor that is similar to an aerial photograph 

of the Earth's land surface. As the sidescan-sonar instrument is towed behind the ship along previously determined 

tracklines, the sonar continuously emits pulses of sound that 

bounce off strips or swaths of the sea floor (fig. 2). The 

width of the swaths depends upon the type of sidescan-sonar 

system and the depth to the sea floor being imaged. 

Low-frequency systems have wide swaths, and highfrequency systems have narrow swaths. When constructing 

a mosaic, tracklines are spaced such that adjacent swaths 

overlap by 10 to 20 percent. The sidescan data are processed by computers, and a digital mosaic of a chosen area of 

the sea floor is progressively built by overlapping and 

joining adjacent swaths. 

The mosaic is in shades of gray that range from very 

dark gray to nearly white; features of the sea floor are 

defined by varying energy levels of sound returned from the 

sea floor to yield an acoustic image. The darker shades 

correspond to low backscatter levels. Many complex factors 

determine how sound is backscattered and reflected from 

the sea floor. The differences in the energy of the backscattered sound are related to bottom-sediment grain size and 

the roughness, hardness, and slope of the sea floor. Consequently, interpretation of the acoustic mosaic is not as 

straightforward as interpretation of an aerial photograph. 

Other data sets (cores, sediment samples, sonic profiles, 

and bottom photographs) must be used to supplement sonar 

data so that the sonar images can be interpreted as accurately as possible (Johnson and Helferty, 1990; Gardner and 

others, 1991 ; Schlee and Robb, 1991). These other data 

provide "ground truth" for checking the interpretation of the 

sonar image. The sea-floor characteristics reflect the host of 

geologic processes that have produced the sea-floor environment. The sonographs are used to define features of the 

study area, to interpret geologic processes operating on the 

deep sea floor, and to provide information relevant to 

environmental issues. 

Scientists use a variety of sidescan-sonar systems to 

map the ocean floor. Low-frequency systems that send 

sound pulses at about 6.5 kilohertz (6.5 kHz or 6 cycles per 

second) are best suited for regional reconnaissance mapping 

of broad areas of the sea floor. Mid-range (30-kHz) and 

high-resolution (100-kHz) sidescan-sonar systems are best 

suited for most environmental monitoring purposes and for 

locating manmade objects on the sea floor. 

Reconnaissance mapping of very large areas of the sea 

floor is accomplished with the GLORIA (Geological LOng 

Range Inclined Asdic) sidescan system developed by the 

Institute of Oceanographic Sciences (lOS), a Britisn scientific governmental organization (Somers and others, 1978; 

Laughton, 1981). The GLORIA tow fish is 26 feet long and 

2 feet in diameter and weighs 2.25 tons in air. This 6.5-kHz 

system is capable of being used at speeds of up to 11 miles 

per hour and imaging swaths of sea floor as wide as 28 

miles. Since 1984, the USGS, in cooperation with lOS, has 

been using GLORIA to survey the offshore areas included 

in the U.S. Exclusive Economic Zone (EEZ). The U.S. 

EEZ was claimed by President Reagan in 1983 and consists 

of submerged areas within 200 nautical miles (230 statute 

miles) of the United States and its territories (fig. 3). 

Results from the surveys of the EEZ have been published as 

atlases of areas off our coast (Bering Sea EEZ-SCAN 

Scientific Staff, 1991: EEZ-SCAN 84 Scientific Staff, 

1986; EEZ-SCAN 85 Scientific Staff, 1987; EEZ-SCAN 87 

Scientific Staff, 1991). 

The system is ideal for constructing sonographic mosaics that show regional-scale features such as submarine 
4 IMAGING THE SEA FLOOR 

almyra Atoll 

Jarvis Island 

PACIFIC OCEAN 

Island 

American 

Samoa 

180° 150°W 120° goo 60° 

0 2000 4000 KILOMETERS 

I I I I I I 

0 1000 2000 NAUTICAL MILES 

Figure 3. Exclusive Economic Zone of the United States and U.S. insular areas. Outlines on map are approximate. Leaders from 

figure numbers indicate areas for which images are presented in this bulletin. 

fans, fracture zones, and seamounts. With the GLORIA 

system, approximately 290,000 square miles of sea floor 

can be surveyed in about 3 months. However, the system 

cannot be used in water depths shallower than 650 feet and 

cannot resolve (distinguish) features smaller than about 160 

to 500 feet. Consequently, it is not the system to use for 

monitoring small-scale physical and biological processes 

and products on the sea floor. 

The mid-frequency (30-k.Hz) sidescan systems are able 

to resolve features as small as a few tens of yards and, under 

optimum conditions, smaller. For many of these systems, 

the fish is towed close to the sea floor, and consequently 

these are called deep-towed systems. These systems typically have swath widths that range from 1,650 feet to 3 

miles, and the fish is towed at speeds between 1. 7 and 5. 7 

miles per hour, with 4 miles per hour being typical. 

, Although these systems cannot map as large an area as 

GLORIA in a day, mid-range systems, like the highfrequency systems described in the next paragraph, are also 

capable of detecting small objects on the sea floor. The 

USGS used a 30-k.Hz system, the SeaMARC 1A, to locate 

barrels of radioactive waste offshore from San Francisco 

Bay (Karl and others, 1992). 

The high-frequency (100-k.Hz) sidescan systems typically map swaths having widths of 650 to 1,300 feet (some 

can map swaths as wide as 3,300 feet) and are capable of 

resolving objects as small as a few feet. The highfrequency, shallow-water sonar systems, because of their 

ability to resolve and detect sunken objects, are often used 

to map the location of pipelines or to locate debris on the sea 

floor (fig. 4), as well as to map geologic features. In 

shallow water, the high-frequency sonars have shown 

complex alternating patterns of sediment type and bottom 

morphology. In Massachusetts Bay off Boston, for example, a sidescan mosaic of the sea floor made with a 100-k.Hz 

sidescan instrument shows areas of fine-grained sediments, 

areas of boulders, and sand-wave fields, which change over 

spatial scales of a few hundred yards or less (fig. 5). The 

spatial complexity of the sea floor revealed in this acoustic 

image would have been impossible to obtain by taking and 

analyzing individual samples of the sea floor (Butman and 

others, 1992). 
NEW TECHNIQUES FOR IMAGING THE SEA FLOOR 5 

Figure 4. Two high-frequency sidescan-sonar images of the shallow sea floor off 

Oregon showing (A) a sunken ship (courtesy of Klein Associates, Salem, N.H.) and 

(B) a sunken military aircraft PBY 4 (courtesy of Williamson and Associates, Seattle, 

Wash.). The bright band across each image is the outgoing sound pulse and marks the 

ship's track across the continental shelf. The shadow adjacent to the plane is caused by 

the tail, wings, and fuselage blocking return of a signal from the area of sea floor 

behind the plane. 
6 IMAGING THE SEA FLOOR 

Figure 5. Mosaic of high-frequency sidescan-sonar images of 

part of the Massachusetts Bay sea floor. The depth contours (in 

feet below mean low water) are superimposed; the contour 

interval is 5 feet. Numbered ticks at bottom and on left side of 

figure indicate distance in feet (1 ,000 feet between ticks). The 

north-trending stripes indicate the ship's track and are not 

removed by the processing techniques used here. The blank gray 

stripes are areas of no data return. Note that high areas of the sea 

floor are associated with a bright sonar return, indicating gravel 

and boulders, and that low areas are associated with a darker 

sonar return, indicating fine-grained sediment. Areas A and B 

considered for the outfall of treated sewage effluent from Boston 

are outlined in center of figure and along right edge. From 

Bothner and others (1992). 
SELECTED VIEWS OF THE DEEP SEA FLOOR 

SHORELINE 

t 

CONTINENTAL SHELF 

0 .....J 

~~I 

;::<l: 

5~ 2 (SCALES IN MILES) 

> 3 0 5 10 15 20 25 30 35 

HORIZONTAL SCALE 

VERTICAL EXAGGERATION 5:1 

7 

SEA LEVEL 

Figure 6. Topographic profile through the submerged part of a typical passive continental margin like that off the U.S. Atlantic Coast. 

Water depth at the shelf-slope boundary is 650 feet. 

SELECTED VIEWS OF THE DEEP SEA FLOOR 

Off most land areas like the Eastern United States, the submerged continental margin 

consists of the shelf, slope, and continental rise (fig. 6). During the GLORIA cruises, 

most areas surveyed were deepwater areas like the rise. What do the GLORIA images 

show about the deep sea floor? The images in the rest of this bulletin provide some 

examples, and one image is compared with a Sea Beam map of the same area. 
8 IMAGING THE SEA FLOOR 

PART OF THE ATLANTIC CONTINENTAL RISE 

The GLORIA mosaic in figure 7 shows the area 

seaward of the continental rise off the Middle Atlantic 

States of New Jersey, Delaware, Maryland, Virginia, and 

North Carolina in deep water beyond the continental shelf. 

The deep sea floor shown by the mosaic covers about 

49,000 square miles, roughly the area of New York State, 

from a water depth of 650 feet (edge of the continental 

shelf) to about 14,750 feet, mainly the gradually deepening 

area of the continental rise. On the image, you can see 

broad meandering valleys and channelways (serpentine, 

light-gray bands) that snake across the mosaic. They have 

been cut by sediment-laden turbidity currents that flow 

underwater intermittently down canyons and across the 

deep sea floor, carrying sand, gravel, and mud downslope 

to their ultimate resting place in broad, deep plains seaward 

of the area shown by the mosaic. These turbidity currents 

have the power to erode valleys and channels, provided the 

slope is steep enough, and to disrupt and break submarine 

transoceanic communication cables where they cross active 

submarine channels. 

If a small part of the mosaic is enlarged (figs. 8 and 9), 

you can see that some of the channels or valleys lose their 

definition once they reach the nearly flat surface of the 

continental rise (the snakelike patterns appear to end). To 

see this better, let us look at a Sea Beam map (fig. 8) that 

depicts in detail a small part of the area shown in the mosaic 

(fig. 7). The map in figure 8 shows the boundary area 

between the uppermost, nearly flat continental rise and the 

lower part of the continental slope; the surface of the 

continental slope commonly dips 4 o -6°. The lines on the 

map connect features at equal depth at an interval of 20 

meters (each meter is approximately 3 feet). You can see 

valleys coming in from the upper left where the contour 

lines are close together (continental slope). Many of the 

valleys, gently curved to straight, continue toward the 

center of the map (where contours are more widely spaced 

on the upper continental rise, a much flatter part of the sea 

floor). At least two submarine canyons (Baltimore and 

Wilmington) continue out onto the rise as meandering 

(sinuous), well-defined channels a fraction of a mile wide 

and 50-150 feet below the adjacent rise. Most of the other 

slope canyons disappear where they enter the rise (widely 

spaced contours). Even the Baltimore and Wilmington 

Canyons lose their clear outlines on the rise, as shown 

toward the right edge of the map. The amount of sediment 

flowing periodically through most of the slope canyons has 

not been sufficient to maintain the channels out on the flat 

area of the rise, and their extensions there have been 

covered by later sediment. 

The GLORIA mosaic of approximately the same area 

greatly enlarged (fig. 9) shows many of the same features as 

the Sea Beam map. Both the Baltimore and Wilmington 

Canyons are evident as linear streaks across the mosaic; 

some meandering of the channels is apparent too. As on the 

Sea Beam map, both canyons lose their outlines toward the 

right side of the mosaic. 
SELECTED VIEWS OF THE DEEP SEA FLOOR 

76° 74° 72° 70° .---.-----------~.-------------,---------------------------~--------------------------~ 

36° 

Figure 7. GLORIA mosaic of the continental slope and upper continental rise off the Middle Atlantic States of New Jersey, Delaware, 

Maryland, Virginia, and North Carolina. Areas of figures 8 and 9, showing the base of the continental slope, are indicated. Mosaic from 

Schlee and Robb (1991 , fig. 2A). 

9 
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38°00' 

0 10 MILES 

0 10 KILOMETERS 

Figure 8. Sea Beam bathymetric map of the lower continental slope and upper 

continental rise near Baltimore and Wilmington Canyons; contours of equal depth (thin 

black lines) show that water gradually deepens to the right across the upper continental 

rise (from B.A. McGregor, U.S. Geological Survey, written commun., 1990). Dark 

areas (upper left) of many canyons are the lower continental slope (major area of depth 

transition to the deep sea). Notice that Baltimore and Wilmington Canyons continue out 

onto the upper continental rise. Location of mapped area is indicated in figure 7. 

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0 10 MILES I I I I I I I I I I I 

0 10 KILOMETERS 

Figure 9. An enlargement of the GLORIA mosaic area outlined in figure 7. The same area is included in the Sea Beam bathymetric map (fig. 8) of the lower continental slope and 

upper continental rise near Baltimore and Wilmington Canyons. These canyons appear as linear streaks across the mosaic. 

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12 IMAGING THE SEA FLOOR 

MONTEREY CANYON OFF CALIFORNIA AND ZHEMCHUG CANYON OFF ALASKA 

Similar channel features are seen off the West Coast 

and Alaska. Two examples are the Monterey and Zhemchug 

Canyons. Monterey Canyon off central California (fig. 10) 

incises a narrow shelf and continues to the west and 

southwest as a tightly to broadly meandering canyon 0.8 to 

2.5 miles wide across the slope and rise. Monterey Canyon 

probably began to form during times of lower sea level, 

when shallow parts of the sea floor were exposed and 

streams flowed across what is now the continental shelf, 

cutting valleys and canyons. After the shelf and canyon 

were drowned as the sea level rose, various oceanographic 

and geologic processes continued to erode the canyon. 

Turbidity currents are among the major agents in the 

development of Monterey Canyon. At the base of the 

canyon, the sediment carried by the turbidity currents is 

deposited to form a submarine fan. 

The largest submarine canyon (by volume) in the 

world (fig. 11) incises the continental shelf and slope of the 

Bering Sea off Alaska (Carlson and Karl, 1988). Zhemchug 

Canyon has a volume of 1,391 cubic miles and appears to 

have originated by the failure of massive blocks that now lie 

at the mouth of the canyon. The immense size of the 

Zhemchug is realized by comparing its volume to the 

volumes of 17 cubic miles for Wilmington Canyon and 108 

cubic miles for Monterey Canyon. Zhemchug Canyon 

consists of two main branches that merge to form a single 

large channel. A variety of geologic and oceanographic 

processes are responsible for the origin and development of 

the canyon. Its location and shape are controlled by faults. 

Faults have produced zones of weakness that erode more 

easily than the surrounding sediments and rock strata. 

Numerous underwater landslides have been major factors in 

the excavation of Zhemchug Canyon. 

Figure 10. GLORIA mosaic of the sea floor off California .... 

showing the upper part of Monterey Canyon. The canyon 

begins or heads very close to the shore and continues for more 

than 250 miles across the ocean floor. The dimensions of 

Monterey Canyon near the shelf break are very similar to the 

dimensions (width and depth) of the Grand Canyon of the 

Colorado. The image of the land area is from Landsat 2 

multispectral scanner, band 5. 
Area of 

figure 

SELECTED VIEWS OF THE DEEP SEA FLOOR 

10 20 

10 20 30 KILOMETERS 

13 

30 MILES 
14 IMAGING THE SEA FLOOR 

58° 

Area of 

ligure"o 

10 

10 

20 

Figure 11. GLORIA mosaic of the continental margin in the 

Bering Sea off Alaska showing Zhemchug Canyon. Enormous 

submarine canyons dominate the continental margin of the 

Bering Sea. Zhemchug Canyon is over 250 miles from the 

175° 

20 30 MILES 

30 KILOMETERS 

closest shoreline. The canyon is 60 miles wide and more than 

1.5 miles deep at the edge of the continental shelf; on the basis 

of volume, it is the largest submarine canyon in the world. 
SELECTED VIEWS OF THE DEEP SEA FLOOR 15 

CASCADIA SEA CHANNEL OFF WASHINGTON AND OREGON 

Off northern California, Washington, and Oregon, 

channels issue from the mouths of some canyons and wind 

their way for many hundreds of miles across the deep sea 

floor. One such channel is Cascadia Seachannel off Washington and Oregon (figs. 12 and 13). Cascadia Seachannel 

begins at the mouth of Nitinat Canyon and gently meanders 

across the abyssal ocean floor until it makes a sharp 

right-angle bend to enter the Blanco Fracture Zone (defined 

below). It leaves the Blanco Fracture Zone to terminate on 

the Tufts Plain. The origin of large deep-sea channels is 

controversial. Turbidity currents that episodically flow 

down the length of the channels undoubtedly contributed to 

the formation and growth of these channels. 
16 IMAGING THE SEA FLOOR 

TUFTS PLAIN 

_,() 

-<~c~, ~ EXPLANATION C' 

Spreading ridges and ..0(-1.,._ 

fracture zone / !;:' 

- Seamounts 

~ Major troughs 

100 MILES 

100 KILOMETERS 

Contours in meters 

Figure 12. Major sea-floor features off Washington and Oregon, including those that are discussed in the 

text and in figures 13 and 14. Locations of areas imaged in the two figures are shown. Screened areas 

representing spreading ridges and fracture zones lack thick sediment cover, as shown on GLORIA imagery. 

Modified from Karl and others (1989, fig. 1). 
SELECTED VIEWS OF THE DEEP SEA FLOOR 

I 

50 KILOMETERS 

50 MILES 

I 

Figure 13. GLORIA mosaic of the sea floor off Oregon showing part of the Cascadia 

Seachannel. The channel originates at the base of the continental slope off Washington and 

parallels the continental slope southward as far as the Blanco Fracture Zone; there the channel 

turns west, follows the fracture zone for about 100 miles, exits, and terminates on the Tufts 

Plain. 

17 
18 IMAGING THE SEA FLOOR 

PLATE TECTONIC CRUSTAL FEATURES 

The features described thus far are products of the 

geological processes of sedimentation, deposition, and 

erosion. Other features on the sea floor seen on the 

GLORIA images are the products of volcanism and the 

processes that build mountains and cause earthquakes. The 

outermost shell of the Earth, the crust, is made up of many 

thin crustal segments called plates. The plates are constantly 

moving. Individual plates are separated from each other by 

spreading centers (where new sea floor is created), fracture 

zones (where plates move past each other), and subduction 

zones (where old sea floor is destroyed as it is pushed under 

the continents at the deep ocean trenches). 

New oceanic crust is created in the median rift valley 

of the Gorda Ridge (fig. 14) by injections of molten rock 

(magma) from a region called the mantle, deep within the 

Earth. As the magma cools, new oceanic crust forms as a 

series of ridges and valleys that are clearly seen on the 

sonograph. Successive injections of magma push new crust 

away from the rift valley as part of the process called 

sea-floor spreading, and Gorda Ridge is functionally termed 

a spreading center. Because new ocean floor is created at 

the median rift valley, ridges are progressively older farther 

away from the median rift valley and they are less pronounced because they are covered with thicker deposits of 

sediment. The area lacking thick sedimentary cover is 

detectable on GLORIA imagery, which was used to place 

the pattern for spreading ridges and fracture zones in figure 

12. Spreading centers are typically terminated by a special 

type of fault called a transform fault that trends at near-right 

angles to the spreading center. For example, the Blanco 

Fracture Zone is a transform fault at the northern terminus 

of Gorda Ridge. Earthquakes occur along fracture zones as 

crustal plates slide past each other on either side of the 

transform fault and along spreading centers during magma 

injections. 
SELECTED VIEWS OF THE DEEP SEA FLOOR 

126° 

0 25 MILES 

0 25 KILOMETERS 

EXPLANATION 

D o Median rift valley Volcano 

Ridge crest 7 _ _...,_....,,_ 1 Fracture zone 7 

Figure 14. GLORIA mosaic of the sea floor off Oregon showing the Blanco Fracture Zone at the northern terminus of 

Gorda Ridge. The Gorda Ridge, a median rift valley flanked by a series of ridges and valleys, separates two crustal plates. 

19 
20 IMAGING THE SEA FLOOR 

VOLCANIC AND LANDSLIDE FEATURES 

Some undersea volcanoes are in the middle of plates; 

in some areas, like the Hawaiian Islands, these volcanoes 

extend above sea level. The North Arch lava field, and 

associated Cretaceous ridges and seamounts, and volcanic 

fissures and vents are shown in figure 15. The total field 

covers an area of about 9, 700 square miles. The lava flows 

are extensive and relatively flat. The vents for the extensive 

flows have not been identified, but they appear to be located 

near the bases of Cretaceous ridges and isolated volcanoes 

within the volcanic field. Flows of several different ages are 

distinguished on the basis of the backscatter contrasts 

observed on the mosaic; typically, the older flows have 

lower backscatter. The flows are seen to have traveled off 

the arch, both to the north and south, in a downslope 

direction. Individual narrow flows that are seen to occupy 

topographic lows on the sea floor have traveled up to 60 

miles. More than 200 new volcanoes were discovered off 

the West Coast of the United States during the 1984 

GLORIA surveys. Most of the undersea volcanoes seen on 

the GLORIA images are not active; they are either extinct or 

dormant. 

In addition to the volcanic features, immense landslides have been discovered off the Hawaiian Islands (fig. 

16). These submarine landslides are among the largest on 

Earth and are exposed over an area slightly larger than five 

times the land area of the Hawaiian Islands; the landslides 

whose extent is partly shown in figure 17 cover 10,000 

square miles (Moore and others, 1989), an area slightly 

larger than the State of Vermont. The Hawaiian landslides 

are of two kinds-slumps, which have moved slowly and 

intermittently, and debris avalanches, which can involve the 

failure of hundreds of cubic miles of rock and sediment that 

have disintegrated into relatively small pieces (which may 

still be quite large) and have moved rapidly and catastrophically. Debris blocks range in size from Tuscaloosa Seamount, the large (greater than 12 miles across) tabular block 

shown near the center of figure 17 (extending northwest 

from lat 22°N., long 157°W.), down to small blocks of talus 

below the resolution of the GLORIA system. The relatively 

thin, irregular, downslope-trending light returns immediately offshore of the islands are from numerous submerged 

canyons. 

Figure 15. GLORIA mosaic of lavas on the sea floor north of ...,.. 

the island of Oahu, Hawaii. GLORIA sidescan-sonar surveys 

conducted in the Exclusive Economic Zone surrounding the 

Hawaiian Islands discovered the North Arch lava field, an 

extensive lava flow field north of Oahu. The volcanic field is 

on the Hawaiian Arch, a 650-foot-high flexural arch formed by 

the loading of the ocean's crust by the weight of the Hawaiian 

Islands. The region is characterized by high acoustic backscatter, and the GLORIA images are constructed so that areas of 

high and low acoustic backscatter are shown by light and dark 

tones, respectively. Lava flows, ridges, and seamounts typically appear lighter (brighter) than the sediment because they 

provide a greater acoustic backscatter. 
159° 2501JSiiDI ___ _ 

Area of 

figure 

"' D 

SELECTED VIEWS OF THE DEEP SEA FLOOR 

EXPLANATION 

Ridge, dashed where inferred 

Volcano 

Large, old or eroded or rugged seamount, 

probably Cretaceous; dashed where 

inferred 

21 
22 IMAGING THE SEA FLOOR 

~- :'-' :'._. 

·~:·:::{:)~:> .. I 

I , 

100 KILOMETERS 

I 

I 

100 MILES 

Figure 16. Major submarine landslides-consisting of slumps 

and debris avalanches-off the Hawaiian Islands. Area of figure 

17, showing Nuuanu and Wailau debris avalanches, is indicated. 

Extent of landslides is shown by dashed lines. Dot-patterned areas 

represent hummocky ground associated with debris avalanches; 

dots are more widely spaced where topography is subdued. 

Hachured lines indicate scarps associated with slumps. Thin, 

downslope-directed lines represent submarine canyons and their 

subaerial counterparts. Modified from Moore and others (1989, 

fig . 2). 

Figure 17. GLORIA mosaic showing the coalesced Nuuanu ~ 

and Wailau debris avalanches, which are submarine landslides 

north of the islands of Oahu and Molokai, Hawaii. The 

Tuscaloosa Seamount is a large debris block in the landslides. 

The lighter shades designate regions of greater acoustic backscatter, either because of a steep slope facing the incoming 

signal or because of the blocky talus of debris avalanche 

deposits. Mosaic from Moore and others (1989). Location of 

mosaicked area is shown in figure 16. 
SELECTED VIEWS OF THE DEEP SEA FLOOR 23 
24 IMAGING THE SEA FLOOR 

CONCLUSIONS 

GLORIA images show a complex series of landforms 

dominated by slumping and sedimentation in the North 

Atlantic and Pacific and by plate tectonics and volcanism in 

the actively growing part of the deep oceanic basins. Sea 

Beam maps are relatively new, but some have been published by NOAA (U.S. National Oceanic and Atmospheric 

Administration, 1991) for some areas of the continental 

slope in the Gulf of Mexico and off Washington, Virginia, 

and Alaska. With acoustic technology, geologists are learning about the 70 percent of the Earth's surface that they 

have never seen, the sea floor. Acoustic technology is 

lighting the way in the remote darkness. We hope this brief 

description of the technology and some of the sea-floor 

features will interest you to look further at the atlases and 

maps cited in this bulletin. 

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1987, Sidescan sonar as a tool for detection of demersal fish 

habitats: Fisheries Bulletin, v. 85, no. 4, p. 725-736. 

Bering Sea EEZ-SCAN Scientific Staff, 1991, Atlas of the U.S. 

Exclusive Economic Zone, Bering Sea: U.S. Geological 

Survey Miscellaneous Investigations Series I-2053, 145 p., 

scale 1:500,000. 

Bothner, M.H., Parmenter, C.M., Twichell, D.C., Polloni, C.F., 

and Knebel, H.J., 1992, A geologic map of the sea floor in 

western Massachusetts Bay, constructed from digital 

sidescan-sonar images, photography, and sediment samples: 

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Butman, Bradford, Bothner, M.H., Hathaway, J.C., Jenter, H.L., 

Knebel, H.J., Manheim, F.T., and Signell, R.P., 1992, 

Contaminant transport and accumulation in Massachusetts 

Bay and Boston Harbor: A summary of U.S. Geological 

Survey studies: U.S. Geological Survey Open-File Report 

92-202, 42 p. 

Carlson, P.R., and Karl, H.A., 1988, Development of large 

submarine canyons in the Bering Sea, indicated by morphologic, seismic, and sedimentologic characteristics: Geological Society of America Bulletin, v. 100, p. 1594--1615. 

Davis, E.E., Currie, R.G., Sawyer, B.S., and Kosalos, J.G., 

1986, The use of swath bathymetric and acoustic image 

mapping tools in marine geoscience: Marine Technology 

Society Journal, v. 20, no. 4, p. 17-27. 

EEZ-SCAN 84 Scientific Staff, 1986, Atlas of the Exclusive 

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Geological Survey Miscellaneous Investigations Series 

I-1792, 152 p., scale 1:500,000. 

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U.S. Geological Survey Miscellaneous Investigations Series 

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Gardner, J.V., Field, M.E., Lee, H., Ewards, B.E., Masson, 

D.G., Kenyon, N., and Kidd, R.B., 1991, Ground truthing 

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5955-5974. 

Heezen, B.C., Tharp, Marie, and Ewing, Maurice, 1959, The 

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Johnson, H.P., and Helferty, Maryann, 1990, The geological 

interpretation of side-scan sonar: Reviews of Geophysics, v. 

28, no. 4, p. 357-380. 

Karl, H.A., Hampton, M.A., and Kenyon, N.H., 1989, Lateral 

migration of Cascadia Channel in response to accretionary 

tectonics: Geology, v. 17, no. 2, p. 144--147. 

Karl, H.A., Schwab, W.C., Drake, D.E., and Chin, J.L., 1992, 

Detection of barrels that contain low-level radioactive waste 

in Farallon Island Radioactive Waste Dumpsite using side-· 

scan sonar and underwater-optical systems-Preliminary 

interpretation of barrel distribution: U.S. Geological Survey 

Open-File Report 92-178, 29 p. 

Laughton, A.S., 1981, The first decade of GLORIA: Journal of 

Geophysical Research, v. 86, no. B12, p. 11511-11534. 

Moore, J.G., Clague, D.A., Holcomb, R.T., Lipman, P.W., 

Normark, W.R., and Torresan, M.E., 1989, Prodigious 

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Geophysical Research, v. 94, no. B12, p. 17465-17484. 

Schlee, J.S., and Robb, J.M., 1991, Submarine processes of the 

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Geological Society of America Bulletin, v. 103, p. 

1090-1103. 

Somers, M.L., Carson, R.M., Revie, J.A., Edge, R.H., Barrow, 

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Tyee, R.C., 1986, Deep seafloor mapping systems-A review: 

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*U.S. G.P.0.:1995-387-030:66