Tracy Vallier
1254

Hess Rise


Hess Rise, a submerged mountain range in the north central Pacific Ocean, is one of the largest oceanic plateaus in the world's oceans. The geology of Hess Rise is similar to that of several other oceanic plateaus, particularly those in the Pacific and Indian oceans. This entirely submerged feature was named for Professor Harry Hess, a prominent geologist who discovered many sea floor features during World War II while he served as an officer on U.S. Navy ships in the Pacific Ocean. Professor Hess studied echo sounder records and became interested in the vast array of sea floor features that appeared on those records.


Location and Description

Hess Rise is a spectacular submerged mountain range. The plateau has an anchor, or inverted T, shape with the shank of the anchor, nearly 1000 kilometers (600 miles) long, elongated to the NNW at about 327° and the arms, over 1000 kilometers (600 miles) long, trend nearly east-west. It is bounded by latitudes 33° N. to 42° N. and longitudes 174° W. to 173° E. and has a surface area of about 400,000 square kilometers (144,000 square miles). Less than 100 kilometers (60 miles) west of the plateau's northwestern ridge, the Emperor Seamounts leave a mountainous trail as they trek northward with Hess Rise on the Pacific plate. The plateau is truncated along its south side by the longest fracture zone (scar of an ancient transform fault) in the North Pacific Ocean, the Mendocino Fracture Zone. The east end of this fracture zone is colliding with northern California at Cape Mendocino.

Bathymetric charts of Hess Rise show that the plateau rises from an ocean floor depth of about 5200 meters (17,000 feet) to only 119 meters (390 feet) below sea level at Mellish Bank. Mellish Bank, previously thought to be a high point on the southern edge of the plateau, has been searched for by ships during the past two decades, but its existence has not been confirmed. One pinnacle on the southern part of the plateau, however, does rise to a depth of only 1400 meters (4600 feet). If Mellish Bank is not included because of the premise that it may not exist, then the relief of the plateau, from its lowest to highest point, is still about 3800 meters (approximately 12,500 feet). For comparison purposes, if Hess Rise were on land it would occupy most of the area that is included in the states of Washington and Oregon. Its relief is similar to that of the Cascade Mountains in Oregon and Washington. For example, Mt. Adams, one of the Cascade volcanoes located in south central Washington, rises to a height of about 3750 meters (12,307 feet) above sea level.


Composition

Hess Rise is a sediment-covered plateau with a volcanic base. It is buoyed up by a much thicker crust than most other crust of the ocean basins. Hess Rise was born about 100 to 110 million years ago along an ancient oceanic spreading center that formed a boundary between the Pacific and Farallon lithospheric plates. It originated near the equator in the South Pacific and subsequently traveled northward on the Pacific plate to its present position. The plateau cooled and subsided while it traveled northward. Pelagic sediments rained down on the feature and draped over it like a blanket. These sediments consisted mostly of calcareous nannoplankton, like coccolithophores, and foraminifera, radiolaria, and clay.

Marine geophysical surveys, completed on the oceanic plateau in the middle 1970s, collected bathymetric, stratigraphic, magnetic, and gravity data. Basically, these geophysical studies established that Hess Rise is a large feature and that it has a sediment section overlying a volcanic pedestal. Based in part by the results from these geophysical surveys, four sites were drilled and cored in the middle to late 1970s by the GLOMAR CHALLENGER as part of the Deep Sea Drilling Project. One site, 464, was located on the northern part, site 310 was located on the central part, and two sites, 465 and 466, were drilled and cored on the southern arm. Igneous rocks of the volcanic pedestal, recovered beneath sedimentary rocks at sites 464 and 465, consisted of basalt and trachyte respectively.


Geologic History

From oldest to youngest, and lowest to highest, the rock and sediment section on Hess Rise records a simple and intriguing geologic history. The stratigraphic, or layered, succession has 110 to 100 million year old volcanic rocks overlain by Cretaceous to Recent limestone, chert, chalk, calcareous ooze, and brown clay. The volcanic rocks were erupted onto the sea floor, in places building large volcanic cones that grew upward through the column of sea water to become islands. Subsequently, erosion occurred as the plateau began its long northward trip on the back of the Pacific plate, from near the equator to its present position. The islands subsided until all of Hess Rise was below sea level. As the plateau passed slowly through the equatorial zone, where the divergence of ocean currents created zones of upwelling with attendant high biologic productivity, pelagic sediments were deposited. These pelagic sediments included thick successions of foraminifera and calcareous nannoplankton that formed calcareous ooze, chalk, and ultimately limestone. Siliceous radiolaria became porcellanite and chert. Those parts of the plateau that subsided below the calcium carbonate compensation depth, a level in the ocean below which calcareous materials dissolve, are now covered by pelagic brown clay. The brown clay, here and elsewhere in most parts of the oceans, is derived from the constant deluge of very fine-grained sediments that are dropped into the ocean from winds. Most of these fine-grained particles were eroded from the world's great deserts, and some were blasted into the earth's jet streams by far off volcanic eruptions.

The Cretaceous-Tertiary boundary was recovered in a Deep Sea Drilling Project core from Hess Rise. Studies of the boundary indicate that a significant decrease in both surface temperature and biologic productivity occurred at the end of the Cretaceous period. Many species became extinct. Furthermore, an abrupt increase in transition metals and iridium suggests that an extraterrestrial source was responsible for these characteristics. Although Hess Rise is far removed from the great meteor impact that closed the Cretaceous period, the existence of this small layer of debris at the boundary does indicate that dust from the impact site in Mexico reached most, and perhaps all, of the world through its winds. 


Future Possibilities

If the Pacific plate continues to move northwestward, like it has for the past approximately 42 million years, then Hess Rise will someday reach the Kuril Trench and its adjacent subduction zone. The plateau will break apart and most of it will probably be subducted into the earth's mantle to be recycled. Perhaps the high parts will not be subducted, but will be zippered to the front of the Kuril Islands. When Hess Rise reaches the Kuril Trench, its rocks and sediments will have passively recorded about 150 million years of earth history.


Further Reading

Larson, Roger, et al. Initial Reports of the Deep Sea Drilling Project, Volume 32. Washington, D.C.: U.S. Government Printing Office, 1975.

Nemoto, Kenji, and Loren Kroenke. "Marine geology of Hess Rise: Bathymetry, surface sediment distribution, and environment of deposition." Journal of Geophysical Research, v. 86, p. 10,734-10,752, 1981.

Pringle, Malcolm, William Sager, William Sliter, and Seth Stein, eds. The Mesozoic Pacific: Geology, Tectonics, and Volcanism. Geophysical Monograph 79, American Geophysical Union, 1993.

Thiede, Jörn, et al. Initial Reports of the Deep Sea Drilling Project, Volume 62. Washington, D.C.: U. S. Government Printing Office, 1981.

Vallier, Tracy, Walter Dean, David Rea, and Jörn Thiede. "Geologic evolution of Hess Rise, central North Pacific Ocean:" Geological Society of America Bulletin, v. 94, p. 1289-1307, 1983.


Related Topics

Calcium Carbonate Compensation Depth, Emperor Seamounts, Fracture Zone, Mass Extinctions, Oceanic Plateau, Subduction, Transform Fault