Active Margin Geomorphology

About the Book

N.Christian Smoot, GEOSTREAMS, Ltd., 306 Kipukai Pl., Honolulu, HI 96825, USA

Dong R. Choi, Consulting Geologist, 6 Mann Place, Higgins, ACT 2615, Australia

M. Ismail Bhat, Department of Geology and Geophysics, University of Kashmir, Srinagar, India

Any study of Earth geodynamics must of necessity address events that have and are occurring to create the marine geomorphology. One problem with a geomorphologic study of the Earth=s surface is that the ocean basins are inconveniently filled with water, and that constitutes at least 70% of Earth´s surface. This treatise on active margin geomorphology shows the different major ocean floor feature types associated with active margins, their geography, and the processes which construct them based on the vast amount of information gathered since 1966.

Geomorphology deals with the relief that features exhibit on the Earth´s surface and is based on the principle that all landforms can be related to a particular geologic process or processes. A certain sequence of events explains the evolution of a feature that is dependent on time and is also predictable. The science of geomorphology is defined by the principle of uniformitarianism, a principle theorized by J. Hutton in 1785 in which the same landforms repeat themselves given enough tries. A prime example is that mountains will erode level. Many detours exist, so that uniformitarianism may be hard to define on a microscale due to such parameters as reef building, subsequent volcanic activity such as multiple vent building, parasite cone formation, and different material makeup. Charles Lyell wrote Principles of Geology in 1833, a book Steven Jay Gould (1999) calls Aperhaps the most important scientific textbook ever written.@ Lyell based his book on two fundamental hypotheses, those of gradualism and a steady-state, non-directional Earth. These are both used to expand on uniformitarianism. Lyell said that change neither speeds up nor slows down over geologic time. William Morris Davis synthesized many early ideas and published a series of articles between 1889 and 1934, molding them into a unified system for the study of landforms (Davis, 1909). That system is based on structure, process, and time. Penck=s (1924) ideas included tectonic uplift as a causal agent. Others were published and are generally included in the science of geomorphology. All are based on terrestrial events, with climate being the primary factor.

During the 1800s, another group espoused the idea of catastrophism. This hypothesis stated that most geological episodes, such as volcanism, mountain building (orogenies), earthquakes, and floods, occurred quickly and almost singularly. A worldwide event of orogeny would have necessarily been followed by a period of quiescence. The theory held that these catastrophies had decreased over time (Gould, 1999). Essentially, Earth was formed as a fireball spun off the Sun followed by an overall cooling. The outer shell cooled first, while the core remained hot. Due to the effects of gravity, the cooling outer crust collapsed in on the mantle. Unfortunately for the Acatastrophists@ of old, they linked their theory with the religious dogmas of the day and declared Ussher to be right; the Earth was no more than a few thousand years old.

During the 1960s an idea called plate tectonics gelled from all the previously gathered data. This hypothesis, based primarily on geophysical studies, revealed an overly simplified conveyor-belt explanation. Midocean ridges, or spreading centers, are zones of shallow seismicity where magma wells up to the surface from the asthenosphere and forms new oceanic crust, comprised primarily of basalt. The spreading center is a divergent plate boundary, and Aridge push@ is one of the three driving forces of plate tectonics. The rate of movement varies but is measured in centimeters per year. The crust moves and ages w

About the Author

N.C. Smoot retired from 30 years of deep ocean surveying and mapping work with the Ocean Survey Program of the US Naval Oceanographic Office, rising to the rank of senior scientist. He spent 10 cruises in the Med, 25 in the north Pacific, and 32 in the Atlantic. D.R. Choi has worked in the oceans around Australia and is now a minerals exploration consultant in Asia and Australia. M.I. Bhat spent 25 years doing field work and analysis with the Wadia Institute of Himalayan Geeology, rising to the rank of senior scientist before leaving to start the Department of Geology and Geophysics at Kashmir University two years ago.