The flattest and arguably dullest place on Earth. Mid-ocean ridges : The topographic expression of sea floor spreading centers and divergent plate boundaries. They rise above the abyssal plain because they are made of rock that is still relatively warm. The top of the ridge bears a rift valley - the site of spreading. Seamounts : The ocean floor is punctuated by hot-spot volcanoes. When these rise above sea level they are islands.
It they don't they are seamounts. An active margin : Similar to passive, with these contrasts: Continental shelf : Narrow, uneven, or downright nonexistent. Instead the continental slope begins directly off shore.
Trench : The topographic expression of a convergent margin and subduction zone. The lowest elevations on Earth occur in oceanic trenches. We will consider the geology of these regions in order of increasing depth: Continental shelf and surroundings : Shallow ocean waters roughly correspond to the photic zone - the region penetrated by sunlight.
This extends from m. Note: "Continental shelf" may be a useful term to describe topography, but from the standpoint of geologic processes , the photic zone is more important because the major depositional processes in this depth zone are mediated by living things.
Clastic deposition : Sediment rich i. For this reason, shelf environments are typically biogenic. Generally sediments shed into the water by rivers are spread up and down adjacent shorelines and don't make it into the shelf environment proper. The muddy environments around the mouths of rivers are considered part of the deltaic environment.
One exception : Because sea levels change over time, however, clastic sediments can find their way to lower depths. For example, off the east coast of North America at about 80 m. How did they get theree? During the last ice age, most continental shelf was exposed above water. These sands are the drowned remains of Pleistocene low-stand age beaches.
Biogenic deposition : In clear sunlit water, organisms are the primary source of sediment. Major sediment producers include: Algae - Calcium carbonate Calcareous algae with and without soft tissue.
Sponges - Calcium carbonate Corals - Calcium carbonate Bryozoans - Calcium carbonate Molluscs - Calcium carbonate Echinoderms - Calcium carbonate Such organisms encrust and build up on any stable surface. This invariably results in the deposition of layers of limestone. Sometimes, organisms build impressive piles of biogenic sediment that stand out topographically. These take two forms: Mudmounds : Piles of loose, unconsolidated, skeletal material and sediment that stand above their surroundings.
Reefs : Piles of intergrown organismal skeletons and sediment that form a rigid framework. Images of contemporary reefs often don't give a sense of a reef's potential scale. Reef builders : Reefs have been around since the Cambrian. Major reef builders throughout the Phanerozoic include.
Formation of new oceanic crust along a spreading center associated with a mid-ocean ridge. Some spreading centers appear on land. For example, a portion of the Mid-Atlantic Ridge is exposed as Iceland. What is the difference between a mid-ocean ridge and a mid-ocean rise? The two features are basically the same except for their shape topography and how fast they form.
Bathymetric image of the spreading center on the East Pacific Rise. Deep-Ocean Basins Deep-ocean basins cover the greatest portion of the Earth's surface. Trenches Trenches are long, relatively narrow canyon-like features that run parallel to continental margins. An animated view of part of the Marianas Trench. See a NOAA animation. Abyssal Plains An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and meters that extends from the continental rise continental Lithogenous sediments accumulate along continental margins to the distant deep ocean basin where continental-derived sediment deposition is not significant.
Seamounts, Islands, Atolls, and Guyots A seamount is any isolated mountain-sized feature that rises above the seafloor. The South Pacific region has numerous islands, seamounts, atolls, and guyots. All of them started forming as undersea volcanoes. Hawaii is the youngest volcanic island of the Emperor Seamount Chain. Many of the older seamounts were once volcanic islands but are now atolls or guyots. A satellite view of an atoll displaying a fringing carbonate reef platform with islands surrounding an eroding central volcanic peak.
Most atolls of the world are located in tropical regions of the South Pacific and Indian Oceans. Caryl-Sue, National Geographic Society. For information on user permissions, please read our Terms of Service. If you have questions about how to cite anything on our website in your project or classroom presentation, please contact your teacher. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource.
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The movements of these plates can account for noticeable geologic events such as earthquakes, volcanic eruptions, and more subtle yet sublime events, like the building of mountains. Teach your students about plate tectonics using these classroom resources. Students read about the establishment of the Mariana Trench Marine National Monument and discuss why it is important to preserve the Mariana Trench and surrounding area.
Students locate the Mariana Trench on a map, discuss who has jurisdiction over it, and identify the challenges of exploring the deepest place on Earth. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content.
Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Links map The Peru-Chile Trench stretches along the west coast of South America, where the oceanic crust of the Nazca plate is subducting beneath the continental crust of the South American plate. Visit our MapMaker Interactive map to learn more. Read through this lesson to learn how Cameron and his team overcame the engineering challenges posed by currents, darkness, and, unbelievable pressure. Ocean trench es are long, narrow depression s on the seafloor.
These chasm s are the deepest parts of the ocean—and some of the deepest natural spots on Earth. In particular, ocean trenches are a feature of convergent plate boundaries, where two or more tectonic plate s meet. At many convergent plate boundaries, dense lithosphere melts or slides beneath less-dense lithosphere in a process called subduction , creating a trench. Ocean trenches occupy the deepest layer of the ocean, the hadalpelagic zone.
The intense pressure, lack of sunlight, and frigid temperatures of the hadalpelagic zone make ocean trenches some of the most unique habitat s on Earth. When the leading edge of a dense tectonic plate meets the leading edge of a less-dense plate, the denser plate bends downward. This place where the denser plate subducts is called a subduction zone. Oceanic subduction zones almost always feature a small hill preceding the ocean trench itself.
This hill, called the outer trench swell , marks the region where the subducting plate begins to buckle and fall beneath the more buoyant plate. Some ocean trenches are formed by subduction between a plate carrying continental crust and a plate carrying oceanic crust.
Continental crust is always much more buoyant than oceanic crust, and oceanic crust will always subduct. Ocean trenches formed by this continental-oceanic boundary are asymmetric al. On the inner slope continental side , the trench walls are much more steep. The types of rocks found in these ocean trenches are also asymmetrical. The oceanic side is dominate d by thick sedimentary rock s, while the continental side generally has a more igneous and metamorphic composition.
Some of the most familiar ocean trenches are the result of this type of convergent plate boundary. The Peru-Chile Trench off the west coast of South America is formed by the oceanic crust of the Nazca plate subducting beneath the continental crust of the South American plate.
The Ryukyu Trench, stretching out from southern Japan, is formed as the oceanic crust of the Philippine plate subducts beneath the continental crust of the Eurasian plate. More rarely, ocean trenches can be formed when two plates carrying oceanic crust meet. The Mariana Trench, in the South Pacific Ocean, is formed as the mighty Pacific plate subducts beneath the smaller, less-dense Philippine plate. In a subduction zone, some of the molten material—the former seafloor—can rise through volcanoes located near the trench.
The volcanoes often build volcanic arc s—island mountain range s that lie parallel to the trench. The Aleutian Islands form a volcanic arc that swings out from the Alaskan Peninsula and just north of the Aleutian Trench.
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