Continental margins describe the area of transition from the land also to the deep seafloor, i.e. between continental and oceanic crust. In an active continental margin, the boundary between the continent and also the ocean is also a tectonic plate boundary, so tright here is the majority of geological task roughly the margin. The west coastline of the United States is an example of an active margin, wbelow the coastline corresponds through the boundary in between the Pacific and also The United States and Canada Plates. A passive continental margin occurs wbelow the change from land to sea is not linked via a plate boundary. The east coast of the USA is a great example; the plate boundary is situated alengthy the mid Atlantic ridge, far from the coastline. Passive margins are less geologically active. Figure 1.2.1 reflects an idealized passive margin. When examining this figure, and also others like it, note that tright here is substantial vertical exaggeration; the depth scale covers roughly 5000 m, while the horizontal range extends around 300 km. This provides the functions look a lot steeper than they actually are. The bar at the bottom of Figure 1.2.1 reflects what a passive margin would certainly look like without this exaggeration; tright here is a much more gradual transition to depth.

You are watching: Why do passive margins have no trenches

*
Figure 1.2.1 Features of a passive continental margin (modified by PW from Steven Earle, “Physical Geology”).

The continental shelf is the shpermit, flooded edge of the continent. Geologically the shelf is still component of the continental crust, however it is frequently overlassist via marine sediments. On average, the shelf exhas a tendency around 80 km from the coast; some margins have extremely bit shelf, while the Siberian Shelf in the Arctic exhas a tendency approximately 1500 km. The depth of the shelf primarily stays listed below about 150 m, and also the floor of the shelf is fairly flat. The level topography is the outcome of transforms in sea level; throughout history the shelves have actually been both submerged and exposed, and as sea level climbed and also dropped, wave action, ice sheets, and other erosional procedures smoothed out the shelf surface. Wave action and the motion of sediments over the shelf have ongoing this smoothing process. Continental shelves only consist of about 6% of the ocean’s surchallenge area, but they are biologically one of the richest components of the ocean; their shenable depth avoids nutrients from sinking out, and their proximity to the shore gives significant nutrient input. The continental shelf ends at the shelf break, which is the suggest wright here the angle of the seafloor starts to acquire steeper. The shelf break averages around 135 m deep.

After the shelf break, the seafloor takes on a steeper angle (around 4o) as it descends to the deep ocean. This steeper portion of the margin is the continental slope, and also it extends from the shelf break down to 3000-5000m. In some parts of the sea, huge submarine canyons have actually been sculpted right into the continental slope; for example, Monterey Canyon in Monterey Bay, The golden state, is a submarine canyon similar in size to the Grand Canyon! These canyons may be carved out by turbidity currents, which are fundamentally landslides of sediment, rocks, and other debris down the confront of the slope.

At the bottom of the slope is the continental rise. This location represents wright here the continental crust meets the oceanic crust, as the slope begins to level off to come to be the deep ocean floor. The climb consists of a thick layer of collected sediment coming from the continent, so it is difficult to tell wright here the slope ends and the climb begins.

After the increase comes the abyssal plain, or the deep sea floor, lying in between 4500 – 6000 m. The abyssal plain has most of the sea floor, and is the flattest area on Planet. It is flat as a result of countless years of sediment accumulation on the bottom, which buries many kind of bottom functions (Figure 1.2.2).

*
Figure 1.2.2 Topography of the North Atlantic. Shenable continental shelf areas are shown in red, and also the abyssal plain is presented in blue. Alengthy the eastern coast of the USA the continental slope can be viewed in green (Stalso Earle, “Physical Geology”).

See more: What Do Stage Directions Provide For The Reader, Drama Review Flashcards

Passive margins, as described above, have wide shelves, gentle slopes, and a well-occurred climb. Since passive margins are not plate borders, they suffer long durations of family member stability which can bring about the advance of these attributes. Active margins have comparable functions to passive margins, however the plate boundary affects the properties of the features. Active margins, choose the Pacific coastline of The United States and Canada, have actually narrower shelves, steeper slopes, and also little bit to no increase, specifically in convergent limits. Trenches associated via subduction zones act as sediment traps, avoiding the buildup of a continental rise, and maintaining sediments off of the abyssal levels.


* “Physical Geology” by Steven Earle offered under a CC-BY 4.0 worldwide license. Download this book for free at http://open up.bccampus.ca