The Oceans

Planet earth has two types of crust one is continental crust the one on which we stay and the oceanic crust which consists of all the water in the planet. most plates have an area of both the continental and the oceanic crust except the Nazca Plate, it has an only oceanic crust and the pacific plate has a relatively lower continental crust. oceanic crust is comparatively thinner and less dense than continental crust it has an average thickness of 7-8 km.  

The oceanic crust is continuously being created at the ridges and it also descends back into the asthenosphere at the trenches(Subduction zone). between the ridges and the trenches lies the sediments covered Abssayal plain, and many plateaus and hills. the ocean begins at the edge of the continent with a continental margin. there are two types of continental margins and they are referred to as Passive continental margin and Active continental margin.

Ocean Relief

Ocean relief is formed due to tectonic, volcanic, erosional, depositional processes and their interactions. The ocean basins have many features similar to the topography of the land surface. ocean reliefs are divided into Major and Minor Relief features

Major Relief features

i) continental shelf

the part of the continent(landmass) that is entering into the ocean. in other words, it's just a Seaward extension of the continental plate, it has a very small slope of around 0.1º. mainly covered with variable thickness of sediments brought down by rivers glaciers, etc. sedimentary deposition received over a long period of time by the shelf become the source for fossil fuel(petroleum) hence it is an important location for the extraction of petroleum. it has an average width of 70-80 km the depth varies between 60m - 300m. it almost covers 7-8% of the total ocean area.it is an . the continental shelf ends at a very steep slope known as the shelf break. After the shelf break, the shelf starts to steeply descend towards the deep ocean floor in what is called the continental slope.

The Siberian shelf in the arctic ocean is the largest continental shelf on the planet with a width of 1500km. continental shelves are mainly formed due to the sedimentary depositions brought by the rivers, and due to the rise in sea level.

significance of continental shelf

• Almost all of the marine food comes from the continental shelf
• Riches fishing grounds 
• potential sites for economic grounds (20% of the world production of petroleum comes from shelves)

ii)continental slope

The continental slope connects the continental shelf and ocean basin, it begins at the shelf break. in this region the gradient varies between 2º-5º and the depth varies between 200m-3000m. the seaward edge of the continental slope loses gradient and gives rise to continental rise, hence we can say that at some point in time in the past the continental rise was part of the continental slope, but due to the process of erosion the gradient decreases and it becomes continental rise. canyons and trenches are observed in this region. the continent slope boundary indicated the end of the continent and the beginning of the ocean bed.

iii)continental rise

as we move deep down the continental slope loses its gradient and it reaches a value between 0.5º-1º it is referred to as continental rise. with more increase in-depth the rise becomes flat and merges with the abyssal plain. 

iv)Abyssal plain 

this is the smoothest and the flattest region of the world due to terrigenous and shallow water sediments that buries the irregular topography, it almost covers 40% of the ocean floor. varies in-depth(3000m-6000m). this region is covered with fine-grained sediments like clay and silt.

these plains are more common in the Atlantic region and rare in the pacific region 

Minor Relief features

i)Seamounts: these are underwater mountains with pointed summits that do not reach the surface of the ocean, these are formed due to volcanic activities. these can be 3000-4500m tall.

ii)Guyots: these are underwater flat-topped mountains and can be taller than 300m. they can be isolated or part of a long mountain chain 

Seamounts and Guyots are very common in the pacific ocean.

iii)Mid Oceanic Ridges: Mid Oceanic Ridge system is a chain of mountains In the middle of the ocean formed when two plates are moving away from each other, this process is known as seafloor spreading, it wraps around the globe up to 65,000 km and covers almost 23% of earth's surface. The average depth to the crest (top) of the ridge is 2500 m, but it rises above sea level in Iceland and is more than 4000 m deep in the Cayman Trough.

Distribution of  Mid Oceanic ridges

The Atlantic Ocean has the following notable ridges.

1. The Mid Atlantic Ridge
2. The Reykjanes Ridge
3. The Madeira Ridge
4. The Atlantic Indian Ridge
5. The South Scotia Ridge
6. The Zapiola Ridge
7. The America – Antarctica Ridge
8. The Astrid Ridge
9. The Parnaiba Ridge
10. Belem Ridge
11. The Ceard Ridge
12. The Barracuda Ridge
13. The Blake Bahama Ridge
14. The Eirik Ridge
15. The West Scotia Ridge
16. The Newfound land Ridge
17. The Walvis Ridge
18. The Hatton Ridge

The Pacific Ocean has the following notable ridges:

1. Gorda Ridge
2. Kyushu – Palau Ridge
3. South Honshu Ridge
4. West Norfolk Ridge and Norfolk Ridge
5. Kermadec Ridge
6. Robbie Ridge
7. Hawaiian Ridge
8. Christmas Ridge and Necker Ridge
9. Tehuantepec Ridge
10. Colon Ridge
11. Pacific Antarctica Ridge
12. Nazca Ridge
13. Society Ridge 
14. Tuamotu Ridge 
15. Macquarie Ridge
16. Caroline Ridge
17. Galapagos Ridge

The  Indian Ocean has the following notable ridges:

1. Ninety East Ridge
2. Investigator Ridge
3. Chain Ridge
4. Amirantle Ridge
5. Davie Ridge
6. Murray Ridge
7. Carlsberg Ridge
8. Sheba Ridge
9. Atlantic Indian Ridge
10. Southwest Indian Ridge
11. Mid Indian Ridge
12. Broken Ridge
13. Southeast Indian Ridge
14. Hartog Ridge
15. East Indiaman Ridge

iv)Trenches: An oceanic trench is a type of convergent boundary at which two lithospheric Plates meet, then the dense lithosphere melts or slides beneath less-dense lithosphere in a process called subduction, creating a trench. they are some 3-5 km deep than the surrounding ocean floor. trenches are very common in the pacific ocean and almost form a continuous ring in the Eastern and Western margins of the pacific ocean. marina trench is the deepest trench (approx 11km deep)