Figure 2.9: Summary of major landforms on continents and features of coastlines. ( 11 )As rivers and streams flow across continents, they cut away at rock, formingriver valleys
(Figure2.9). The bits and pieces of rock carried by rivers are deposited where rivers meet
the oceans. These can formdeltas, like the Mississippi River delta andbarrier islands,
like Padre Island in Texas. Our rivers bring sand to the shore which forms ourbeaches.
Ocean Basins
Theoceanbasinsbeginwheretheoceanmeetstheland. Thenamesforthepartsoftheocean
nearesttotheshorestillhavetheword“continental”attachedtothembecausethecontinents
form the edge of the ocean. Thecontinental marginis the part of the ocean basin that
begins at the coastline and goes down to the ocean floor. It starts with thecontinental
shelf, which is a part of the continent that is underwater today. The continental shelf
usually goes out about 100 – 200 kilometers and is about 100-200 meters deep, which is a
very shallow area of the ocean (Figure2.10).
From the edge of the continental shelf, thecontinental slopeis the hill that forms the
edge of the continent. As we travel down the continental slope, before we get all the way to
the ocean floor, there is often a large pile of sediments brought from rivers, which forms the
continental rise. The continental rise ends at the ocean floor, which is called theabyssal
plain.
The ocean floor itself is not totally flat. Small hills rise above the thick layers of mud that
cover the ocean floor. In many areas, small undersea volcanoes, calledseamounts(Figure
2.11) rise more than 1000 m above the seafloor. Besides seamounts, there are long, very tall
(about 2 km) mountain ranges that form along the middle parts of all the oceans. They are
connected in huge ridge systems calledmid-ocean ridges(Figure2.12). The mid-ocean
ridges are formed from volcanic eruptions, when molten rock from inside the Earth breaks
through the crust, flows out as lava and forms the mountains.