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Sima – Oceanic Crust

The top layer of the Earth’s surface is called the crust. There are two different kinds of crust. One is called oceanic crust or sima. The other is called continental crust or sial.

Image of a coral reef sea floor, which is part of the oceanic crust.

All coral reefs, including this one, are part of the oceanic crust.

Sima is different than continental crust. Sima is thinner, denser, younger and made of a different chemical composition.

With that said, a lot of things happen on the continental crust.  For example, on the continental crust, we find volcanoes erupting, plants growingearthquakes, and life living!  In fact, did you know that ALL of these things happen on the oceanic crust too? Well, you do now!

Now, let’s talk about the sima!


As the name oceanic crust suggests, sima is on the ocean floor. Sima is extremely dense and made out of two basic substances; silica, and magnesium.  The word sima is a combination of these two names, silica, and magnesium.

Sima forms at the divergent plate boundaries on oceanic ridges. Its formation at these spreading centers leads to what we call seafloor spreading. Additionally, scientists believe that sima about 4 miles in thickness and has several layers. This does not include the overlying sediment. The top layer is about 1,650 feet thick. It consists of hardened lava which we call igneous rock.

The next layer is made of feeder dikes which are 0.6 miles thick. Dikes serve as the plumbing system of the seafloor, moving magma up to produce lava.

What we know about the oceanic crust comes from a few different places. One, for example, is the sampling of the ocean floor.  From this sampling, we know the composition of the oceanic crust (lava, magnesium, and silica). Unfortunately, the actual structure of the crust and how much of what substances are there is unclear.

Rock data combined with seismic waves experiments have given us a snapshot.  In order to understand the structure of the crust and what it is made of, scientists use this data.  We call this research stratigraphy.

Illustration of the seafloor.

The oceanic crust, sima.

The Ocean Floor

The floor of the ocean is broken into three basic levels.  Firstly, the deep ocean floor is called the abyssal plain.  Secondly, the middle layer is the mid-oceanic ridge. And finally, the deepest layer is the ocean trench.  Many scientists believe that a layer of sima is also below continental plates.

Ocean Pun

Question: What did the sea say to the shore?
Answer: Nothing, it just waved.

Other Great Resources:

Bill Nye: The Earth’s Crust:

Oceanic vs. Continental Crust:

Easy Science for Kids: http://easyscienceforkids.com/all-about-earths-crust/

Written By: Monica Siegenthaler.