Re: How much magma is in the Earth's core?

That’s a great question and the answer really tells us a lot about what the inside of the Earth is like. I’m going to answer your question by first talking about how big the Earth is and where, exactly, the core is. Then I’ll tell you about what type of material is in the core of the Earth and whether that material can be considered "magma" or not. Finally, I’ll explain where the "magma" that we see come out of volcanoes like Hawaii comes from.

The Earth is a really big place. It is broken up into three main sections, so that if you cut the Earth in half, if would sort of look like a hard- boiled egg. The shell of the egg is very thin and represents the Earth’s crust. The Earth’s crust (the part of the Earth that we walk on and live on and the only part of the Earth that humans have been to) is only 22 miles thick (35 km) and can even be much thinner in places. The white part of the boiled egg represents the Earth’s mantle. The mantle is 1790 miles thick and so it ends 1812 miles below the surface of the Earth (2900 km). That is where the Earth’s core (or the yellow part of the egg) starts. The Earth’s core continues all the way to the center of the Earth, which is 3984 miles below the surface (6374 km). Compared to the crust, the mantle and the core of the Earth are extremely huge! There is much much more mantle and core material on the Earth than all the rocks we see and touch and pick up on the crust of the Earth. For a good example of this, check out the cut-away image of the Earth (from the Tech Museum of Innovation’s web page).

Each layer of the Earth is made up of slightly different material (just like the egg). The crust of the Earth is made up of very light materials while the Earth’s core is made up of very dense (or heavy) materials. Now, when you first think of rocks that you hold in your hand, they seem pretty heavy. But the material in the core of the Earth is made up of solid iron, and a handful of solid iron is much heavier than a similar-sized rock you might pick up somewhere (go out and try it – hold the metal end of a hammer in one hand and a hand-sized rock in the other!).

The core of the Earth is made up mostly of iron and some nickel and a small bit of other stuff. The core’s temperature is extremely hot, about 7000° F (compare this to a warm summer day when it is only 80° F). The pressures at the Earth’s core are very high as well, so the iron that makes up the core is extremely hot and being squeezed really hard, because it is buried underneath thousands of miles of rocks. Because of these huge pressures and temperatures, the Earth’s core is actually split into two sections, the outer core and the inner core. The outer core is molten iron, a liquid (just like when you see pictures of people melting iron, and the iron is red hot and flows like thick soup). But at the inner core, the pressures are so high that the molten iron is squeezed so much that it becomes a solid again. The inner core is solid iron and weighs about 100 million million million tons!!

You may have noticed that I haven’t answered your question yet. The simple answer to how much "magma" there is in the core is 100% (for the outer core where the iron is all melted) and 0% (for the inner core where the iron is solid). However, the magma that comes out of volcanoes doesn’t come from the core of the Earth. Rather, it comes from the mantle part of the Earth, the part above the core and right below the crust. And the mantle is mostly not melted! The mantle is only about 10-20% melted and all the magma in the world comes from this small portion of the mantle.

To try and think about how much magma is in the mantle (20%), fill up a container with one cup of sand. Then, add one-quarter cup of water. The sand represents the solid rock in the mantle and the water represents all the magma or melted rock in the mantle. The magma can flow around through the rocks by flowing through little pathways and cracks, just like the water in the cup can flow between the sand grains. But think of how difficult it must be to get a bunch of water to pool under one particular part of the sand so that you can get enough magma (or water) in one place to form a volcano! It is very difficult indeed, but it clearly happens on Earth and scientists are trying hard to figure out how that happens. By asking these sorts of questions and trying to understand the inside of the Earth, you are acting just like a scientist too!