NAME:_____________________________________________________
Environmental Science
The Dynamic Earth: Earth’s 3 different compositional layers
Read the following information about Earth’s layers and answer the questions that follow.
Earth has a core, but this is really two distinct parts: the inner core and the outer core. Both parts of the core are
made up of mostly iron and some nickel. The difference is that in the inner core, those minerals are solid, and in the
outer core, they're liquid.
The inner core of the earth is incredibly hot. What's amazing about the inner core is that even though it's about as
hot as the surface of the sun, there's so much pressure from the weight of the world pushing down on it that it can't
melt.
The outer core is also made up of iron and nickel, but it's quite different because it is a liquid. This is because there is
much less pressure on this layer than the one below it (the outer core adds a lot of pressure to the inner core!).
Though the flow of this liquid layer is very slow moving (about a few kilometers a year), it is what produces Earth's
magnetic field. Our North and South Poles exist because of this liquid outer core, even though it's almost 2,000
miles below us.
Sitting on top of the outer core, we find the mantle. This layer is by far the thickest layer of Earth, about 1,800 miles
thick! It also makes up about 85% of Earth's volume. Like the core, the mantle contains mostly iron, but in the form
of silicate rocks (rocks containing silicon). You might be surprised to learn that this rock actually moves like a fluid,
similar to how silly putty moves. If you poke silly putty hard, it acts like a solid, but if you slowly pull it apart, it acts
like a liquid. We call this ability of rock to move without breaking plasticity.
The mantle can also be divided into two portions, the upper mantle and the lower mantle. The lower mantle is
completely solid because, like the solid inner core of Earth, the pressure is just too great for it to melt and flow. The
upper mantle is also known as the asthenosphere, which flows from convection currents. Convection occurs in all
fluids and is the rising of warm particles and sinking of cool particles. So, as the material in the upper mantle warms,
it rises straight up, and as it rises, it cools and then sinks back down.
This convection flow of the asthenosphere has a large impact on Earth's lithosphere, the outermost layer of the
planet. The lithosphere is only about 60 miles thick and contains both the crust and a small portion of the upper
mantle. The lithosphere is very rigid; it does not flow like the asthenosphere but instead floats on top of it like ice on
a pond.
This layer of Earth is broken up into several different pieces, like a jigsaw puzzle called tectonic plates. The
convection currents below in the mantle move these plates around on the surface of Earth. Though the 'flow' of the
asthenosphere is on par with a snail's pace, these enormous plates running into each other is what causes
earthquakes, volcanoes and mountain range formation.
The crust is contained in the lithosphere and is composed of oxygen mostly and some silicon (Together, oxygen and
silicon are 84% earths crust). It can also be divided into two parts. We live on the continental crust, and the ocean
floor is made up of the oceanic crust. The continental crust is thicker than the oceanic crust, but it's made up of rock
that is less dense than the oceanic crust, so it sits on top of it, above sea level.
It might help to think of the earth's crust like a cargo ship in the ocean. An empty ship floats higher in the water than
a ship that has been loaded up with goods to carry across the sea, right? Well, in this analogy, the continental crust
is the empty ship sitting higher on top of the liquid mantle and the oceanic crust is the fully loaded ship sinking
down lower into this liquid layer. It's all because of gravity!
Questions:
1. Explain why there is a difference in the physical state of the inner and outer core.
2. The atomic weight of nickel is 58.69 amu (atomic mass units). The weight of iron is 55 amu. The
weight of silicon is 28 amu and oxygen’s atomic weight is 15.99 amu. Using these atomic weights,
compare the composition of the core, mantle and crust, and explain why certain chemicals ended
up where they did. (For example, why did nickel end up in the core and not in the crust?)
3. What is convection? How does mantle rock move? How does this affect the tectonic plates above
the mantle?
4. What is the difference between the lithosphere and the asthenosphere?
5. Explain why oceanic crust always slides underneath continental crust.