Earth Layering:  Compositions, Properties, Locations

The geosphere is a term used to describe the “solid earth” as a means to distinguish it from the hydrosphere (oceans) and the atmosphere.
Most people recognize that Earth’s interior has a kind of layered structure.
Here, we’ll learn about the classification of the various layers and their material composition;  but a good question to keep in mind (we’ll get to it eventually!) is simply why the earth (and other planets) are in fact layered as opposed to being homogeneous, and has the earth always been layered?

Characteristics of the Crust

 

In a nutshell (and it does kind of resemble a nut!)– scientists have come to recognized three major compositional layers to planet earth.  The outermost layer is the crust, atop which we live, and it is by far the thinnest.  The crust varies in thickness, but it is only rarely more than about 50 km thick, which is nothing compared to the nearly 6370 km to earth’s very center.  In a volumetric sense, the crust is really a lot like the skin of an apple compared to the rest of the apple.  We live on this “skin,” i.e. crust, but it really makes up very little of the whole earth!

The bulk of the earth is the mantle, which (contrary to popular belief) is nearly 100% solid rock (very little in the way of melt, or magma).   There is a region in the upper mantle (around 100-250km in depth, but variable) where the mantle is near its melting temperature, and because of this it has rather mushy-gushy properties, like toothpaste or taffy or silly putty– it’s called the asthenosphere.  The ductile properties of the asthenosphere are enormously important, because this allows the tectonic plates to slide around on top, resulting in the motions of plate tectonics!

Finally, the deepest interior region is the core (made mostly of iron), separated into the outer core and inner core.
It’s a clear separation, because the outer core is liquid and the inner core is solid.
This might seem kind of odd– inasmuch as the outer core is actually cooler than the inner core.

Temperatures in earth’s center, the inner core, are in the range of 5000-6000 degrees C.
BUT, due to such incredible pressure, the material (largely iron metal) is NOT liquid, but instead solid!
The outer core is somewhat cooler, but there is less pressure, and this allows the outer core (again, largely iron metal) to be in a liquid state!
Key Idea– Melting Temperature is pressure dependent.  Where the pressure is high, the melting temperature is also very high.            

OUTER CORE IS LIQUID METAL,, INNER CORE IS SOLID METAL!

 

 

 

 

OK, enough about the CORE,
Back to Earth’s CRUST

Note the two different types in image above, continental and oceanic.  Earth’s outermost zone, the crust, is thin and (compared to interior regions) relatively cold, and hence it behaves in a ridged fashion (rocks break more often than bend).  a cold, thin, brittle outer shell made of rock.

There are two very different types of crust, each with its own distinctive physical and chemical properties, which are summarized in Table 1.

Crust	Thickness	Density	Composition	Rock Types
Oceanic	5–12 km (3–8 mi)	3.0 g/cm3	Mafic	Basalt and gabbro
Continental	Avg. 35 km (22 mi)	2.7 g/cm3	Felsic	All types

Oceanic Crust

Usually, ocean crust is subducted at plate boundaries.  There are, however, rare instances of oceanic crust being “obducted”, in other words pushed on top of the continental crust.  The pieces of ocean crust found sitting on continental crust are called ophiolites.  This unusual occurrence provides geologists with a view of what a slice through the ocean crust might look like.
See below, a typical ophiolite sequence (“top-down” view):

 

 

 

Oceanic crust is composed of mafic magma that erupts on the seafloor to create basalt lava flows or cools deeper down to create the intrusive igneous rock gabbro (Figure 1).

Sediments, primarily mud and the shells of tiny sea creatures, coat the seafloor. Sediment is thickest near the shore, where it comes off the continents in rivers and on wind currents.

The oceanic crust is relatively thin and lies above the mantle. The cross section of oceanic crust in Figure 2 shows the layers that grade from sediments at the top to extrusive basalt lava, to the sheeted dikes that feed lava to the surface, to deeper intrusive gabbro, and finally to the mantle.  This is basically the same thing as the ophiolite sequence, seen above!

Continental Crust

Continental crust is made up of many different types of igneous, metamorphic, and sedimentary rocks. The average composition is granite, which is much less dense than the mafic rocks of the oceanic crust (Figure 3). Because it is thick and has relatively low density, continental crust rises higher on the mantle than oceanic crust, which sinks into the mantle to form basins. When filled with water, these basins form the planet’s oceans.

Summary

• Oceanic crust is thinner and denser than continental crust.
• Oceanic crust is more mafic, continental crust is more felsic.
• Crust is very thin relative to Earth’s radius.

Interactive Practice

Visit Annenberg Learner– cool site for investigating earth interior

Characteristics of the Mantle

Two big ideas regarding the mantle:

1.  This is a region of very high temperatures and pressures.
2. Although hot, for the most part the mantle is SOLID.  Only in relatively rare locations does the mantle melt!  (As you’ll learn later– it melts where it rises, e.g. mid-ocean ridges, and it melts where fluids lower the normally super-high temperatures required to melt rocks under pressure.

Solid Rock

Scientists know that the mantle is made of rock based on evidence from seismic waves, heat flow, and meteorites. The properties fit the ultramafic rock peridotite, which is made of the iron- and magnesium-rich silicate minerals (Figure 4). Peridotite is rarely found at Earth’s surface.

Heat Flow

Scientists know that the mantle is extremely hot because of the heat flowing outward from it and because of its physical properties.

Heat flows in two different ways within the Earth:

1. Conduction: Heat is transferred through rapid collisions of atoms, which can only happen if the material is solid. Heat flows from warmer to cooler places until all are the same temperature. The mantle is hot mostly because of heat conducted from the core.  STICK A METAL ROD INTO A FIRE!
2. Convection: If a material is able to move, even if it moves very slowly, convection currents can form.

Convection in the mantle is the same as convection in a pot of water on a stove.  SOUP RICE ANALOGY

Convection currents within Earth’s mantle form as material near the core heats up. As the core heats the bottom layer of mantle material, particles move more rapidly, decreasing its density and causing it to rise. The rising material begins the convection current. When the warm material reaches the surface, it spreads horizontally. The material cools because it is no longer near the core. It eventually becomes cool and dense enough to sink back down into the mantle. At the bottom of the mantle, the material travels horizontally and is heated by the core. It reaches the location where warm mantle material rises, and the mantle convection cell is complete (Figure 5).

Characteristics of the Inner and Outer Core

At the planet’s center lies a dense metallic core. Scientists know that the core is metal because:

1. The density of Earth’s surface layers is much less than the overall density of the planet, as calculated from the planet’s rotation. If the surface layers are less dense than average, then the interior must be denser than average. Calculations indicate that the core is about 85% iron metal with nickel metal making up much of the remaining 15%.
2. Metallic meteorites are thought to be representative of the core. The 85% iron/15% nickel calculation above is also seen in metallic meteorites (Figure 6).

If Earth’s core were not metal, the planet would not have a magnetic field. Metals such as iron are magnetic, but rock, which makes up the mantle and crust, is not.

Scientists know that the outer core is liquid and the inner core is solid because:

1. S-waves do not go through the outer core.
2. The strong magnetic field is caused by convection in the liquid outer core. Convection currents in the outer core are due to heat from the even hotter inner core.

The heat that keeps the outer core from solidifying is produced by the breakdown of radioactive elements in the inner core.

Explore More

Use this resource to answer the questions that follow.

1. What materials can P-waves travel through?
2. What materials can S-waves travel through?
3. How do we know the outer core is liquid?
4. What happens to P-waves when they go through a liquid?
5. What do P-waves tell about the inner core?

The Lithosphere and Asthenosphere

PLATE TECTONICS=> this is just the motion of the outer portion of the earth (i.e. the lithosphere), sliding about, and carrying continents and also opening and closing ocean basins!
Plate tectonics WORKS because the outer RIGID part of the earth is the Lithosphere.  That’s the part that moves — see “convection diagram” above.  The lithosphere is the outermost (relatively cool) mantle and the crust.
It slides around at velocities between 1 and 15cm per year.  Slow stuff, but over long periods of time can account for big changes in continental positions.  (In general, plate velocities are on the order of about 5cm per year.)

RIGID block?  What’s that mean?
As per above, The lithosphere behaves like a rigid block.  Push on any piece, and the other side of that piece moves!
In contrast, the asthenosphere is a bit more unusual.  It’s gushy.  It’s mushy.
In science-speak, the asthenosphere is a material that deforms in a plastic manner.
It’s not a molten substance, merely something that lacks rigidity.
Examples from our everyday life include toothpaste and (perhaps more everyday for kids) silly-putty!

   

Lithosphere

The lithosphere is composed of both the crust and the portion of the upper mantle that behaves as a brittle, rigid solid. The lithosphere is the outermost mechanical layer, which behaves as a brittle, rigid solid. The lithosphere is about 100 kilometers thick. How are crust and lithosphere different from each other?

The definition of the lithosphere is based on how Earth materials behave, so it includes the crust and the uppermost mantle, which are both brittle. Since it is rigid and brittle, when stresses act on the lithosphere, it breaks. This is what we experience as an earthquake.

Although we sometimes refer to Earth’s plates as being plates of crust, the plates are actually made of lithosphere.

Asthenosphere

The asthenosphere is solid upper mantle material that is so hot that it behaves plastically and can flow. The lithosphere rides on the asthenosphere.

Summary

• The lithosphere is the brittle crust and uppermost mantle.
• The asthenosphere is a solid but it can flow, like toothpaste.
• The lithosphere rests on the asthenosphere.

Explore More

Watch this video.  Think about how you might answers questions that follow–

1. What does he mean by the mechanical properties of a layer?
2. In the compositional model: What is the outermost layer? What are the two types of this layer and what are their main features?
3. What is the lithosphere?
4. What are the mechanical properties of the material below the lithosphere and what is the layer called?
5. What is the composition and mechanical property of the mesosphere relative to the asthenosphere?

.