Layers of the Earth Notes
Different layers exist from the surface of Earth to its very center. These different layers formed
because 4.5 billion years ago, most of Earth was in a fluid state. While Earth was cooling,
heavier materials sank to the center and lighter materials floated up to the surface of Earth
(density). It is because of this separation that Earth has layers. These layers are the crust,
mantle, and core, with the core being divided into an inner and an outer core. We can tell
these layers apart based on their chemical composition and the state of the matter (Whether
it is solid or liquid) within each.
Core- Found at the very center of Earth. It makes up 16%of the Earth’s volume.




The core is made up of very dense elements, such as iron and nickel.
It begins about 2900 km below the surface, extending to Earth’s center, (6400
km below the surface).
The outer portion of the core is molten or liquid, while the inner portion is solid.
This difference in state divides the core into two regions:
o the outer core (liquid) and
o The inner core (solid). The inner core is solid because at the center of
Earth, there is so much pressure it is impossible for iron to melt.
Mantle- Found directly above the core. The mantle makes up the majority of Earth’s volume
(83%).




It ranges from 40-2,900 km below Earth’s surface.
The mantle consists of elements of a medium density (not as dense as the
elements found in the core, but more dense than those in the crust).
The mantle is composed of rocks known as silicates. The main elements in
silicates are silicon (Si) and oxygen (O). Silicates in the mantle include heavier
elements like magnesium (Mg) and iron (Fe).
The state of matter within the mantle varies from a brittle, solid state to a soft,
molten state, somewhat like Silly Putty known as the asthenosphere.
Crust- it is the very top layer of Earth; it accounts for only 1% of the Earth’s volume. On
average, Earth’s crust is only 40 km thick, extremely thin when compared to the core or
mantle. Most of the rocks in the crust are made up of lighter elements, such as silicates
combined with aluminum (Al), calcium (Ca), magnesium (Mg), potassium (K), and sodium
(Na). The crust has very little iron (Fe) (6%) when compared with Earth as a whole (35% of the
entire planet).

The crust is very thin when compared to the other layers of Earth. Earth’s crust is
either:
o Continental (crust where the
surface is land) - Continental
crust is thick, ranging from 25 –
70 km, and is comparatively
less dense, since it is made up
of many different types of rocks
(sedimentary, metamorphic,
and igneous) that are relatively lighter than the igneous rocks found in
oceanic crust.
o Oceanic (crust where the surface is under the ocean). Oceanic crust is
relatively thin (~5 km), and dense since it is made up of heavy igneous
rocks.
Each layer in Earth has a unique combination of composition, thickness, and state (liquid
versus solid). We can use these different characteristics to make a model of Earth’s interior.
One visual model we can think of to help us understand Earth is a hard-boiled egg. The shell
represents the crust; the white egg represents the mantle; the yellow yolk represents the
core, with the darker yellow in the center and lighter yellow surrounding it representing the
inner and outer core, respectively.
Another way scientists look at the upper portion of Earth is in
terms of the lithosphere and asthenosphere. The lithosphere
includes the crust and the uppermost portion of the mantle,
and is solid and rigid. The asthenosphere lies directly below
the lithosphere but does not extend very deep in the upper
mantle. It is also solid but is more plastic and slowly flows like
Silly Putty.
Earth’s lithosphere is divided into pieces known as tectonic plates. These plates move on top
of the asthenosphere in relation to one another. This movement causes Earth’s surface to
change at the boundaries between plates. The major
plates on Earth are:







African plate,
Antarctic plate,
Eurasian plate,
Indo-Australian plate,
North American plate,
South American plate, and
Pacific plate.
Tectonic plates are usually larger than the land masses for which they are named. For
example, the Eurasian plate includes the Europe and Asian continents, but also includes
parts of the Atlantic Ocean, North Sea, and Mediterranean Sea. The scientific theory that
explains the motion of the plates and continuous changing of Earth’s surface is known as
Plate Tectonic Theory.
The theory of plate tectonics
This theory has evolved over the past 100 years as scientists have discovered new
pieces of evidence. In the early 1900s, Alfred Wegener first proposed the theory of
continental drift; that all of the continents had once been connected in one, massive supercontinent called Pangaea and for some reason had split and drifted apart. Wegener had
made several observations that supported this theory. He found that if you looked at the
coasts of the continents, it looks like they could fit together like pieces of a puzzle. For
example, the east coast of South America looks like it fits together with the west coast of
Africa. Another piece of evidence he found is that you can find fossils of the same animals
on different continents. For example, there are mesosaurus fossils both in South America and
on the west coast of Africa, suggesting that these two continents were connected when the
mesosaurus was alive. Wegener also noticed that the rocks found on the east coast of South
America matched the rocks found on the west coast of Africa. Rocks found on the east
coast of North America, are also found in Greenland, Great Britain, the north coast of Africa,
and Scandinavia. This suggests that these areas were connected when these rocks formed,
and then they split apart. Despite all the evidence Wegener had suggesting that the
continents were once together, he could not describe how or why they would move apart,
so many scientists dismissed his theory.
Wegener’s theory got the support it need during the 1960s when scientists discovered
seafloor spreading. Scientists were able to use new technology (echo sounding) to find
ocean features such as trenches, seamount chains (mountains on the sea floor), and midocean ridges.
Harry Hess proposed that:

At Mid-ocean ridges, magma flows up from the mantle and forms new crust on
both sides of the ridge. This new crust makes the ocean floor wider and pushes
the continents surrounding it further away from each other. Drilling of the sea
floor confirmed that oceans are youngest along mid-ocean ridges and oldest
along their edges. The mid-Atlantic ocean ridge has been making new oceanic
crust for millions of years, allowing the Atlantic Ocean to get bigger and pushing
North and South America further away from Europe and Africa.
The plate tectonics theory also describes the three main types of motion that cause changes
to Earth’s surface and are responsible for many of the land and ocean features we see
today.
These motions occur at plate boundaries and are:


Divergent boundary- where plates move apart, we
find mid-ocean ridges, which look like large rifts in the
ocean floor. An example of this occurs at the midAtlantic ridge. When a divergent boundary occurs on
land we get rifts or canyons like the Great Rift Valley in
East Africa (unlike The Grand Canyon which formed by weathering and
erosion).
Convergent boundary- where plates push together, we can find mountains and
trenches. If two plates with continental crust (continental-continental) are
colliding, we get high mountains, like the Himalayas in Asia where the IndoAustralian plate is colliding into the Eurasian plate. Since oceanic crust is so
much denser than continental crust, when these two collide (or come together,
smash together); this is known as Subduction, the oceanic crust sinks beneath
the continental crust (oceanic – continental). Where the oceanic crust dips
beneath the continental crust, a trench is formed on the seafloor. In addition, as

this oceanic crust sinks (subducts) into the mantle and is heated, it begins to
melt and rise to the surface, causing mountains and volcanoes to form. This is
happening where the Pacific plate (oceanic crust) is colliding with and sinking
beneath the South American plate, forming the Andes Mountains.
Transform boundary- plates slide or scrape past each other. The tension
between these plates is released when they slide past and this leads to faulting,
earthquakes, and changes in Earth’s surface. This happens often in California
along the San Andreas Fault.
We can use land features, such as mountains, trenches, and
ridges, and monitor seismic activity (i.e. volcanoes and
earthquakes) to determine where plate boundaries are located,
what type of boundary it is, and even how fast the plates are
moving in relation to each other.
The entire edge of the Pacific plate has so much seismic activity (earthquakes and
volcanoes), due to the motion along these plate boundaries, that it is called the “Ring of
Fire.”
Earth’s surface is constantly changing shape. Scientists use satellites in space to observe how
the surface of Earth is changing. They can also measure the seismic activity that is caused by
plate movement in order to track how the plates are changing. They can use this data to
know how fast the plates are moving or if certain areas along plate boundaries are due for a
volcanic eruption or even an earthquake. Plate tectonics have changed the appearance
of Earth’s surface and will continue to do so in the future.