THE COMPOSITION AND FORMATION OF EARTH
Why do we care about how the Earth formed, and what does this have to do with dinosaurs (since
this is fundamentally a course on dinosaurs)? Well, the Earth is integrated with the things living
on it. The evolution of life is dependent of the evolution of the Earth itself...which have to do
with its composition.
The heavier elements are towards the center of the Earth and the lighter ones towards the surface.
The mantle is 83% of the volume of the earth. The mantle is dynamic. Above the mantle is the
crust. The crust is the rigid outermost layer of the planet. It is composed of minerals. There are two
types of crusts. There is continental crust, which were standing upon. It is mostly silicon and
aluminum. It is thick. Then, there is oceanic crust. It is thin, but dense. It is primarily iron and
magnesium. This crust that were standing on, that forms the floor of ocean basins is sitting on top
of a thin layer of the mantle. The crust + the thin layer of mantle right below are together called the
lithosphere. The lithosphere is sitting on top of the asthenosphere. All of the outer crust is
flowing/moving. The crust of the planet is moving. It is broken into a series of plates, which are
composed of either continental crust, or oceanic crust, or both...and these plates are moving...but
theyre not moving in the same direction.
This process is called plate tectonics. Plate tectonics is responsible for the origin of life on
Earth. Amongst other things, plate tectonics is a recycling mechanism (for the destruction and
creation of new plates).
Plate tectonics is the formation of new plates and crust and the destruction of old plates and crust.
It is a cycle.
Plates are moving around over the mantle. The reason these plates move is because theyre being
driven around b/c of heat transfer at the center of the planet. As they move around, the plates are
breaking, converging, diverging away from each other...basically, theyre moving!
When plates converge, a process called subduction takes place. Subduction is the process by
which heavier plates dip under lighter plates. It is almost always the case that the heavier, denser,
oceanic crust subducts under the lighter, less dense continental crust. When this happens, the
oceanic crust descends back down into the mantle, where it decomposes and simply reduces back
to its elemental composition as part of the mantle. So, plates subduct under each other. However,
keep in mind that this is what happens at one end of the plate. If a plate is moving towards one
plate, then it is moving away from another. When two plates move away from each other, you have
a rift/vent between these plates. As a result, magma comes up from the upper mantle, cools down,
and forms new crust.
Again, the general pattern is that heavier, denser, oceanic crust subducts under lighter, less dense
continental crust. It is destroyed at one end and more is created at the other end. Now, moving into
further detail, as a piece of oceanic crust is descending, a couple of things are happening. Its
minerals are breaking down. Some of the minerals have water in them, which lowers the
temperature . These start to rise up, carving a way through the continental crust, and manifest in the form of volcanoes. When you have two plates moving against another plate, you get
earthquakes. You have subduction at one end, and the formation of new crust at the other. The
formation of new crust at a mid-ocean ridge is called seafloor spreading. What this shows is that
there is a recycling pattern of oceanic crust. Continental crust is generally much older and it
generally is not subducted (i.e., continental crust is lighter and is rarely pushed below the heavier
oceanic crust). It is generally destroyed by erosion. It weathers away.
How do we get these basins?
What happens is that the heat energy underneath continental crust starts to pull apart the
continental crust. As it separates, it will start to form a series of rifts. Eventually, the rifting will
become so great that magma will rise and form new crust. When this happens, the magma forms
oceanic crust, not continental crust. However, because these are topographically low areas, water
If you look at the red sea, it is one of these rifts. The Arabian land mass was previously part of
Africa but it has moved away.
This process of plate tectonics creates and generates oceanic crust. It is a dynamic process. As a
result, there are changes on the surface of the planet. Because there are living beings on the planet,
they will respond to these changes.
Plates are usually continental plates with pieces of oceanic plates around it.
What we have is plates moving against each other. What is also going on is that in the core of the
planet, you have the outer core moving over the inner core, which generates an electromagnetic
field. The Earth has magnetic poles. It is bipolar. It has a magnetic north and a magnetic south.
Plate tectonics is a paradigm of modern geology. It is an overarching unifying theory.
The magnetic field is not stationary over time. We have a positive pole at the north and a negative
pole at south.
Over time, the position of the poles has changed. This is called polar wander. The poles will
wander, BUT they will always be at opposite ends. The intensity of the Earths magnetic field will
get stronger and weaker over time. The wander of these poles over time, including changes in their
position and intensity, is a great tool for looking at the history of the Earth. What happens with the
orientation of these poles is that as they wander around, sometimes the orientation of the poles will
switch. The north will become south, and vice versa.
As oceanic ridges spread apart, magma is coming up and forming new crust. Everything in earth is
in its magnetic field. Just like a compass needle, magma is coming up and has magnetically
charged elements in it. Those elements are aligning themselves with the Earths magnetic field and
orientation AT THAT TIME. Once they cool, they lock into position, and preserve the orientation
of the Earths magnetic field AT THAT TIME. This happens again as the ridge widens further. This is evidence of the formation of new pieces of crust. This is also a method for figuring out the
ages of rocks all over the planet. So, you have on either side of the ridge new crust being formed.
Where is the oldest crust? It is farther away from the ridge. The farther you go, the older the crust.
At the same time, you can see the signal of the earths magnetic field at that time.
Everywhere on Earths surface, the barcode is being preserved. This means that you can take a
rock with a given barcode and compare it to any other rock on the planet, and if the signals line up,
the rocks are of the same age.
The oldest oceanic crust is about a 190 million years old. The oldest continental crust is about 3.4
billion years old. Continental crust is older because it is recycled more slowly (through erosion)
than oceanic crust (which is recycled relatively quickly through the process of subduction).
Oceanic crust subducts under continental crust? Why? The plates move because of heat flow in the
mantle. Were getting heat from two directions. One is from the sun. We also get from the center
of the planet. What that means is that within the core and the mantle, heat is flowing out. The
mantle is not liquid. This is a misunderstanding. Its superheated. The mantle heats up...what
happens when it heats up? It rises. You have heat moving from the core heating up the mantle. As
it heats up, it begins to rise towards the underside of the lithosphere. As it heats up, it is moving in
convections cells and moving towards the underside of the lithosphere. This causes the plates to
move around. Eventually, after imparting energy on the plates and causing plate movement, it
cools down, descends, heats up again, and rises to the underside of the lithosphere, causing
movement in the plates once again...and so on. Its a cycle.
Sometimes, mantle material will rise and hit the underside of the lithosphere and keep going right
through the lithosphere. This will form an island.
Hotspots (places where mantle material tends to rise right through the lithosphere) can be
distributed along plate boundaries, but sometimes can occur in the middle of plates too. We can
actually look at the minerals that make oceanic crust. This is because the magma that comes up is
from right below the crust. At hot spots, however, the magma that comes up is from deep within
the Earth. PLATE TECTONICS
Plate tectonics is the movement of rigid tectonic plates that compose the outer crust of the Earth.
The crust is composed of continental and oceanic crust. Oceanic crust is heavier, denser, and
younger than continental crust.
As we discussed last class, oceanic crust subducts under continental crust.
With plate tectonics comes plate rotation and plate movement. As these plates are rotating around,
they are creating new connections. Plates are moving. The continent that were on has not always
been the North American continent. There is a cycle of continental construction and
Approximately 200 million years ago, a large continent called Pangaea deconstructed into two
smaller continents (Gondwana and Laurasia). About 100 million years ago Gondwana fragmented
into Africa, South America, India, Australia, and Antarctica. At the same time, Laurasia
fragmented into North America and Eurasia.
India and Australia are on one large plate called the Indo-Australian plate. About 100 million years
ago, India and Australia start to fracture off and begin to rotate. India moves up as an isolated
continent and docks with Southern Asia about 40 million years ago. The oceanic crust on the
outskirts of the Indian plate comes into contact with the continental crust of the Eurasian plate and
subducts under it (i.e., oc