Convection is not only occurring at the surface of the Earth but inside as well. How is it possible?
What is the internal structure of the Earth?
The Earth's interior consists of a very thin and brittle crust, the mantle, and the core. The outermost layer of the Earth is called the crust. It is a thin layer of rock that covers the globe. There are two kinds of crust. There is the oceanic crust which lies under the oceans and the continental crust which comprises the continents. One of the main differences between the two is their density. The lighter continental crust lays higher on the earth's surface and accounts for the land masses, and mountains. The heavier oceanic crust sits lower on the planet and forms the natural basins for the vast oceans which cover the Earth.
Also of importance beside the density of the two crusts is their thickness. The heavier oceanic crust is relatively thin only about 7 miles. The lighter continental crust while thicker is lighter in density so it can be supported by the material underneath. The continental crust averages about 20 miles thick but can be up to 40 miles thick in certain places especially where there are mountain ranges.4
Below the crust is the mantle. The mantle is divided into two sections. The upper part of the mantle and the crust make up what is known as the lithosphere. Both of these parts are rigid and cooler in comparison to the material below in the lower part of the mantle or asthenosphere. The asthenosphere or "soft" area is between 100 and 200 kilometers below the Earth's surface and the temperature is near the melting point. The entire mantle beneath the lithosphere acts like plastic that is pliable enough to make land masses or plates move across it.5
The core is made of two distinct parts. The inner core is a solid mass which is surrounded by the outer core which is mostly liquid. While the core and mantle are about equal in thickness, the core forms only 15% of the Earth's volume whereas the mantle covers about 84% of its volume and 63% of its mass. The remaining 1% is the crust.6
What is the source of heat inside the Earth?
The heat within the Earth is the result of the natural process that occurred during the planet's development. The Earth formed from a cloud of a swirling mass of particles and gas some 4.57 billion years ago. As the Earth took shape gravitation pulled some of the denser pieces to the core. As the process continued it created tremendous amounts of heat which caused less dense material to rise and form layers. Therefore the core is a dense solid mass of almost pure iron. The heat from inside the Earth is the result of leftover heat from the process of "accretion" during which the Earth absorbed the orbital energy of colliding/sticking bodies (like asteroids) and from the decay of radioactive elements like uranium, thorium, and a heavy isotope of potassium. Thus half of the Earth's internal heat supply comes from radioactive decay and half comes from primordial heat left over from the planet's formation. As the Earth assumed a shape closer to what it is today it was covered by a low-density crust, oceans, and an atmosphere. 7
What is plate tectonics?
This theory was first suggested by Alfed Wegener a German meteorologist in his 1915 book
: The Origin of Continents and Oceans
. In his book Wegener discussed his puzzlement concerning the fact that he had collected similar fossils and rocks on different continents. How could this be unless all of the continents had once been joined together in a supercontinent he called Pangaea. His speculation was not new because Sir Francis Bacon had written in 1620 that it was obviously apparent from looking at maps that the South American coast line fit side by side with the coast of Africa. Despite these curious observations this theory of continental drift was deemed crazy primarily because Wegener could not account for how the continents could break off and slide away from each other. Many years later when studies showed that a portion of the ocean floor was spreading, Wegener's ideas didn't sound so foolish. By 1960 scientists began to put together the "plate tectonic theory".8
As previously stated the Earth's outer surface is covered with a thin crust. This crust lies on top of the asthenosphere which has been compared to a plastic like material. By plastic it is not meant to conjure up the idea of a rigid material but something almost of the order of silly putty which is solid but pliable. The idea of mantle convection - that there are convection cells of hot rock circulating beneath the Earth's crust and causing the crust to split and move about - was first proposed by an English geologist named Arthur Holmes around 1929.
Plate tectonic theory says Earth's crust is broken into 12 major plates all moving relative to each other. The 12 major plates are the result of the stiff lithosphere cracking as it slips and slides on top of the putty-like asthenosphere. The rising and falling (convection) of mantle rock helps to propel the plates. The example which is most of used is comparing the crust and lithosphere to the shell of an egg. If it is cracked it breaks into small pieces or plates but it sits on the moving white of the egg which most resembles the asthenosphere. Most of the activity whether earthquakes or volcanism occur at the edge of the boundaries of the plates.
How do cooling and convection inside the Earth lead to Earthquakes?
We have already discussed the idea of convection, which is the rising of heated fluid and the falling of cooled fluid. Within the Earth the process takes place in the asthenosphere. The explanation of this movement is referred to as Mantle Convection Theory. The theory states that within Earth's mantle there is rock rising and falling in a circular fashion. The convection currents result from different temperatures in mantle materials. These hot upwellings and cold downwellings avoid each other and set up a pattern. At first it was believed that this material rises in the asthenosphere and moves laterally across under the lithosphere moving the plates like they are on a conveyor belt. It seemed that the spreading seafloor was what set the plates in continual motion. The older heavier edge of the plate on the other side would sink or "subduct" as the ocean floor spread apart. Now scientists believe the reverse seems to be the catalyst for the plates moving. The "subducting" edge of the plate sinks down and is gradually absorbed back into the mantle dragging along the rest of the plate.
While the exact cause of plate movement cannot be positively certain there is no doubt that the plates often bump into one another and move in different directions. It is from this bumping and jostling that earthquakes occur. There are three different kinds of plate boundaries: convergent, divergent, and transform. Most geological activity takes place on these boundaries all over the world.
Convergent boundaries are places where the boundaries of plates are moving toward each other and bump or crash into each other resulting in the destruction of the Earth's crust. Here is where the greatest earthquakes in the world occur. If two continental plates collide the edges of the plates will buckle and form mountains. If an oceanic plate and a continental plate crash the dense oceanic plate will dip under the edge of the other. This process is called subduction. An example of this can be seen along the coast of South America where the oceanic Nazca Plate crashed into and is diving beneath South America. The result of this crash was the formation of the Andes Mountains.
Remember that oceanic plates will subduct (be drawn down) into the mantle because of their greater density but a continent cannot subduct because of its lighter density. Thus oceanic plates will recycle themselves by returning to the molten asthenosphere while continents are never destroyed but will reconfigure themselves as they move about and slam into other continents.
Divergent boundaries are places where the plates are moving apart and new crust is being created. As the plates separate the block between them usually sinks into the softer plastic like interior of the asthenosphere. The falling land forms a valley or rift. Then magma seeps up and fills in the cracks. This is what is happening between the South American plate and the African plate. As the plates separate new crust is forming along these faults. When it was first noticed that a new sea-floor was being made scientist wondered if the Earth was going to get larger. That didn't happen because as the new sea-floor is created the old is subducted back down into the mantle where it is recycled.
Transform boundaries are places where plates slide by each other. The plates slide by each other in a horizontal motion and because the edges of the plates are usually uneven there is the potential for large eruptions such as that occurring near the San Andreas Fault in California. One of the strongest earthquakes along this fault was the 1906 San Francisco Earthquake.
How do cooling and convection inside the Earth lead to Volcanoes?
Most of the Earth's volcanoes form where the ocean plates are separating from each other (divergent boundaries). These spreading center volcanoes are caused when the oceanic plates are pulled back and oceanic rifts occur. We can see this in the case of the Mid-Atlantic ridge where the sea floor is spreading apart. Over 80% of the Earth's magma comes out of the Earth in the oceans. As the plate pulls apart some of the asthenosphere liquefies and rises to fill in the gap.9
The classic volcanic cone which we are use to seeing occurs in 7% to 13% of the time at a subduction zone.10 Just the process of the plate diving back into the mantle involves a tremendous amount of energy which results in Earth's greatest earthquakes. While these volcanoes, such as Vesuvius, are spectacular, they produce far less magma than the oceanic volcanoes.
Transform faults in which the boundaries of plates slide past each other have little or no association with volcanism. The horizontal sliding does not allow a rift to develop where magma could be released.
Sometimes volcanoes occur in the middle of a plate. To account for these volcanoes scientist have suggested the plume theory. In this theory a hot narrow plume of mantle rises from the very hot core-mantle boundary and when it reaches the surface it forms what is called a hot spot. At the hotspot, mantle rock melts when it gets close to the surface (where pressure is lower and it's easier to melt). The melted rock or magma will then erupt and build a volcano. If the eruptions take place on a moving plate it will develop a long line of volcanic islands like the Hawaiian Islands.
An eruption can last a few minutes, hours, or even days. The eruption can consist of oozing lava, or perhaps a spectacular explosion. This all depends on the type of magma that the volcano extrudes. Magma can have different viscosity or resistance to flow depending on its content of silica, and also depending on its temperature. In other words high viscosity means the lava is thick and may trap gases and build up pressure leading to a dangerous explosion. Low viscosity means that the lava is thin and can flow freely without trapping rocks or gases; an eruption with this sort of lava will be calmer.
There are a number of types of eruptions. An eruption of lava with high viscosity will be very damaging resulting in what are called a Vulcanian or Plinian type of eruption. A low viscosity eruption will result in a calmer event which would be either Icelandic, Hawaiian, or Strombolian.
There are a number of types of volcanoes but I have chosen to limit discussion to 3 main types based on their eruption patterns and their general form. Their eruption pattern is determined largely by the type of magma (called lava if it reaches the surface) they produce. They are the, Shield Volcano, Scoria Cone, and Stratovolcano.
The Shield volcano has very gentle slopes that convex upward. They are broad, low profile volcanoes. Their shape comes from the fact that they are built by Icelandic or Hawaiian type (low viscosity) eruptions (Mauna Loa, Haleakala).
The Scoria cone has straight sides with steep slopes and a large summit crater. They are also called cinder cones and are the most common type of volcano. They are developed as a result of Strombolian-type eruptions where the magma that is released is of medium viscosity. (Paricutin Volcano and Stromboli Volcano)
The Stratovolcano has gentle lower slopes but the upper portion usually rises steeply with a small summit crater. They are also known as composite cones. They are the most picturesque and most deadly volcanoes and are associated with explosive plinian eruptions. Their lava is high-viscosity so it tends to solidify and form a protective cap. (Mt. Fuji, Mt. St. Helens, and Mt .Kilimanjaro)