Looking At Crusts
From creation to destruction – and the underlying processes of heat.
When you start looking at our Earth, or our moon, our solar system's rocky planets, or our dwarf planets, even certain asteroids, you first notice the outermost solid shells – their crusts.
Here's pictures of a few of them.
Figure 1. "The Blue Marble" is the famous photograph of the Earth taken on December 7, 1972 by the crew of the Apollo 17 spacecraft. In it, you can see Africa, Antarctica, and the Arabian Peninsula.
Figure 2. Full Moon view from Earth in Belgium (Hamois).
Figure 3. Full color image of from first MESSENGER flyby, 30 January 2008.
The Moon - Io
Figure 4. NASA's Galileo spacecraft acquired its highest resolution images of Jupiter's moon Io on 3 July 1999 during its closest pass to Io since orbit insertion in late 1995.
The Differentiated Asteroid - Vesta
Figure 5. Vesta is the second-most massive body in the asteroid belt, though only 28% as massive as Ceres. It has a differentiated interior, and is similar to 2 Pallas in volume but about 25% more massive.
All of these planetary bodies have different surface features – but they have the same process that forms them.
This process separates the components that make up these planetary bodies. It creates a core and mantle, and sometimes a chemically distinct crust on top.
But this all depends upon having enough heat. Heat to melt the rocks and minerals so they flow and separate themselves in layers.
So, how did this happen?
A Side Trip – Differentiation
When the Sun ignited in the solar nebula, hydrogen, helium and other volatile materials were evaporated in the area near the Sun. The solar wind streaming away from the sun as well as its light pressure forced the dust and other low-density material away.
These pressures also stripped early atmospheres from the growing rocky bodies, but those bodies continued to accumulate material, and they continued to build toward protoplanets.
Figure 6. In this illustration, the rocky inner planets form from our sun's protoplanetary disk, picking up dust, stones, and other materials as they circle the growing star.
The elements of heat
In time, these pre-planets accumulated higher concentrations of radioactive elements which produced heat.
Many of the accumulating material also produced heat due to the velocity and impact pressures produced as they collided with these planetary bodies.
And another factor began to weigh in; as these bodies grew larger, with their increasing size, such growth produced increasing gravitational pressure which provided even more heat.
All of these elements melted parts of these early planetary bodies, and in the melted zones, the denser materials sank, while the lighter materials rose.
Computer modeling of this process shows us that many proto planetary bodies were hot enough to melt and boil off the ices, then to begin melting minerals together.
Eventually, iron was reduced to metallic form and sank to the cores, further releasing more gravitational energy and melting the silicate rocks even further.
In the end of the process, we find an outer crust, lighter material in the mantle, and the heaviest elements at the core – sometimes having an outer liquid core and a solid inner core.
Here's an ongoing example, a cut away of Mars.
The core is mostly made of iron with some possible lighter elements such as sulfur. The mantle is the darker material between the core and the thin crust.
Figure 7. Analyzing three years of radio tracking data from the Mars Global Surveyor spacecraft, researchers at NASA's Jet Propulsion Laboratory concluded that Mars has not cooled to a completely solid iron core.
Our Crust In Motion
As we have seen, our Earth and the rest of our solar system, was formed by accretion from a rotating disk of dust and gas.
The immense amount of heat energy released from gravitational energy and from the decay of radioactive elements melted our entire planet, and it, like Mars, is still cooling off today.
Denser materials like iron (Fe) sank into the core of the Earth, while lighter silicates (Si), other oxygen (O) compounds, and water (H2O) rose near the surface.
Figure 8. Note the terms Lithosphere and Asthenosphere. These top layers play key roles in the crust of our Earth – from earthquakes to volcanoes.
From the center of our Earth to its surface, let's look at this more closely.
As you can see, the earth is divided into four main layers:
The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulphur (S).
The inner core is under such extreme pressure that it remains solid.
Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) – known as silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner.
The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle, so it fractures from the pressures of the hot, plastic, moving mantle, which then produces earthquakes.
A Closer Look at the Mantle
The release of heat from the Earth's core creates convection currents in the mantle.
This works very much like a lava lamp (hence its name). Let's take a look before we go on.
The light bulb at the base of the lamp heats up wax droplets, which expand in volume. When the wax expands, its density decreases, and the wax rises.
When at the top of the lamp, and farther away from the heat, the wax cools. Cooling means that the volume of the wax contracts – which increases its density - and causes it to sink.
This is how convection currents in the mantle work.
Convection and the release of heat from the Earth's core drives convection cells in the mantle. Convection in the mantle moves areas of the crust, called plates.
It is possible to use sesmic waves traveling through the mantle to map out this convection, much like a hospital CAT scan can map out bones and organs with x-rays.
Here's an illustration of this.
Figure 10. This image was developed in the Havard Univ. Seismology Lab.
The blue blobs show where colder, denser material is sinking into the mantle.
Here's another illustration from the lab, this time of the hot, rising mantle.
Figure 11. This image was also developed in the Havard Univ. Seismology Lab.
The red blobs are warmer plumes of less dense material, rising principally into the ocean-ridge spreading centers.
The study of these convection currents of the mantle is called plate tectonics.
Let's return to the illustration of the structures of the Earth.
Figure 12. Note the top layers of the Earth.
The crustal plates consist of the outer layers of the Earth, the lithosphere, which is cool enough to behave as a more or less rigid shell and the hot asthenosphere.
The two of these, powered by the heat of the core, are part of all earthquakes and volcanoes.
- The lithosphere has the strength and the brittle behavior to fracture into a number of “plates.”
There are two types of plates: continental plates comprised mostly of granitic-type rock rich in silica and aluminum and oceanic plates comprised mostly of basaltic-type rocks rich in magnesium and aluminum.
Earthquake and volcanic activity is common along the boundaries of these plates.
Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot. The satellite image below shows the volcanic islands of the Galapagos hotspot.
Figure 13. Image by NASA.
Seahorse-shaped Isabella and more rounded Fernandina are volcanic islands generated by a mantle hotspot offshore from Ecuador. The hotspot rises at the junction of the Cocos oceanic plate on the north and the Nazca plate on the south and east.
In 1963, J. Tuzo Wilson, a Canadian geophysicist, noted that in certain locations around the world, such as the Galapagos, volcanism has been active for very long periods of time. This could only happen, he reasoned, if relatively small, long-lasting, and exceptionally hot regions -- called hotspots -- existed below the plates that would provide localized sources of high heat energy (thermal plumes) to sustain volcanism.
Besides this addition to plate tectonics, here's an illustration, a map of the current arrangement of the tectonic plates scattered around the Earth’s crust.
As you can see, the red lines show each tectonic plate - slabs of rock which form the Earth’s crust and float on the mantle.
Because they float on the mantle, they move (only inches per year), and depending on the direction of that movement, they collide, forming deep ocean trenches, mountains, volcanoes, and generate earthquakes.
The continental and oceanic plates are different. A continental plate is less dense compared to an oceanic plate. It also cannot sink into the mantle. The continental plates form the land while the oceanic plate forms the sea bed.
Besides this, the red lines also show the boundaries between the plates, and these boundaries have distinct features.
There are several types of plate boundaries.
Here the crust is destroyed and recycled back into the interior of the Earth as one plate dives under another. These are known as Subduction Zones - mountains and volcanoes are often found where plates converge.
When an oceanic plate pushes into and subducts under a continental plate, the overriding continental plate is lifted up and a mountain range is created. Even though the oceanic plate as a whole sinks smoothly and continuously into the subduction trench, the deepest part of the subducting plate breaks into smaller pieces.
These smaller pieces become locked in place for long periods of time before moving suddenly and generating large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters.
When two oceanic plates converge one is usually subducted under the other and in the process a deep oceanic trench is formed. The Marianas Trench, for example, is a deep trench created as the result of the Phillipine Plate subducting under the Pacific Plate.
Oceanic-oceanic plate convergence also results in the formation of undersea volcanoes. Over millions of years, however, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs.
Divergent Boundary also known as a Constructive Boundary
At divergent boundaries new crust is created as two or more plates pull away from each other.
Oceans are born and grow wider where plates diverge or pull apart.
As seen below, when a diverging boundary occurs on land a "rift," or separation will arise and over time that mass of land will break apart into distinct land masses and the surrounding water will fill the space between them.
Iceland is splitting along the Mid-Atlantic Ridge -- a divergent boundary between the North American and Eurasian Plates. As North America moves westward and Eurasia eastward, new crust is created on both sides of the diverging boundary. While the creation of new crust adds mass to Iceland on both sides of the boundary, it also creates a rift along the boundary. Iceland will inevitably break apart into two separate land masses at some point in the future, as the Atlantic waters eventually rush in to fill the widening and deepening space between.
This is when two tectonic plates move in different directions or at different speeds. Plates are locked together by friction while pressure builds up. Finally, the plate breaks along the fault line.
Most transform faults are found on the ocean floor. They commonly offset active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes.
A few, however, occur on land. The San Andreas fault zone in California is a transform fault.
Collision Boundary (Continental-continental Convergence)
This is when two continental plates move together. The two plates are too light to sink into the mantle. Therefore, the plates buckle up and form mountains.
The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights.
So, this is how our Earth changes the shape of its landmasses. It is another indication of how old our planet is, that through slow movements of crustal plates and through billions of years our landmasses have moved thousands of miles.
Plate tectonics is a basic process of our world, but can we find evidence of this on other planets?
Plate Tectonics on Mars
Scientists first found evidence of plate tectonics on Mars in 1999. Unfortunately, this evidence was done with the Mars Global Surveyor's magnetometer, which only covered only one region in the Southern Hemisphere.
The residual ice cap of Mars' south pole (in white) tops the smooth layered deposits that overlie the cratered southern highlands.
NASA/MOLA Science Team
However, a new high resolution magnetic field map has been compiled, covering the entire surface of Mars, and is based on four years of data taken in a constant orbit. Each region on the surface has been sampled many times, and all of the information gathered lends support to and expands on the 1999 results.
Plate tectonics is alive and well on Mars.
But what about other planetary bodies?
Well, the answer is yes: Tectonic processes on other planets and moons in our solar system do occur to varying degrees – but nothing like the plate tectonics as on Earth.
Moon and Mercury
Impact craters form when meteors crash into the surface. When the solar system was young, there was lots of debris to impact planets. With less debris, fewer impact craters now form.
What we have learned from this, is that planets or moons with few impact craters have had significant tectonic activity. And the reverse: Worlds saturated with impact craters have had little tectonic activity reshaping their surfaces.
Both Mercury and Earth's Moon have many impact craters, so their tectonic activity has been limited. These small worlds cooled quickly. (Think about a cup of coffee cooling faster than the larger pot.) Cool planetary interiors lack the heat needed to power tectonic activity.
Figure 21. The Earth-Moon system is a rarity in the universe.
The lunar maria, which are large smooth areas, formed from tectonic activity billions of years ago. Large impacts from space formed large basins which filled in as lava oozed from the interior. The Moon also has rilles which were once lava rivers.
Mercury has no maria, but billions of years ago lava flows formed intercrater plains. Mercury also has cliffs, called scarps, that are hundreds of kilometers long. Scarps may have formed when Mercury's crust shrank and buckled as its interior cooled.
Figure 22. Mercury – Close Up, NASA
Venus shows considerable tectonic activity. It has about 1600 large volcanoes, and perhaps hundreds of thousands of smaller volcanic features. About 80% of the surface is covered with solidified lava flows. The larger volcanoes are often shield volcanoes, similar to Earth's, which form from lava flowing up through the volcano's center.
Venus has many smaller volcanoes called pancake or dome volcanoes – And all of these volcanic structures form when lava flows up from the mantle and swells the crust.
Figure 23. Venus (in back) vs Mars (in front), NASA.
Currently, Astronomers do not know if Venus still has active volcanoes because thick clouds obscure the surface. However geologic evidence suggests that tectonic activity shaped the surface of Venus as recently as a few hundred million years ago.
The Galileo mission observed about 60 active volcanoes on Jupiter's closest large moon, Io. And you can see some of them in the photo above. However, Io does not have tectonic plates so the volcanoes are randomly distributed.
Europa, Jupiter's second major moon, has an icy crust surrounding a water mantle. The icy crust has many cracks exhibiting an unusual type of tectonic activity.
Figure 24. Europa's Surface: Voyager Project, JPL, NASA,
Copyright Calvin J. Hamilton
Tidal forces from nearby Jupiter supply the heat needed to cause tectonic activity on these moons.
The asteroid belt is the region of our Solar System located roughly between the orbits of Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets.
Asteroids can range in diameter from a few centimeters to a thousand kilometers. Every so often, they shed off fragments of rock, which sometimes fall to earth as meteorites.
And from this, scientists have recently unearthed two meteorites in the Antarctic that expand our knowledge on how asteroids were formed.
As we talked about, planets like Earth have a molten core with plates of crust that float on the mantle and can thus move. These tectonic plates rub together, create friction, cause earthquakes and are connected to forming volcanoes.
Certain types of rocks created by heat, known as igneous rocks, turn up in these areas. One type of igneous rock is called andesite, and until now, the only place it was known in the solar system was on Earth. But andesite has turned up in two relatively new meteorites – and researchers have seen nothing like them before.
Andesites were only thought to form in relatively large planets, since they require large-scale processes like plate tectonics to cook them up.
Since asteroids are small, researchers thought the new meteorites might have come from another planet, or even from the moon. But analysis of the oxygen molecules in the rocks showed they must have come from an asteroid.
Figure 25. The asteroid Gaspara © NASA.
The team also discovered the rocks were more than 4.5 billion years old – meaning they formed just after the birth of our solar system.
The researchers think the parent asteroid that produced the meteorites was larger than 100 kilometers in diameter, which could hold enough heat to melt some of the rocks within it. But not big enough to have large-scale plate tectonics.
Researchers believe melted rocks within the asteroid bubbled up to the surface, solidifying and forming andesites, which then broke off to make the meteorites.
And this is an important point.
Plate tectonics isn’t the only way to make such rocks like adesites, nor the only way to produce rock melting heat. (But that's an impact story for another time.)
Here's a link to an excellent site: Plate Tectonics:
Copyright 1996 - 2005 platetectonics.com. All rights reserved.
Here's a wonderful cross section illustration of the main types of plate boundaries - from the U.S. Geological Survey:
Here's a good reference dictionary:
Here's an extensive site about Asteroids. But please overlook the wild and seemingly, at times, strange ads posted:
Here's a geology glossary:
Figures & Acknowledgments
We want to thank all of the wonderful sites which helped illustrate this discussion, and we wish that all of you visit them as part of your reading. We are humbled by the intelligence and grace of our science communities.
Figure 1. en.wikipedia.org
Figure 2. en.wikipedia.org
Figure 3. en.wikipedia.org
Figure 4. en.wikipedia.org
Figure 5. solarsystem.nasa.gov
Figure 6. www.solstation.com
Figure 7. www.solstation.com
Figure 8. www.uwsp.edu
Figure 9. pubs.usgs.gov
Figure 10. www.seismo.unr.edu
Figure 11. www.seismo.unr.edu
Figure 12. www.uwsp.edu
Figure 13. Image by NASA. gosouthamerica.about.com
Figure 14. www.worldatlas.com
Figures 15 – 19. pubs.usgs.gov
Figure 20. www.aaas.org
Figure 21. www.theregister.co.uk
Figure 22. astronomy.nmsu.edu
Figure 23. marsprogram.jpl.nasa.gov
Figure 24. apod.nasa.gov
Figure 25. www.thenakedscientists.com