by Kari Hetcher and Scott Hughes

Introduction: A Planetary Perspective

Imagine an interstellar space traveler finding her way to our solar system. She approaches a medium sized, mid-life star, and counts eight planets (or is that nine?) rotating around it, plus a large number of moons rotating around the larger planets. The planetary bodies are of varying colors and sizes, but one strikes her as different in some way. It is a rather small rocky planet, the third one out from that star. It looks blue, green, and white from space, is continually changing, and its dark side twinkles with thousands of faint pinpoints of light. How unique and beautiful that planet is! The traveler has found the original home of Earthlings.

This incredible scenario of a space traveler conjures mental images of our own fascination with far-away mystical places. Indeed, the story may be fiction, yet we envision that it could be true someday -- to have our home planet rediscovered as a land of beauty, a unique dynamic world where internal and surficial processes are diverse, like so many other planetary bodies in the universe!

View two images of our home, planet Earth: (NOTE: One image is a photograph from space provided by NASA; the other one is a computer image modeled from global topography provided by NOAA.)

What geologic features can you immediately recognize from these images?

Check out more images at the JSC Digital Image Collection website.

For updated information on NASA programs and activities, visit NASA News and Features.

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Internal and External (Surficial) Planetary Processes

The two pictures below illustrate internal AND external Earth processes. On the left is South Sister volcano in Oregon, which may erupt again someday in the not-too-distant future. The right picture is Mount St. Helens volcano in Washington viewed south into the well-known amphitheater crater created during an explosive eruption in 1980.  Both volcanoes are composite cones in the Cascade Range, a roughly north-south string of volcanoes parallel to the west coast of North America.  These volcanoes belong to a continental magmatic arc, which is the surface manifestation of a subduction zone caused by converging oceanic and continental lithospheric plates.

A lot can be learned about planetary geology from images such as these. Learn to recognize processes, and relate them to mechanisms that are responsible.  What is different about the evolutionary stages of these two volcanoes?  What can be inferred by their morphologic features?  What can be inferred about their relative surface ages?  Even the clouds tell you something about how these mountains evolve through time.

• Examples of Internal Processes: subduction, magma genesis, magma ascent, heat flow, dome growth, explosive volcanism.
• Examples of Surficial Processes: eolian deposition(wind-carried ash and other tephra deposits), glaciation, stream erosion, chemical and physical weathering.

Much of what we know about the Earth as part of a planetary system has been discovered only in the past few centuries. We've come a long way from imagining the Earth as the center of the universe to understanding its meager position in the solar system. The pictures above of two Cascades volcanoes reminds us that we live on a dynamic planet. It is very much alive with internal and external activity. These mountains are composite volcanoes that rise above a platform of older volcanic rocks. They are part of a magmatic arc that is the surface manifestation of internal processes occurring many kilometers deep where crust and mantle interact with each other and higher temperatures and pressures prevail.

Click on the image to the right to see or download a rather large image of the Pacific Ocean topography from the NOAA National Geophysical Data Center.

There is no way that everything we know about the workings of our planet can be covered in this course, but think about the following topics and how they fit into our understanding of the formation of the Earth:

We will use examples from Earth in subsequent modules to evaluate these processes on other planetary bodies.

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Birth Of The Earth

Most of what we know about the Earth's interior is derived from studies of rocks at the surface. Internal variations in compositional and physical properties are inferred from studies of other planets, seismic waves, and meteorites.

Composition refers to mineralogy, i.e. the types and abundances of minerals (which are used to classify rock types) and rock chemistry, i.e. the abundances of major-, minor-, and trace-elements, as well as certain compounds, such as H₂O and CO₂). Mineral and chemical compositions change from one region in the Earth to another.

Physical Properties = density, rheology (response to stresses), and states of matter (solid, liquid, gas). Physical properties also change from one region to another, although the changes do not always coincide with compositional changes.

Most geologists and geochemists believe that the bulk composition of the Earth is approximately chondritic. Planetary bodies in the Solar System accreted from, and are composed of, primordial Solar Nebula material represented by carbonaceous chondrites, which are meteorites comprised of early condensed solar grains and dust. Using isotopic dating (radiometric) techniques, geoscientists have determined the time since meteorites formed in the cooling solar nebula. These techniques yield an age of the Earth around 4.6 billion years old (4.6 x 10⁹ years). In geochronological terms, this would be 4.6 Ga.

During the formation of the Solar System, the inner terrestrial planets formed in the hotter region close to the Sun, while giant gaseous Jovian planets formed in a cooler part of the solar nebula. Refer to Module Two for details. Earth’s average distance from the Sun (1 AU) is about 150 million kilometers (93 million miles), a distance favorable for terrestrial planet formation as well as for condensation of water-bearing minerals.

Accretion of chondritic material into the early planet Earth was followed by internal heating which caused differentiation. Light silicate minerals, such as feldspars and quartz, separated from the heavier metallic minerals and rose to the surface of the planet, eventually forming the continental crust and mantle. The denser materials sank to the center of the planet to form the iron-rich core. This scenario is much like that of the other inner planets which we believe are similarly differentiated.

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Plate Tectonics

Answer the questions at the end of this section after visiting several Earth websites and reading about the interior of the Earth in these sites, your textbook and other available resources. You may want to locate an introductory geology textbook if you are not familiar with some of the terms and concepts in this module.

Many of the older geology professors today did not study plate tectonics in their college years because the idea was not yet accepted widely. By the same token, some of the processes we use to explain planetary features may be obsolete in a few years. However, this does not prevent us from exploring all possibilities, keeping our minds open, and learning what comprises our planetary system. To this end we use our home planet as a means of understanding planetary processes, albeit many are not easily recognized on other planetary bodies.

Our knowledge has grown considerably in the past three decades, and it is continually growing and changing. Much of what we know about Earth came from our travels to Earth orbit, and to the Moon during the Apollo missions. The photo of the Tibetan Plateau on Earth, taken from space, illustrates how geology as a planetary science suddenly emerged with space travel.

In 1915, Alfred Wegner, a German meteorologist and geophysicist published the book "Origin of Continents and Oceans" in which he proposed the idea of Continental Drift. His evidence included (1) the puzzle-like fit of South America and Africa, (2) evidence of former glaciers in the hot land of Australia, (3) similar fossils found on different continents, and, (4) similar rock structures and mountain ranges on different parts of the world.

Wegner proposed that a supercontinent called Pangaea (all land) once contained all the continents we now know, and that they began to split apart about 200 million years ago.

This idea was not well received. Who could imagine the continents floating along the planet? What could possibly cause this drift? In the past, people had wondered why there were mountains and huge oceans. Some folks thought that the Earth was once molten, so when it cooled it contracted and shrank leaving continents separated by gigantic chasms. The outer skin of the Earth responded like a piece of dried fruit, folding up and down to accommodate this shrinking. When you think about it, this was a pretty imaginative idea! Wegner also had to contend with strong opposition from religious leaders and the belief that the world was created in 4004 B.C., a date derived by Charles Usher from Biblical accounts of family trees.

Not until the 1950's and 60's did the theory of Plate Tectonics start to become widely believed and accepted. Detailed mapping of the ocean floor showed raised relief ridges that we know to be where oceanic crust is undergoing seafloor spreading. Work by Hess and Vine on magnetic 'stripes' of the ocean floor added credence to this idea by illustrating how magnetic polar reversals are recorded in basalts. Many concepts in geology now fit within the paradigm of plate tectonics. Thus, we have much evidence to show that continents moved around the planet, slowly but surely, slipping along three main types of plate boundaries.

 

Plate Boundaries – Review Figures 8 and 9, p. 118 in the textbook.

 

This figure, from the NASA Goddard Space Flight Center, shows plate boundaries in light blue, measured earthquakes in yellow dots, and recently active volcanoes in red triangles. Study it well in order to make associations of various tectonic regions with the features present in them.

• Study the age relations of oceanic crust and the Mid Atlantic Ridge and the Pacific Ocean floor of the Pacific from the National Geophysical Data Center of NOAA (National Oceanic and Atmospheric Administration).
• Geomorphology from Space is an online geomorphology book created by NASA. Many of the following examples can be looked up here using the index to lookup specific places.
• Divergent Boundary: An area where two lithospheric plates are separating, allowing magma to rise and form new oceanic crust. An example of a divergent boundary, or spreading center, is the Mid Atlantic Ridge. Here, the ocean floor is moving away from an area of upwelling mantle, producing a warm, less dense ridge that appears to be an underwater mountain range. Upwelling of the mantle and rise of overlying lithosphere is caused by large slowly-moving convection currents in the mantle. Divergent boundaries are found in all levels of maturity, from a juvenile East African Rift  to the Red Sea to full ocean basins around the world. Visit Understanding Plate Motions, a chapter from This Dynamic Earth, an online book from the United States Geological Survey, for an excellent overview of plate motions.
• Convergent Boundary: At a convergent boundary, two plates are being pushed or pulled together. There are two sub-types of convergence which lead to either a subduction zone and volcanism or a crustal orogeny (mountain building event).

• At a subduction zone, cool and dense ocean crust comes in contact with less dense continental crust. The ocean crust is heavier, so when the two plates exert force in opposite directions, the ocean crust begins to subduct, or sink below the continental crust. This type of boundary forms the Cascades of North America and the Andes of South America.
• The Nazca Plate is subducting beneath the South American Plate to form the high volcanic mountains of the Andes.
• If both plates are continental crust, neither will give way and sink beneath the other, so they will form huge mountains as the continent crust deforms. This type of convergence occurred on the East Coast of North America during the formation of the Appalachian Mountains during the Ordovician and Devonian time periods. A current example of continent-continent convergence is the Himalayan Mountains.

• More Himalayan pictures can be found at Geomorphology from Space.  The subcontinent of India is riding the Indian-Australian Plate which is colliding with the Eurasian Plate, pushing up the highest mountain range in the world like a hallway rug pushed up against the wall.
• Transform Boundary: A plate margin where neither plate is subducted or pushed up; they simply slide past each other. Another name for this type of boundary is a strike-slip zone. The most famous example of this type is the San Andreas Fault in California. There are two types of strike-slip, or transform, faults: left lateral and right lateral. You can tell which one you are looking at by imagining yourself standing on one side of the fault looking across to the other side. Whichever direction the other side is moving is the type of strike-slip fault.

TASK 1: Answer the Study Questions

• What are the major layers of Earth's interior? What evidence is used to make this interpretation? [Earth Figure (same as Figure 4, Chapter 9, in your textbook)]
• Of what significance is the mantle to Earth's evolution? Write your answer in terms of bulk mineralogy, bulk chemical composition, layering, proportion of Earth's mass, temperature gradient, and pressure gradient.
• What is the lithosphere? What is the aesthenosphere? What is the difference between these two?
• How does oceanic lithosphere differ from continental lithosphere?
• What causes true polar wander and apparent polar wander?
• What kind(s) of faults would you expect to see at a convergent plate boundary?  A divergent boundary?  A transform boundary?   Would there be only one kind of fault?  Why?
• Which type of fault is the San Andreas, right or left lateral?
• What is the age range of the ocean floor?
• Give definitions for each of the following components of the Earth's interior. (see Table 1, p. 114 in your textbook): continental crust, oceanic crust, upper mantle, transition mantle, lower mantle, outer core, inner core.

HELPFUL LINKS

This Dynamic Earth: The Story of Plate Tectonics from the United States Geological Survey,an online book by Jacquelyne Kious and Robert I. Tilling

An Introduction to The ABC's of Plate Tectonic by Donald L. Blanchard

Plate Tectonics: Chemical and physical layers of the Earth, historical development of the theory, and descriptions of the location and types of plate boundaries from Volcano World at the University of North Dakota.

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Volcanism