The Dynamic Earth

After the Great Bombardment ended and differentiation took place, the Earth cooled and took much the same form as we see it today. However, there is much evidence that, over a long time scale, the Earth is still changing and that there are strong forces still at work within the Earth. This evidence is in several forms:

• Mountain ranges: From dating of rocks, it is estimated that the Earth is about 4.5 billion years old. This presents a paradox of sorts concerning how mountain ranges persist, given that reasonable estimates suggest that water erosion will wear down a typical mountain in about 200 million years. How do mountains form?    
• Volcanoes and Earthquakes: The presence of volcanoes and earthquakes within the Earth suggest that powerful forces are at work.  
• Continental Drift: Qualitatively, it appears from the shape of the continents that at one time in the distant past that the continents were joined, and subsequently drifted apart.

  
  

   

  Figure 14.3: Continental drift

  
  
  

  This drift has subsequently been quantitatively measured by comparing data taken from astronomical data gathered from radio telescopes on different continents; by this means, North America and Europe are found to be drifting apart at a rate of about 5 cm per year. What forces would lead to such a drift?

• Ocean floors: Rather than being flat plains eroded by millions of years of water action, the ocean floors exhibit many of the same features, such as valleys and mountains, that occur on land.  
• Magnetic reversals: On the ocean floor are ridges from which molten magma comes out from deep within the Earth. Within this magma is magnetite, which is a magnetic rock whose atoms subsequently align themselves with the Earth's magnetic field. The magnetic alignment of this magnetite this provides a snapshot of the direction that the Earth's magnetic field was pointing at the time the magnetite hardened. There are places in the ocean floor where adjacent layers of magnetite are aligned in opposite directions, indicating that the Earth's magnetic field reversed itself. In fact, it is estimated that over 300 such reversals have occured over the Earth's history. What forces would cause the Earth's magnetic poles to reverse?
• Rock dating: Like Carbon-14 used for dating of once-living artifacts, other radioactive isotopes with longer half-lives can be used to date rocks. It is found that the age of rocks range from the very old (3-4 billion years old) to the very young (3-4 million years old). Thus, new rocks must be forming.