Getting started

• Go to the NAVDAT website and click on the "Animations" link.
• This exercise focuses on the "QuickTime Animation of magmatism in the western United States over the past 65 million years" (filename "WUS.mov"). It is the second animation from the top.
• Before watching the animation, read the brief explanatory text and be aware of the following:
  • While the NAVDAT database contains a very large number of samples (tens of thousands), it is not complete, nor is the sample coverage the same throughout North America. Presently, NAVDAT's coverage is relatively high for certain areas (e.g., eastern California and southwestern New Mexico), but relatively low in other areas (e.g., the Pacific Northwest).
  • The clustering of sample locations in an area on the animated map is solely related to the sample coverage in that area, and has no relationship to the volume of material erupted.
  • The NAVDAT animations map past volcanic activity relative to the present-day geography of North America, and do not take into account significant tectonic and paleographic changes that have affected the region over geologic time. For example, during the last ~30 million years, tectonic extension has significantly widened the Basin and Range province, and the San Andreas transform system has translated portions of westernmost California and Baja California considerable distances northward along the western margin of the North American Plate. Neither of these effects is seen in the NAVDAT animations.
    
    The brief introductory text next to the animation mentions the plate tectonic reconstructions of Tanya Atwater, a geology professor at UC-Santa Barbara. Back in the 1970's, Dr. Atwater realized that the pattern of magnetic anomalies in the Pacific Ocean could be used to reconstruct the geometry of the ancient Farallon Plate and the development of the San Andreas transform system as the Farallon-Pacific ridge interacted with western North America. To learn more, go here for Dr. Atwater's animation showing the plate tectonic history of western North America from 38 Ma to present. After seeing her animation, you will be able to recognize the various tectonic elements shown on the NAVDAT animation, such as the red ridge-transform system between the Pacific and Farallon Plates and the western North American plate boundary shown in purple.
• When the animation downloads and plays for the first time, pay particular attention to the legend for the symbols and the overall pattern of volcanism. The animated array of colored dots may appear random and have little meaning to you now, but after you work through the questions on the next page, you should have a better understanding of the Cenozoic volcanic history in this part of the world!