The Villarica volcano in southern Chile, one of South America’s most active volcanos, changes the earth’s surface by expelling molten lava from the earth’s interior.

For a long time, geology was considered an art and not a science, the reason being that geology was mainly a descriptive activity. It lacked any meaningful quantitative approach and did not have a unifying theory that would tie together the various geologic processes. This changed in the mid-1960s with the acceptance of the theory of plate tectonics as well as various quantitative techniques.

Professor Edgar J. MCullough, a retired member of the University of Arizona Department of Geosciences, will present the fourth of four lectures on the Evolution of the Earth in the Great Room of the Arizona Senior Academy, at 2:30 p.m. Thursday (Sept.7).

In the Sept. 21 session, McCullough will discuss geologic history. In that lecture, McCullough is expected to describe how geologists are able to talk about things that have happened in the past 4.6 billion years.  For example, how do we know when and where mountain building has taken place; where oceans have covered large portions of continental areas; why we have “missing” time.

Edgar J. McCullough

The energy for the constructive processes comes from the interior of the Earth. This energy is thought to owe its origin to the initial compaction of the Earth, meteorite impacts on the forming earth and/or decay of radioactive minerals over time. Chemical changes occur over time that result in the formation of rocks like granite and the development of continents. The processes include volcanism, earthquakes, deformation and mountain building.

McCullough began teaching at the UA in the 1960s later became head of the UA Geosciences Department and dean of the College of Science.

Written by Charles Prewitt, Academy Village Volunteer

More Info on attending an event
Academy Village is an active-adult community located off Old Spanish Trail six miles southeast of Saguaro National Park East. Its residents support the Arizona Senior Academy, a non-profit charitable organization whose mission includes offering free concerts and lectures to the public.

These events are held in the Great Room of The ASA Building adjacent to the Academy Village Community Center. Due to the popularity of cultural events, non-residents who wish to ensure priority seating are advised to make reservations by email at or by phone at (520) 647-0980. To learn more about the Academy, go to

Parking for visitors is in the lot behind the Community Center. All parking spaces in front of the Academy building are reserved.

How Geology Helps Interpret Earth’s History: September 2017
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