Smithsonian Journeys Dispatches

A Q&A with Expert Anthony Coates

Q: Tony, you will soon embark on a truly epic adventure with our Smithsonian travelers. Exploring remote lands by private jet is a thrilling and exciting way to experience the world. Travelers will be immersed in an array of cultures and they'll also have a chance to stand at or on some of the Earth's great geological wonders. Your first stop will be Machu Picchu. We know it is the "Lost City of the Incas," but can you tell us about the dramatic geology of the region? 

A: Machu Picchu lies in the heart of the central Andes, a region famous for the mining of precious metals, especially tin, copper, and silver. There was even an associated mining culture with its own literature and poetry. I will show how and why this came about. On a larger scale, I will trace the rise of the Andes over the last ten million years and show how that "tilted" South America more and more towards the east so that all the western rivers began to be ponded into a gigantic interior sea, called Pebus. This vast body of water finally drained eastward and evolved into the Amazon river system, equal in size to the entire United States. 

Q: Continuing to Easter Island, travelers will visit one of the most remote places on Earth. The island is known for the hauntingly beautiful and massive stone moai. What are the statues made of? And has Easter Island always been an island or was it once part of South America? 

A: Easter Island is a geological "hotspot," analogous to the Hawaian Islands but no longer active (the last explosion was 10,000 years ago). The moai were hewn from the basalt that forms the island which is only about 0.7 million years old. Hotspots occur when a "plume" of hot magma from the interior of the earth penetrates a moving tectonic plate. Easter Island is a point of penetration with a huge submarine volcanic mountain, 1.2 miles high, built up on the seafloor. Leading from Easter Island, a line of submerged extinct volcanoes forms a submarine ridge that gets older towards the east (called the Sala and Gomez Ridge), and traces the movement of the Nazca tectonic plate over the Easter Island hotspot. It runs for 1,300 miles until it disappears under Peru.

Q: I understand that the Great Barrier Reef is the world's largest single structure made by living organisms. It is made up of many species of hard and soft corals, plus the reef supports an infinite variety of marine life including more than 30 species of whale, six turtle species, and countless species of fish. Why did the reef form off of Northeast Australia and why aren't there more reefs like it in the world? 

A: Coral reefs are rather picky as to the conditions under which they flourish. For their best development, they need 1) the sea to be low in nutrients; 2) the water not deeper than 50 meters (they need light); 3) the seawater temperature should be around 80 degrees Fahrenheit (so they are restricted to latitudes 30 degrees north and south of the Equator); and 4) (for a structure as geographically wide and deep as the Great Barrier Reef) a continental shelf that is tectonically stable for millions of years. These conditions do not occur together very often, hence the rarity of a reef as extensive (2,600 kilometers) and thick as the Great Barrier Reef. However, there are major reefs in other parts of the world, for example the Belize barrier reef (about 1000 kilometers), the New Caledonia Reef (1500 kilometers long), and the Andros, Bahamas reef (1000 kilometers).

Q: Stopping in India you'll see some of the country's most fabled places including Jaipur, known for its dazzling palaces, massive forts, and colorful markets and Agra, home to the legendary Taj Mahal. India is unique in its myriad combinations of cultures. But is India also unique as a land mass? 

A: 230 million years ago India was united with Africa, Australia, Antarctica, and South America to form an ancient supercontinent called Pangea. We know this because the sequence of strata and several small freshwater fossils are found in each of the separate regions, and this only makes sense if they were formerly united in one region on one land mass. This landmass began to break up about 180 million years ago and India moved geologically rapidly northwards until, in an extraordinary plate tectonic collision, it ran into Asia about 45 million years ago forming the highest mountain range in the world.

Q: You'll visit Ngorongoro Crater in Tanzania, a magnificent UNESCO World Heritage Site and home to the "Big Five," plus you'll see huge herds of wildebeest, zebras, and gazelle. Tell us a bit more about this great grazing ground. Is the crater actually an extinct volcano? And how long ago was it active? 

A: The Ngorongoro crator is one of many now extinct volcanoes that were active about 25 million years ago. They were the result of two tectonic plates separating along a line of what is now the 4000 mile long African Rift Valley which actually stretches from Syria in the north to the Zambezi River in the south. As the plates separated, molten lava poured up into the gap forming large and very active but unique volcanoes, all fed by a huge magma chamber beneath. About 2.5 million years ago this system dramatically changed. The molten rock drained down, and the then high mountains (perhaps 15,000-feet high) collapsed to form enormous craters called calderas, of which the 100-square-mile Ngorongoro Caldera is a superb example.