When living near a plate boundary, one must expect geologic hazards such as earthquakes and volcanic eruptions. For example, along the west coast of the U.S. a transform plate boundary (San Andreas fault) causes earthquakes throughout California. To the north, along the coast of northern California, Oregon and Washington, a convergent (subduction) plate boundary causes volcanic eruptions in the Cascade Range and also some earthquakes.
A subduction plate boundary also extends along the entire coast of Chile, so of course there are many earthquakes and volcanic eruptions. See my last post for a brief introduction to the geology of South America and a map of earthquakes along its west coast: https://landscapes-revealed.net/back-to-chile-a-geologic-wonderland/.
Our trip started with five days in Santiago, the capital city of Chile where ~40% of the country’s population lives. With a population of ~7 million people, it seems prudent to ask what geologic hazards residents can anticipate. In this post, I explore the hazards and show some photos from our time in Santiago.
Earthquakes in Santiago
Residents experience frequent tremors; in fact, we experienced one on the first morning of our stay in Santiago! It was a magnitude 6.2 earthquake, but was located about 400 km (240 miles) north of the city so the shaking was mild.
The largest Chilean earthquakes have been farther south. For example, the strongest earthquake ever recorded worldwide was in 1960 near Valdivia, located about 1000 km (600 miles) to the south of Santiago. That earthquake had a magnitude of 9.5. The most recent major earthquake was in 2010 and had a magnitude of 8.8. Although the epicenter was ~335 km (200 miles) to the south of Santiago, a friend who was living here then said the shaking and damage occurred over a large area, including in Santiago.
There is no getting away from the reality of frequent and large earthquakes when living adjacent to a subduction zone where our planet’s largest magnitude earthquakes occur.
What about volcanic eruptions?
After Indonesia, Chile is the country with the largest number of active volcanoes. But the hazard varies along the length of the country because of the varying behavior of the down-going oceanic plate. Where the oceanic plate descends at a high angle, the down-going rocks and water cause melting. That melted rock, called magma, rises up through the continent and some of it makes it all the way to the surface to form volcanoes. But where the oceanic plate descends at a low angle, it doesn’t get very deep and magma doesn’t form to rise up and make volcanoes, as shown in the map below.
This map shows the Central Volcanic Zone in northern Chile and the Southern Volcanic Zone (SVZ) in central Chile. The location of the the Chile-Peru trench is the plate boundary where the oceanic plate is descending beneath the continental plate. The numbers show the convergent rate of the two plates. Notice that Santiago is located at the northern end of the SVZ. Farther north is the “flat-slab zone”, where the oceanic plate is descending at a shallow angle and there are no active volcanoes.
The map below shows why there is a “flat-slab zone”.
This Google Earth image shows the location of Santiago (red circle) and Chile’s boundary with Argentina (yellow line). In the ocean the dark blue area extending parallel to the coast is the Chile-Peru trench. The darker the blue, the deeper the water. To the west of the trench is a line of seamounts that are moving eastward with the Pacific plate and being subducted beneath the continent. Seamounts are formerly active volcanoes that are now peaks on the seafloor. As the seamounts descend into the trench, they “clog up” the subduction zone and prevent the oceanic plate from sinking deeply. It is these seamounts that are preventing volcanoes from forming in central Chile.
There are several volcanoes at the north end of the Southern Volcanic Zone (SVZ) that have been active within the past few thousand years but none are as active as some of the volcanoes farther south in the SVZ. So, in general, earthquakes tend to be a larger hazard for Santiago than volcanoes.
But there used to be active volcanoes in Santiago
We stayed in the Providencia neighborhood that is close to San Cristóbal and Santa Lucia Hills, both of which are remnants of active volcanoes that erupted about 15 million years ago.
At subduction zone plate boundaries, there are many differences in the oceanic plate that can cause differences in the location and types of volcanic eruptions over time. In Santiago from about 30–15 million years ago, plate interactions were such that active volcanoes formed in this valley. But with changes in plate interactions, volcanism is now located in the chain of volcanoes that make up the Andes, the summit of which is the boundary between Chile and Argentina.
We went to the top of two hills that are the remnants of the ancient Santiago volcanoes.
Jay is standing on a terrace near the top of Santa Lucia Hill. On the upper left are the natural outcrops of basalt that make up the hill. Many structures, such as the castle at the top of the image, have been built on the hill over the years. The hill is a popular place for Santiago residents to get some exercise and enjoy the views. It is the remnant of a 15 million-year-old volcano that is now highly eroded.Along the path to the top of Santa Lucia Hill is this exposure of basalt columns. When basalt lava cools, it contracts. The column edges are the cracks formed by cooling. They are common in many outcrops of basaltic rock, including those in my home state of Oregon.On Santa Lucia Hill is a garden (jardin) named for Charles Darwin, who explored this part of Chile from the sea to the top of the Andes when he was journeying as the naturalist on the Beagle. There is also a plaque to Darwin, who was on Santa Lucia Hill on the 21th of August in 1834. He described the hill as follows (translated back into English from Spanish): “An inexhaustible source of pleasure is to climb Santa Lucia Hill, a little rocky hill that rises in the center of the city. From here the view is truly impressive and unique”. The government of the United Kingdom gave the plaque to the inhabitants of Santiago in April 1997.
The view he saw was no doubt quite different than what we saw this week.
Here is the view eastward toward the Andes from Santa Lucia Hill. In Darwin’s day there would have been fewer buildings, and certainly no high-rises. Even in the 20 years since we lived in Santiago the number of high-rises has increased, as the population has expanded by about a million people, from 6 to 7 million. The nearby hill (left side of photo) is San Cristóbal, another remnant of a volcano that was active about 15 million years ago. Our Airbnb was close to the southern end of that hill.This view is from San Cristóbal Hill, looking southwest over the city. The green strip is Parque Forestal along the Mapocho River. Santa Lucia Hill is located on the other side of the river toward the left side of the photo.
More photos in Santiago
Statue of Salvador Allende, the democratically-elected president who was deposed in a military coup on September 11, 1973. His quote on the statue: “I have faith in Chile and its destiny”. These were some of the words he spoke to the crowd from The Moneda Palace (state government building) as the military was storming the building. The Moneda Palace is currently being remodeled and is located to the right of the statue. The Chilean flag is flying in front of the building for the Ministry of Justice and Human Rights. We also visited the Museum of Memory and Human Rights that documents the subsequent dictatorship of General Pinochet that lasted from 1973–1988.In my first post I showed a photo of the geology students I taught at the University of Chile 20 years ago. Happily, five of them were able to join us for a convivial dinner. It was fabulous to see them again, now with families and geologic careers. Unfortunately, I forgot to get a photo before two of them left. Chile is known for wine, but also for pisco, a spirit made from wine that is similar to grappa. We found this restaurant called “Chipe Libre – República Independiente del Pisco” that had an amazing selection of piscos. We tried some different types of pisco sours and had delicious dinners of seafood ceviche. Highly recommended! Anita and Ralf, friends from San Francisco, are traveling with us on most of this trip. So you will see more of them in subsequent posts.We celebrated Jay’s birthday (February 14) at Peumayén Ancestral Foods. This restaurant features ancestral (indigenous) foods presented in creative tasting menus. We chose the Patagonian tasting menu. Each course was paired with an excellent Chilean wine. Left side: our first course of locos (abalone) plus cuttlefish and a few other seafood pieces, topped with a crab claw, and paired with a sparkling wine from the Valle de Casablanca, located a short distance west of Santiago. Right side: Jay got a special desert for his birthday. It was paired with a lovely dessert wine. We had a truly excellent experience.
Tomorrow we fly south to Punta Arenas, where the temperatures will be quite a bit cooler.
Happy Birthday, Jay! It looks like you a very memorable celebration!
Jay says thanks! It was fun to have his birthday here.
Happy Birthday, Jay! And thank you, Karen, for your great geologues!
Thanks for reading Jane!
Happy birthday, Jay. I drooled over your birthday dinner post!
Ha Ha, It was really good!