Trekking through Cinque Terre turbidites
Cinque Terre National Park, with its five charming villages clinging to steep rocky slopes along the edge of the turquoise-colored Ligurian Sea, is a wildly popular tourist destination. For example, here is the southernmost town of Riomaggiore.
Below is a map of the five villages, with the protected park area in a darker green color. The Cinque Terre is located in the Ligurian region of northwestern Italy, and this northern part of the Tyrrhenian Sea is referred to as the Ligurian Sea. The map is from https://thetrustedtraveller.com.
Most of the rocks along this part of the coast are marine sedimentary rocks that are referred to as “turbidites” because they are deposited by turbidity flows that move rapidly under water and carry sand from the coast to deeper parts of the ocean. The ribbon-like appearance of the sedimentary layers (also called “beds”) is due to the inter-layering of sand and mud—the sand deposited by the turbidity flows and the mud settling out of suspension during the time between flows. Here is a closer-up view of the interbedded sand (light-colored layers) and mud (dark-colored layers). This photo also shows how the layers have been folded by compressive forces—notice the thick sand layer above Jay’s head that is horizontal above his head but then folded upward to near vertical in the upper half of the photo.
To understand the process of turbidity flows, imagine an avalanche, landslide, or debris flow that gets initiated on the sea floor. Because of the ocean water, the sediments in the flow entrain a lot of water, which reduces friction and enables the sediment/water slurry to flow for long distances downslope. Wikipedia has a decent explanation of turbidity flows: https://en.m.wikipedia.org/wiki/Turbidity_current.
Because these flows occur on a slope, they sometimes move, even after they have been deposited. In the photo below, the light colored sand layers (the turbidity flows) are contorted, indicating that they moved a little after being deposited on the sea floor. On the other hand, the mud layers are not contorted because they were deposited very slowly, as small particles settled to the sea floor during the time between the rapidly-deposited turbidity-flows.
Contortions that involve all of the layers occurred after the sediments had been hardened into rock. These contortions are the result of compressive forces that deformed all of the rocks in this region. The photo below, of the north end of the Vernazza harbor, has a fault on the right side and several folds in the middle to left side.
A popular activity in the Cinque Terre is hiking on the myriad paths that connect the five towns. Because the sandstone layers separate easily from the mudstone layers, they are useful for creating paths and walls. Here is an example of sandstone layers that have been used to create a stairway on a hiking trail and a wall behind the trail (photo by Jay Ach).
Because these flows are turbulent and in deep water, they do not contain many fossils. However, between the turbidity flows, when the sea floor is calm, critters crawl about mining the sediment for organic material. On the top of turbidity flows, it is often possible to see the tracks and trails of organisms that were scouring the sediment in organized patterns. These remains are called trace fossils because they are traces of organisms that lack preservation of the organism itself. Here is an example of a trace fossil on the surface of a “turbidite” sandstone layer in Riomaggiore.
Despite the steepness of the terrane, people have been farming the slopes for thousands of years. Many of the slopes are terraced to grow olive and fruit trees, vegetable gardens, and grapes. Cinque Terre wine is admired for its unique qualities, including a minerality inherited from the rocky soils. Here is an example of the terraced slopes, taken on the trail between Corniglia and Manorola. The town of Corniglia is at the bottom of the slope.
Because the slopes are so high and steep, and because the rock is weak, the greatest geologic hazard in the area is landslides. In 2011 a large amount of rain caused extensive landsliding along the entire Cinque Terre coastline. Many of the trails were damaged; indeed, the coastal trails between Riomaggiore, Manorola, and Corniglia remain closed. Unfortunately, this includes the Via dell’ Amore (Lovers Lane) segment between Riomaggiore and Manorola, which has some of the best turbidite exposures. The town of Vernazza was severely impacted by a debris flow that inundated the main street through town and the ground floor of businesses and houses. Below is the sign that is displayed near the train station to demonstrate the damage in the town. The photo was taken in the area of the port, looking eastward, up the main street. The structures against the sky are a crane with claw that is cleaning up the debris from the town.
Because the rocks are so weak, steel cages and other reinforcements are often found covering the rocks to prevent them from falling on the trails. Here is Jay attempting to climb up a rock face with a steel cage covering it.
Here is a photo of the town where we stayed—Vernazza—sometimes called “the jewel of Cinque Terre”. The view is looking north toward the town of Monterosso.
How did these “turbidite” sedimentary rocks get here and why are they so deformed? Like the rocks in the Swiss Alps, these rocks record the collision between Europe and Africa. The turbidites formed at the edge of the Tethys Sea, where erosion of rocks on the adjacent continent caused sediments to flow to the coastline and to make their way farther seaward, ultimately arriving on the sea floor via turbidity flows. Later, as the sea was closing and the continents were colliding, these sedimentary layers were compressed and uplifted as they were folded and faulted. This area is the southernmost part of the Alps and is referred to as the Maritime Alps.
Final note on the collision between Europe and Africa. This collision has been highly complex and has occurred at different times throughout the Mediterranean during the past 100 million years. There were a variety of micro plates and various seaways between Europe and Africa that resulted from complexities associated with the rifting of Europe and Africa from North America to create the Atlantic Ocean, starting nearly 200 million years ago. As plates continually move about the planet, seaways open and close. So as the Atlantic has grown larger, the Tethys Sea has nearly disappeared, as the various small plates between Europe and Africa have collided and created the mountain ranges of the Swiss Alps and the Maritime Alps, among many others. This collision is still proceeding and eating up the Mediterranean Sea, which is the remnant of the once much larger Tethys Sea.
It is worth mentioning that there are other scenic towns on steep rocky slopes in this part of Italy that lack the tourist throngs of the Cinque Terre towns. For example, we stayed in Sestri Levante, located about 50 kilometers north of Vernazza. It has interesting hiking trails, a cute pedestrian shopping area, and excellent seafood (photo by Jay Ach).