Monday, December 5, 2016

The trees of Calbuco

-Alison
Most of my research can be described as looking at rocks to figure out what happened in the past.  There are many deposits from volcanic eruptions that don't just contain rocks. As volcanic soils are very fertile, many volcanoes are forested which means that falling ash or debris flows interact with trees and other plants. The way trees are damaged by the eruption can tell us a lot about what happened. The trees in the blast zone of Mount St. Helens are a dramatic example.
Trees blown down by the 1980 later blast at Mt St Helens (image from 2015).
I was recently lucky enough to visit Calbuco Volcano in the lake region of Chile. You may remember the impressive pictures of Calbuco erupting at sunset on April 22, 2015.  This heavily forested stratovolcano produced a large plume (which dropped tephra, coarse scoria on the slopes of the volcano and ash all over eastern Chile and Argentina), pyroclastic flows, and lahars (debris flows) from melting glaciers and later rain. Janine did a post right after the eruption that contains lots of amazing videos and photographs of the impacts of the ash on people who live near the volcano.

The deposits from this eruption provide ample chances to see how falling and flowing rock interacted with trees. The different types of damaged to the trees helps us figure out more about what happened on the volcano and in what order. The falling scoria buried trees and fences on the slopes of the volcano. This knocked branches and leaves off of many trees and killed many pine trees, but many trees continued to grow! We know the scoria fell mostly straight down, or there would have been more damage to these threes.
These trees were twice as tall before being covered by 60 cm (or roughly 2 feet) of tephra (scoria, cooled gassy magma). The tree is still growing a year and half later.

Pyroclastic density currents are both fast and hot. These currents form when the column of hot rock and gas collapses and instead of gently raining rocks down like the tephra, they form currents of debris that travel down the volcanic slopes. They snap trees in half and burn healthy wood to charcoal as they pass. To scorch wood the flows are typically expected to be 300 C (572 F) or more. For older volcanic deposits this charcoal is very useful to date the eruption, as well as confirm it was hot.
Burnt wood carried by a pyroclastic flow is now part of the deposit. This log is 60 cm or almost 2 feet long.This tree was knocked over and broken by the flow, so we know the flow was hot and fairly dense.

This tree was burnt and broken, but still standing after the 2015 Calbuco eruption.This means the tree was damaged by hot pyroclastic density currents, but they may not have been dense enough or carrying large enough clasts to knock the tree completely over.

This tree was buried by hot pyroclastic debris, but it didn't fall over until after the base had been completely burnt. This helps us reconstruct the timing of the eruption and how powerful the flows were.
These trees were killed by the pyroclastic flow that burnt the lower part of the trunks and singed the tops, but were later exposed when rainfall eroded the debris without knocking them over.

We also saw trees that had been damaged by passing lahars (debris flows). These are slurries of debris and water formed when the hot pyroclastic flows melted glaciers and mixed with water. These flows may be hot near the source, but cool down as they travel and incorporate more water. Lahars also formed after the eruption ended with heavy rains mixed with all the loose debris of the pyroclastic flows. These mixtures can look like fast moving cement and carry a mixture of sand up to big boulders. These flows knock things down, erode deep channels, and abrade things in their path.

The upstream part of this tree had its bark removed up 2 m above the top of the deposit (more than 6 ft). This also helps us know what direction the main flow was headed.
This large boulder became lodged against this tree trunk at the side of the river valley. You can also see how high the bark was scraped off the tree.
Looking closely at these trees you can spot smaller pebbles embedded in the wood.

This log is part of a root that had grown into previous rocky deposits and been ripped up by a lahar in 2015. The ruler is 20 cm long, or about 8 inches. Logs like this really show how powerful the lahars are to rip up trees with such large roots!

The abrasion of a passing flow can sand a tree down from a circular trunk to one like this. The bark and shape are preserved on the underside of this log, while the top half is almost gone!

The logs then become part of the deposit and stick out of the ground at all sorts of weird angles.

The water from the lahars and later rain storms move the loosest rocks and trees leaving propped logs balancing in their wake.

These trees are a good example as to why geology can be compared to forensics. We can look at lots of different types of evidence after an event to reconstruct what happened and when. Documenting fresh deposits like these also help us do a better job of reconstructing older events so we have a better idea of what volcanoes can and have done in the past, so that we know how to better prepare for future eruptions.

4 comments:

  1. Hi! Interesting article. A question: in the pic shown above the tree with its bark removed, how do you know it was a PF and not a lahar? I ask because the other trees are ok, so I think on a hot lahar that burnt their roots.

    ReplyDelete
    Replies
    1. There are several pictures of trees with the bark removed. When they are abraded it was, like you say, likely a lahar. Which caption is it that seemed to misattribute the abrasion to a PF?

      Delete
    2. Sorry for being late. It's the 6th picture from the beggining.

      Delete
    3. It is possible for lahars to be hot, but rarely do they get hot enough to leave singe marks on trees. In this case the picture isn't very close to show that the trees are burnt (with bark still attached) and the tops are singed. A lahar did pass the area after the pdc to erode the deposits and reveal not only the trunks, but also the roots. Does that help answer your question? Sorry it took me so long to see this and reply.

      Delete