Friday, November 15, 2013

Are tsunamis and volcanic eruptions a result of other catastrophic natural disasters?

Catastrophes are often related: a drought in southern California leads to wildfires, and within a year the denuded terrain is damaged further by floods and landslides because the vegetation that preserves and protects the ground surface is missing. A massive earthquake in Haiti leads to a devastating cholera epidemic, because people are displaced and water sources are compromised. The following query is from a thoughtful young man trying to understand some of these relationships.

Q: hello, my name is Brendan and i have a question related to geology in which i would like u to answer. so ya, here it is: Why do tsunamis and volcanic eruptions often act as a result of other catastrophic natural disasters? ya so please respond to this. Oh and btw I am a student at endeavor charter school, just to let you know. alright well thanks for your time and i hope to get a response. – Brendan J

A: Hi, Brendan - I can provide some brief answers.

Tsunamis are caused by SOME volcanic eruptions and by SOME earthquakes:

1. Thera volcano in the eastern Mediterranean erupted catastrophically around 1,500 BC. It triggered a tsunami that destroyed the Minoan civilization based on Crete. The 1883 eruption of Krakatau volcano in Indonesia sent a tsunami into the island of Java that scoured everything within 10 kilometers of the coastline and swept it all back to the sea. Contemporary descriptions said that you could walk across the Sunda Strait because of all the bodies (people, livestock) and logs floating there. MOST volcanic eruptions, however, do NOT cause tsunamis. For one thing, the volcano must be adjacent to an ocean for this to be possible. 

2. The tsunami that destroyed the Fukushima Di-Ichi nuclear powerplant, and destroyed much of the Sendai, Japan, coast, was caused by the Great Tohoku earthquake of 2011. The tsunami happened because a 200 km by 600 km slab of ocean floor was suddenly uplifted several meters by a sudden slip on a subduction fault offshore. The displaced seawater slopped onto the nearby coast, and a security camera showed a 15-meter (nearly 50 foot) wave crashing into the facility, wreaking nearly incomprehensible damage - that is still evolving as I write this. MOST earthquakes, however, do NOT cause tsunamis. There must be an uplift or down-drop of a large piece of ocean floor for this to happen.

Volcanoes are connected indirectly to subduction earthquakes - the Pacific Ring of Fire is an example of how they are related. A down-going section of oceanic crust, being over-ridden by a lighter-density continental crust (Sendai, Japan, and the coast of Washington State in the US, for instance), gives rise to volcanoes. The oceanic crust is heated up as it works its way deeper and deeper into the hot Mantle, and fluids in that down-going slab of oceanic crust contribute to partial melting. This is where lighter component elements of the oceanic floor float up until they burst through the overlying continental crust. Think of a lava lamp: it's the same general principle. Just offshore of North America, Kamchatka, Japan, the Philippines, and Indonesia (and many other places) there is a subduction fault where a continental section is riding over an oceanic crust. Just INLAND from these subduction faults you will find chains of volcanoes paralleling the same coastal margin - the Cascades Range extending from California to British Columbia is an example. One may not directly cause the other, but they are certainly related nevertheless. 

An earthquake was closely associated with to the 1980 eruption of Mount St Helens, but to this day geologists still argue about it. Did the earthquake trigger the eruption of the highly-unstable volcano? Or did the eruption cause the largest earthquake in recorded human history? 

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