Earth_Magazine_October_2017

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C

hains of volcanoes and a lava

lake pepper the landscape of

the Afar Triangle in north-

eastern Ethiopia, where

eruptions and earthquakes are byprod-

ucts of the rifting that is literally ripping

Africa apart, but recent eruptions have

been docile. Now, scientists studying ash

deposits from the last 40,000 years are

showing that dangerous, explosive erup-

tions present an ongoing hazard, striking

the region every 1,000 years on average.

In Ethiopia, 49 of the country’s

65 volcanoes are active, and almost half

of Ethiopia’s population lives within

100 kilometers of a volcano, including

the capital city Addis Ababa, which sits

just over 40 kilometers from a Holo-

cene-active volcano. Despite clear signs

of geologic upheaval in the past, the vol-

canoes have a benign reputation, says

Catherine Martin-Jones, a geochemist

now at the University of Cambridge in

England and lead author of the new study

in Quaternary Geochronology. “Ethi-

opia is famous for lava lakes, but a lot

of people are really surprised that you

have explosive eruptions in this region,”

Martin-Jones says.

Geologists have known that explosive

eruptions repeatedly showered tiny vol-

canic ash fragments, called tephra, across

the landscape during the past few million

years. But without an assessment of the

past tens of thousands of years, the vol-

canic risk today has been unclear.

To assess this shortfall, Martin-Jones

and her colleagues investigated the his-

tory and tempo of explosive eruptions

over the past 40,000 years in the Afar

Triangle region using the glassy tephra

shards, which paleontologists have long

used to date hominid remains in the

region. Large eruptions disperse ash

across thousands of kilometers, Mar-

tin-Jones says: “It’s effectively deposited

instantaneously, making these ash layers

really useful time markers.”

The scientists collected tephra shards

from sediments in two lakes near the

Afar Triangle by drilling almost 8 meters

below the lake bottoms. Each shard pre-

serves details about the eruption that

produced it, including a unique geochem-

ical fingerprint. These shards suggested

that seven different volcanoes spewed

ash over the region during this period.

Martin-Jones and her team then used

organic material and tephras embedded

in the sediments to mark the timing of

eruptions using radiocarbon dating.

The results were surprising, Mar-

tin-Jones says: In the past 17,000 years,

an explosive eruption struck every

1,000 years, on average. The team also

noted a surge of volcanic activity between

7,500 and 1,600 years ago, when most of

the tephra was deposited. The most recent

eruption occurred 300 to 500 years ago,

but scientists found preserved ash in sed-

iments from one of the lakes revealing at

least three eruptions in the last 1,000 years.

The study is an important first step in

understanding modern volcanic hazards

in Ethiopia, says Andrei Sarna-Wojcicki,

a geologist at the U.S. Geological Survey in

Menlo Park, Calif., who was not involved

with the study. Examining the last 40,

years is a welcome contribution, he says,

since “most people have focused on erup-

tions near the early hominid sites, about

4 million to 1 million years ago.”

Understanding geologically recent

eruptions is crucial to clarifying today’s

hazards in Ethiopia, but until now, no

one has looked at the record from the

Holocene, says Giday WoldeGabriel, a

geologist at Los Alamos National Lab-

oratory in New Mexico who was not

involved with the study. “The area is

ripe for seismic and volcanic hazards,”

WoldeGabriel says.

Martin-Jones and her team intend to

sample sediments from more Ethiopian

lakes to further refine the timing of the

eruptions, which will help volcanologists

refine the picture of hazards in the region.

Sarah Derouin

New active faults found

under Los Angeles

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