The tsunami that washed time away Journal article

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The Tsunami
That Washed
Time Away
High on the clifftops at Henderson Bay,
Northland, you can find millions of
pebbles that were once on the ocean floor.
How did they get on top of the cliffs? This
question has puzzled people for years.
Scientists James Goff and Scott Nichol
wanted to find the answer.
by Jenna Tinkle
Individual images,
text and multimedia
for download →
Photograph: Copyright © S. Nichol, used with permission
Map of NZ showing Henderson Bay: Copyright © Di Fuller, used with permission
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WHY IS THAT?

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The two scientists thought the
answer might have something to
do with the Pacific Ring of Fire. It’s
an area where there are lots of
volcanoes and earthquakes. The
Ring of Fire follows a line around
the Pacific Ocean where tectonic
plates meet. The plates move and
push against each other, which
causes earthquakes and volcanic
eruptions. These can, in turn, cause
tsunamis, or giant waves. New
Zealand is on the Ring of Fire.
The Ring of Fire
PACIFIC RING OF FIRE
→
About 75 percent of the
world’s active volcanoes
are found along the Ring
of Fire, and 90 percent of
the world’s earthquakes
occur there.
Map: By Alan Poole, copyright © Crown 2014

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Finding evidence at Henderson Bay
James and Scott are geologists –
scientists who use evidence from the
earth to understand things that
happened in the past. They found
clues that something suddenly
changed the landscape at Henderson
Bay hundreds of years ago.
A big clue was the shape of the
pebbles on the cliffs. The pebbles were
round. This showed that they had
been shaped by waves. Divers have
brought up similar pebbles from the
ocean floor. And the pebbles couldn’t
have been brought in by the tide – the
cliffs are metres above sea level!
There are many accounts of volcanic
eruptions, earthquakes, and
tsunamis in pūrākau – the stories
that have been passed down by
Māori through generations.
One pūrākau tells how Titipa and all
his fish were swept away by giant
waves in the Bay of Plenty.
Scientists think these stories could
describe real events that happened
in the past. They believe a huge
tsunami struck in about 1450 and
destroyed Māori villages all along the
east coast of New Zealand.
EVIDENCE OF TSUNAMIS IN MĀORI ORAL HISTORY
Illustration: By Scott Pearson, copyright © Crown 2014

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On some of the remaining bits of dune, there are a number of
mounds. Archaeologists call these mounds “middens”. The
middens are made up of shells, bones, and burnt stones – the
remains of an umu (an earth oven).
There were two types of midden. The older one held the
bones of moa. These birds became extinct about 500 years ago.
It also had lots of large tuatua shells. This midden was no
longer a mound. It was spread out across the cliff top.
The other midden sat on top of the older midden’s remains.
This midden was still a mound and was from a much more
recent time. The Māori who lived in the area at that time had
a different diet. The midden held only mud snails and very
small tuatua shells.
These two middens told a story. In the time between the two
middens, something changed the environment at Henderson
Bay. There were no large shellfish in the more recent midden.
One idea is that a tsunami might have wiped out the tuatua
and other sea life and scattered shells, bones, and umu stones
across the landscape. If this happened, any survivors would
have moved away from the coast for a time. However, Māori
returned to Henderson Bay and continued to use the good
fishing spots, freshwater ponds, and umu stones.
A story buried in the ground
Small tuatua
shells and mud
snails were
found in a
mound here.
Large tuatua
shells and moa
bones were
found
scattered here.
↑
Evidence from the middens showed there
had been a change in the environment.
Photograph: Copyright © S. Nichol, used with permission

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James and Scott found another important piece of
evidence in a swamp 600 metres from the sea at
Henderson Bay. James pushed an aluminium pipe
into the ground and took a sample of what lay
beneath the swamp. This is known as taking a core
sample. The sample was a record of 24,000 years of
the swamp’s life. It showed four distinct layers (see
diagram).
James and Scott could see that the third layer held
the key to the puzzle. Between the 600-year-old layer
and the 24,000-year-old layer, there should have been
another thick layer of organic peat, which had been
built up over 23,000 years. It was missing!
The proof is in the swamp
This evidence strongly supported their
hypothesis that a tsunami had rushed over the
swamp about 600 years ago. It washed away
23,000 years of peat and replaced it with a thin
layer of ocean sediment. When it washed
away the peat, the tsunami also washed away
a record of time.
PACIFIC RING OF FIRE
→
Diagram showing
the core sample in
the aluminium pipe
Diagram: By Alan Poole, copyright © Crown 2014

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James and Scott had found evidence to support
their hypothesis about the huge tsunami, but one
question remained unanswered – what caused the
tsunami in the first place?
One possibility is that it was caused by the
eruption of the Healy volcano that sits on the
ocean floor to the north of East Cape. Evidence
provided by the age of pumice from this undersea
volcano showed that it last erupted about 600
years ago.
The Healy volcano
This date matches the evidence
from the core sample. The
eruption triggered a big tsunami.
The wave came to Henderson Bay
and other parts of Northland
(washing away Titipa and all his
fish in the process). Pumice from
the eruption can still be found on
Northland beaches.
Healy volcano cross-section: By Alan Poole, copyright © Crown 2014
NZ map showing Healy volcano: By Alan Poole, copyright © Crown 2014
→
Artist’s impression
of scientists’ theory
(not to scale)

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James and Scott had solved the mystery of the pebbles on
the clifftops at Henderson Bay. By looking at the
evidence, scientists like James and Scott give us a better
understanding of our history and the forces that have
shaped New Zealand. They also give us a better
understanding of where and when tsunamis could
happen in the future and how to plan for them.
Glossary
hypothesis – a proposed explanation
organic peat – natural material
(mainly plants) that has broken down over time
and become spongy
pumice – light, soft rock formed
during volcanic eruptions
sediment – material (sand, dirt, rocks) carried
and then deposited by water
tectonic plates – parts of Earth’s crust that
move very slowly
Photograph: “Crashing wave” by Sheila Sund from http://goo.gl/RUCNU2 is licensed under CC BY 2.0
Why study past tsunamis?
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