to the Galápagos Islands
By Mark Kurz and Karen Harpp
of the most common questions that scientists and nature lovers ask when
they see an interesting rock is: how old is it?
Hundreds of years ago, geologists used the relative position of rock layers to
tell which one was older. Think about a pile of laundry -- the clothes at the
bottom are older (or were worn longer ago) than the ones on top. Similarly, in
a pile of rock layers, the ones on the bottom must be older because they were
there first. This means that each layer is older than the one above it, and younger
than the one below it. The study of the age relationships between different rock
layers, or strata, is a branch of geology called stratigraphy.
Azul vocano on Isabela Island.
sometimes, rock layers can get bent, twisted and folded, and even turned upside
down. Older on the bottom and younger on the top doesnt work
In 1896, Antoine Becquerel discovered radioactivity. In the early 1900s,
scientists found a way to determine the real ages of rocks and minerals using
Some of the elements that make up minerals in rocks are radioactive and breakdown
into other elements. This is known as radioactive decay. Each radioactive parent element
decays to a stable daughter element at a certain rate. Once this
rate is known, scientists can use the production of the daughter element as a
geologic clock. This is a branch of geology called geochronology.
Think about an hourglass. Imagine the sand in the top is a group of parent elements.
The grains move through the narrow part of the hourglass at a constant rate.
When they fall into the bottom, they have changed into daughter elements. If
you know how quickly the sand moves through the glass neck, you can estimate
how much time has passed since someone started the hourglass simply by looking
at how big the pile of sand is at the bottom.
One of the most common methods to date volcanic rocks uses potassium (radioactive
parent) and argon (stable daughter). Potassium is an element found in many minerals
and rocks, and it normally has an atomic mass of 39. But there is a rare form
of potassium that has an atomic number of 40 because it has an extra neutron.
Forms of an element that have the same chemical properties but different masses
are called isotopes. It takes 1.25 billion years for half of the
original potassium-40 isotope to decay to argonthis is called the half
When a volcanic rock is erupted at the Earths surface, the clock starts
ticking. As time passes, the potassium-40 in the rock decays to argon-40. Scientists
can measure the argon-40 produced after eruption, and the amount of potassium-40
responsible for the decay, and then figure out when the lava erupted.
Rocks from around the world have been dated using this method, including the
older volcanoes in the Galápagos Islands. Scientists have also dated rocks
from central Africa that contain fossil evidence of the first human species.
But some volcanic rocks are only hundreds or thousands of years old and they
dont contain much potassium, so the potassium-argon method doesnt
work for them.
So we need a different method for dating young volcanic rocks. We make use of
cosmic rays that come from deep space and are constantly hitting the Earth. When
they hit, they produce tiny amounts of different elements in rock surfaces by
One such element is helium-3. Just like natural radioactivity, if we know the
rate of production of helium-3 and we can measure how much is produced by the
cosmic rays, we can calculate how long the rock has been exposed on the surface
of the Earth.
In our laboratory at Woods Hole Oceanographic Institution, we use an instrument
called a mass spectrometer to measure tiny amounts of helium-3 to determine the
ages of Galápagos lava flows. We have found that the entire surface of
Fernandina volcano, outside of the summit caldera, is less than 4000 years oldyounger
than most geologists thought. Yet other Galápagos lava flows are as young
as a few hundred years.
Knowing ages of lava flows tells us how often a volcano erupts. This can be important
in assessing whether the volcano poses any hazard to surrounding areas.
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