Passage 1 is adapted from Eugene C. Robertson, “The Interior of the Earth.” © 2011 by U.S. Geological Survey. Passage 2 is adapted from USGS, “Hotspots: Mantle Thermal Plumes.” © 1999 by U.S. Geological Survey.
Passage 1
The planet Earth is made up of three main shells: the very
thin, brittle crust, the mantle, and the core; the mantle and core
are each divided into two parts. Although the core and mantle
are about equal in thickness, the core actually forms only 15
percent of the Earth’s volume, whereas the mantle occupies 84
percent. The crust makes up the remaining 1 percent. Our
knowledge of the layering and chemical composition of the
Earth is steadily being improved by earth scientists doing
laboratory experiments on rocks at high pressure and analyzing
earthquake records on computers.
Because the crust is accessible to us, its geology has been
extensively studied, and therefore much more information is
known about its structure and composition than about the
structure and composition of the mantle and core. Within the
crust, intricate patterns are created when rocks are redistributed
and deposited in layers through the geologic processes of
eruption and intrusion of lava, erosion, and consolidation of
rock particles, and solidification and recrystallization of porous
rock.
By the large-scale process of plate tectonics, about twelve
plates, which contain combinations of continents and ocean
basins, have moved around on the Earth’s surface through
much of geologic time. The edges of the plates are marked by
concentrations of earthquakes and volcanoes. Collisions of
plates can produce mountains like the Himalayas, the tallest
range in the world. The plates include the crust and part of the
upper mantle, and they move over a hot, yielding upper mantle
zone at very slow rates of a few centimeters per year, slower
than the rate at which fingernails grow. The crust is much
thinner under the oceans than under continents.
Passage 2
The vast majority of earthquakes and volcanic eruptions
occur near plate boundaries, but there are some exceptions. For
example, the Hawaiian Islands, which are entirely of volcanic
origin, have formed in the middle of the Pacific Ocean more
than 3,200 km from the nearest plate boundary. How do the
Hawaiian Islands and other volcanoes that form in the interior
of plates fit into the plate-tectonics picture?
In 1963, J. Tuzo Wilson, the Canadian geophysicist who
discovered transform faults, came up with an ingenious idea
that became known as the “hotspot” theory. Wilson noted that
in certain locations around the world, such as Hawaii,
volcanism has been active for very long periods of time. This
could only happen, he reasoned, if relatively small, long-
lasting, and exceptionally hot regions—called hotspots—
existed below the plates that would provide localized sources
of high heat energy (thermal plumes) to sustain volcanism.
Specifically, Wilson hypothesized that the distinctive linear
shape of the Hawaiian Island-Emperor Seamounts chain
resulted from the Pacific Plate moving over a deep, stationary
hotspot in the mantle, located beneath the present-day position
of the Island of Hawaii. Heat from this hotspot produced a
persistent source of magma by partly melting the overriding
Pacific Plate. The magma, which is lighter than the surrounding
solid rock, then rises through the mantle and crust to erupt onto
the seafloor, forming an active seamount.
According to Wilson’s hotspot theory, the volcanoes of the
Hawaiian chain should get progressively older and become
more eroded the farther they travel beyond the hotspot. The
oldest volcanic rocks on Kauai, the northwestern most inhabited
Hawaiian island, are about 5.5 million years old and are deeply
eroded. By comparison, on the “Big Island” of Hawaii—
southeastern most in the chain and presumably still positioned
over the hotspot—the oldest exposed rocks are less than 0.7
million years old and new volcanic rock is continually being
formed.
Passage 1
The planet Earth is made up of three main shells: the very
thin, brittle crust, the mantle, and the core; the mantle and core
are each divided into two parts. Although the core and mantle
are about equal in thickness, the core actually forms only 15
percent of the Earth’s volume, whereas the mantle occupies 84
percent. The crust makes up the remaining 1 percent. Our
knowledge of the layering and chemical composition of the
Earth is steadily being improved by earth scientists doing
laboratory experiments on rocks at high pressure and analyzing
earthquake records on computers.
Because the crust is accessible to us, its geology has been
extensively studied, and therefore much more information is
known about its structure and composition than about the
structure and composition of the mantle and core. Within the
crust, intricate patterns are created when rocks are redistributed
and deposited in layers through the geologic processes of
eruption and intrusion of lava, erosion, and consolidation of
rock particles, and solidification and recrystallization of porous
rock.
By the large-scale process of plate tectonics, about twelve
plates, which contain combinations of continents and ocean
basins, have moved around on the Earth’s surface through
much of geologic time. The edges of the plates are marked by
concentrations of earthquakes and volcanoes. Collisions of
plates can produce mountains like the Himalayas, the tallest
range in the world. The plates include the crust and part of the
upper mantle, and they move over a hot, yielding upper mantle
zone at very slow rates of a few centimeters per year, slower
than the rate at which fingernails grow. The crust is much
thinner under the oceans than under continents.
Passage 2
The vast majority of earthquakes and volcanic eruptions
occur near plate boundaries, but there are some exceptions. For
example, the Hawaiian Islands, which are entirely of volcanic
origin, have formed in the middle of the Pacific Ocean more
than 3,200 km from the nearest plate boundary. How do the
Hawaiian Islands and other volcanoes that form in the interior
of plates fit into the plate-tectonics picture?
In 1963, J. Tuzo Wilson, the Canadian geophysicist who
discovered transform faults, came up with an ingenious idea
that became known as the “hotspot” theory. Wilson noted that
in certain locations around the world, such as Hawaii,
volcanism has been active for very long periods of time. This
could only happen, he reasoned, if relatively small, long-
lasting, and exceptionally hot regions—called hotspots—
existed below the plates that would provide localized sources
of high heat energy (thermal plumes) to sustain volcanism.
Specifically, Wilson hypothesized that the distinctive linear
shape of the Hawaiian Island-Emperor Seamounts chain
resulted from the Pacific Plate moving over a deep, stationary
hotspot in the mantle, located beneath the present-day position
of the Island of Hawaii. Heat from this hotspot produced a
persistent source of magma by partly melting the overriding
Pacific Plate. The magma, which is lighter than the surrounding
solid rock, then rises through the mantle and crust to erupt onto
the seafloor, forming an active seamount.
According to Wilson’s hotspot theory, the volcanoes of the
Hawaiian chain should get progressively older and become
more eroded the farther they travel beyond the hotspot. The
oldest volcanic rocks on Kauai, the northwestern most inhabited
Hawaiian island, are about 5.5 million years old and are deeply
eroded. By comparison, on the “Big Island” of Hawaii—
southeastern most in the chain and presumably still positioned
over the hotspot—the oldest exposed rocks are less than 0.7
million years old and new volcanic rock is continually being
formed.