Questions 1-11 are based on the following
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.
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
Line mantle are about equal in thickness, the core actually forms
5 nly 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
10 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
15 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.
20 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
25 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
30 is much thinner under the oceans than under continents.
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
35 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
40 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—
45 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
50 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
55 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 northwesternmost
60 inhabited Hawaiian island, are about 5.5 million years old
and are deeply eroded. By comparison, on the "Big Island" of
Hawaii—southeasternmost 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
65 continually being formed.