SAT Reading - Khan Diagnostic Quiz level 4 - reading 1

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
_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.

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
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.