Reading 47

The Earth’s Crust

The following passage is adapted from an article describing the origins of the ocean floor (© 2009 by Peter Kelemen).

1. At the dark bottom of our cool
2. oceans, 85 percent of the
3. earth’s volcanic eruptions
4. proceed virtually unnoticed.
5. Though unseen, they are hardly
6. insignificant. Submarine
7. volcanoes generate the solid
8. underpinnings of all the world’s
9. oceans: massive slabs of rock
10. seven kilometers thick.
11. Geophysicists first began to
12. appreciate the smoldering
13. origins of the land under the
14. sea, known formally as ocean
15. crust, in the early 1960s.
16. Sonar surveys revealed that
17. volcanoes form nearly
18. continuous ridges that wind
19. around the globe like seams
20. on a baseball. Later, the same
21. scientists strove to explain
22. what fuels these erupting
23. mountain ranges, called
24. mid-ocean ridges. Basic
25. theories suggest that because
26. ocean crust pulls apart along
27. the ridges, hot material deep
28. within the earth’s rocky
29. interior must rise to fill the
30. gap. But details of exactly
31. where the lava originates and
32. how it travels to the surface
33. long remained a mystery.
34. In recent years, mathematical
35. models of the interaction
36. between molten and solid
37. rock have provided some
38. answers, as have examinations
39. of blocks of old seafloor now
40. exposed on the continents.
41. These insights made it
42. possible to develop a detailed
43. theory describing the birth of
44. ocean crust. The process
45. turns out to be quite different
46. from the typical layperson’s
47. idea, in which fiery magma
48. fills an enormous chamber
49. underneath a volcano, then
50. rages upward along a jagged
51. crack. Instead, the process
52. begins dozens of kilometers
53. under the seafloor, where
54. tiny droplets of melted rock
55. ooze through microscopic
56. pores at a rate of about 10
57. centimeters a year, about as
58. fast as fingernails grow.
59. Closer to the surface, the
60. process speeds up, culminating
61. with massive streams of lava
62. pouring over the seafloor
63. with the velocity of a speeding
64. truck. Deciphering how liquid
65. moves through solid rock deep
66. underground not only explains
67. how ocean crust emerges but
68. also may elucidate the behavior
69. of other fluid-transport
70. networks, including the river
71. systems that dissect the
72. planet’s surface.
73. Far below the mid-ocean ridge
74. volcanoes and their countless
75. layers of crust-forming lava
76. is the mantle, a 3,200-
77. kilometer-thick layer of
78. scorching hot rock that forms
79. the earth’s midsection and
80. surrounds its metallic core.
81. At the planet’s cool surface,
82. upthrusted mantle rocks are
83. dark green, but if you could
84. see them in their rightful
85. home, they would be glowing
86. red- or even white-hot. The
87. top of the mantle is about
88. 1,300 degrees Celsius, and it
89. gets about one degree hotter
90. with each kilometer of depth.
91. The weight of overlying rock
92. means the pressure also
93. increases with depth about
94. 1,000 atmospheres for every
95. three kilometers.
96. Knowledge of the intense heat
97. and pressure in the mantle led
98. researchers to hypothesize in
99. the late 1960s that ocean crust
100. originates as tiny amounts of
101. liquid rock known as melt,
102. almost as though the solid
103. rocks were “sweating.” Even
104. a minuscule release of pressure
105. (because of material rising
106. from its original position)
107. causes melt to form in
108. microscopic pores deep within
109. the mantle rock.
110. Explaining how the rock sweat
111. gets to the surface was more
112. difficult. Melt is less dense
113. than the mantle rocks in
114. which it forms, so it will
115. constantly try to migrate
116. upward, toward regions of
117. lower pressure. But what
118. laboratory experiments
119. revealed about the chemical
120. composition of melt did not
121. seem to match up with the
122. composition of rock samples
123. collected from the mid-ocean
124. ridges, where erupted melt
125. hardens.
126. Using specialized equipment
127. to heat and squeeze crystals
128. from mantle rocks in the
129. laboratory, investigators
130. learned that the chemical
131. composition of melt in the
132. mantle varies depending on
133. the depth at which it forms;
134. the composition is controlled
135. by an exchange of atoms
136. between the melt and the
137. minerals that make up the
138. solid rock it passes through.
139. The experiments revealed
140. that as melt rises, it dissolves
141. one kind of mineral,
142. orthopyroxene, and
143. precipitates, or leaves behind,
144. another mineral, olivine.
145. Researchers could thus infer
146. that the higher in the mantle
147. melt formed, the more
148. orthopyroxene it would
149. dissolve, and the more
150. olivine it would leave behind.