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.