</p>Scattered around the globe are more than one hundred regions of volcanic activity known as hot spots. Unlike most volcanoes, hot spots are rarely found along the boundaries of the continental and oceanic plates that comprise the Earth's crust; most hot spots lie deep in the interior of plates and are anchored deep in the layers of the earth's surface. Hot spots are also distinguished from other volcanoes by their lavas, which contain greater amounts of alkali metals than do those from volcanoes at plate margins.
; X8 i* n( ]2 h. e( }In some cases, plates moving past hot spots have left trails of extinct volcanoes in much the same way that wind passing over a chimney carries off puffs of smoke. It appears that the Hawaiian Islands were created in such a manner by a single source of lava, welling up from a hot spot, over which the Pacific Ocean plate passed on a course roughly from the east toward the northwest, carrying off a line of volcanoes of increasing age. Two other pacific island chains- the Austral Ridge and the Tuamotu Ridge-parallel the configuration of the Hawaiian chain; they are also aligned from the east toward the northwest, with the most recent volcanic activity near their eastern terminuses.
, G @% g( E/ n6 c2 M3 W: d8 P; hThat the Pacific plate and the other plates are moving is now beyond dispute; the relative motion of the plates has been reconstructed in detail. However, the relative motion of the plates with respect to the Earth's interior cannot be determined easily. Hot spots provide the measuring instruments for resolving the question of whether two continental plates are moving in opposite directions or whether one is stationary and the other is drifting away from it. The most compelling evidence that a continental plat is stationary is that, at some hot spots, lavas of several ages are superposed instead of being spread out in chronological sequence. Of course, reconstruction of plate motion from the tracks of hot-spot volcanoes assumes that hot spots are immobile, or nearly so. Several studies support such an assumption, including one that has shown that prominent hot spots through- out the world seem not to have moved during the past ten million years.+ Y7 E+ [7 p' o
Beyond acting as frames of reference, hot spots apparently influence the geophysical processes that propel the plates across the globe. When a continental plate comes to rest over a hot spot, material welling up from deeper layers forms a broad dome that, as it grows, develops deep fissures. In some instances, the continental plate may rupture entirely along some of the fissures so that hot spot initiates the formation of a new ocean. Thus, just as earlier theories have explained the mobility of the continental plates, so hot-spot activity may suggest a theory to explain their mutability.
4 w2 B4 }- D$ \( l0 j20. The primary purpose of the passage is to# S) M. `; U2 L6 X
(A) describe the way in which hot spots influence the extinction of volcanoes& r, r5 W* c1 p* `5 [! R/ v
(B) describe and explain the formation of the oceans and continents
+ J4 ]' q% r6 o& G6 d(C) explain how to estimate the age of lava flows from extinct volcanoes, S0 i0 e8 C1 _( q
(D) describe hot spots and explain how they appear to influence and record the motion of plates
6 T" D: C3 W( {$ V(E) describe the formation and orientation of island chains in the Pacific Ocean
) m4 o7 q% f4 `21. According to the passage, hot spots differ from most volcanoes in that hot spots$ I7 D! x# q+ v7 O$ x+ ~0 H, t
(A) can only be found near islands
: R! ]6 \3 `3 u+ l(B) are active whereas all other volcanoes are extinct. e1 h: p- ] P' R% H
(C) are situated closer to the earth's surface
% t6 X! q9 A8 \% u9 ~% K5 f& N(D) can be found along the edges of the plates
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(E) have greater amounts of alkali metals in their lavas |
发表于 2012-8-15 12:40:43
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