Scattered around the globe are more than one
0 g/ w( d1 h: [/ T3 X1 bhundred regions of volcanic activity known as hot
* c& c0 v: I( u5 B& R9 wspots. Unlike most volcanoes, hot spots are rarely ) T7 u9 k4 F- K* }, h! M5 U
found along the boundaries of the continental and 1 v2 P( H; G( h8 {+ b$ |
oceanic plates that comprise the Earth’s crust; most / j( G0 @9 [0 e
hot spots lie deep in the interior of plates and are 2 e' { {2 g# c3 _) f0 F8 h
anchored deep in the layers of the earth’s surface. ) R1 m+ i. [ q: T( a
Hot spots are also distinguished from other volcanoes ' O9 T A, G# g7 O
by their lavas, which contain greater amounts of . a( U- V% U( U5 {/ ^1 Y: Q
alkali metals than do those from volcanoes at plate
1 l' u! ?& |. q( Bmargins.
2 X) Q3 p; o6 B' [# d; H" A In some cases, plates moving past hot spots have ( F! O4 W4 l% b' L: a. u$ j
left trails of extinct volcanoes in much the same way 4 a5 J" `1 @9 s' j. |* y: i( O2 C
that wind passing over a chimney carries off puffs of
" S5 Y2 s) P7 M* Wsmoke. It appears that the Hawaiian Islands were , G6 [2 F v3 s6 ^0 z4 U
created in such a manner by a single source of lava,
( x% G2 Y5 H+ m, ?- l4 Gwelling up from a hot spot, over which the Pacific
& d9 C6 H9 k- e7 }- ^- aOcean plate passed on a course roughly from the east
# G- n; c) O4 P% ltoward the northwest, carrying off a line of volcanoes
U$ ~& q0 e0 b! M8 tof increasing age. Two other pacific island chains—
. ? g; W$ m$ w" O$ R) v0 H, Othe Austral Ridge and the Tuamotu Ridge—parallel $ S1 H$ i1 y E$ H7 p& z
the configuration of the Hawaiian chain; they are also
' O# ^3 [/ N1 S/ t8 z& z. m% ?% Laligned from the east toward the northwest, with the
/ F' R; S; P5 Rmost recent volcanic activity near their eastern ; D8 @/ g1 R, D Y, _! H. O& Z1 C
terminuses.
' _ n1 y2 S" |& A7 @- [ That the Pacific plate and the other plates are
. c @- W3 J& k+ o }0 _moving is now beyond dispute; the relative motion of 0 G4 ]8 E* M l5 p0 X
the plates has been reconstructed in detail. However, / c- L& d2 C7 H) E) {' V$ g
the relative motion of the plates with respect to the
7 y% q$ h1 z# v; zEarth’s interior cannot be determined easily. Hot & M7 c* }7 |$ H( e6 B; w" m4 L4 _
spots provide the measuring instruments for resolving
T# b7 C6 p s3 O- D. b: \% q8 xthe question of whether two continental plates are
) I4 X9 t- n/ b6 Q: Vmoving in opposite directions or whether one is sta-
+ y: {7 U/ g# B0 Z) H6 U% K, ` C2 ]tionary and the other is drifting away from it. The
" d- E0 i/ m6 W# P! umost compelling evidence that a continental plat is & ]; f h& C* e: C
stationary is that, at some hot spots, lavas of several 8 u2 \# b5 r& Y1 J, k: ?
ages are superposed instead of being spread out in
: G0 w. h* f* _chronological sequence. Of course, reconstruction of ) J) o( }! T5 C5 |
plate motion from the tracks of hot-spot volcanoes 0 T2 {) `3 E* p) ]
assumes that hot spots are immobile, or nearly so. . H* p% B( ~) l/ \
Several studies support such an assumption, including
+ {# y- @ M! T# k9 X9 none that has shown that prominent hot spots through-
8 Q2 \5 [; h4 g# E. D; Xout the world seem not to have moved during the past
0 M9 r9 ^* M5 k/ M& Zten million years. |