Scattered around the globe are more than one
- ~9 K, F* O1 l4 r$ Ohundred regions of volcanic activity known as hot
/ N: Y# }& G' r! G5 w* J3 q/ gspots. Unlike most volcanoes, hot spots are rarely
6 D+ i1 V7 {( I) `- ifound along the boundaries of the continental and & y, f+ r5 i0 E
oceanic plates that comprise the Earth’s crust; most 8 y1 W4 T( _7 j/ I+ C0 Y
hot spots lie deep in the interior of plates and are # e. G$ V' t8 q0 o
anchored deep in the layers of the earth’s surface. + q ~+ {) l* n5 U! L. [7 L6 B& D3 |7 w
Hot spots are also distinguished from other volcanoes
4 J; |5 r' j9 o4 {% I5 z# Z8 eby their lavas, which contain greater amounts of
$ y( D/ ?/ O7 a- T* j+ ~alkali metals than do those from volcanoes at plate 1 J0 U% Y3 J4 g6 R
margins.
" D! I9 L0 x; f/ ~# ? In some cases, plates moving past hot spots have
1 \9 U; N }7 sleft trails of extinct volcanoes in much the same way 7 U" K( m( P+ G* `, L
that wind passing over a chimney carries off puffs of
$ {) m9 d8 L4 m* ?) i8 u @% ]smoke. It appears that the Hawaiian Islands were ; W' z$ L; j; [1 Q
created in such a manner by a single source of lava,
; m K8 P6 _3 L# X2 Twelling up from a hot spot, over which the Pacific
% G3 R; x; K6 W- ?, n( ZOcean plate passed on a course roughly from the east % r- E- K: |5 E$ \2 X' ^# `
toward the northwest, carrying off a line of volcanoes / W6 D# [* r/ _3 P
of increasing age. Two other pacific island chains—
" L3 K' V% z7 l1 e( ~( q$ ethe Austral Ridge and the Tuamotu Ridge—parallel % |) Z) U/ U4 p& |$ ~; Q
the configuration of the Hawaiian chain; they are also ; @ H! d/ ]. x# c, a
aligned from the east toward the northwest, with the ! c+ N! I9 h( J! D" Y8 V3 w9 t
most recent volcanic activity near their eastern
. N" L1 l7 ^ b" R. w8 Q' Tterminuses. & Q% M+ [. D9 F* v$ h
That the Pacific plate and the other plates are
& k' E& H( F' q# _# mmoving is now beyond dispute; the relative motion of 5 Q1 I( W5 z; Z; H }
the plates has been reconstructed in detail. However,
5 l& M$ d4 f! t- Pthe relative motion of the plates with respect to the ; X3 N* u/ w3 y" V0 x
Earth’s interior cannot be determined easily. Hot 2 @! ~0 o- j7 |% F2 G. X
spots provide the measuring instruments for resolving ' W0 j* p& a- ~
the question of whether two continental plates are
, T' M/ A: L, [1 `9 O9 Q& Zmoving in opposite directions or whether one is sta- - P- g* u }5 ?! K# W
tionary and the other is drifting away from it. The
* T& k! h, U+ V0 Z# S8 m' r1 T; ymost compelling evidence that a continental plat is ( _% p( O0 n3 u; n
stationary is that, at some hot spots, lavas of several ( S0 s7 h. r& ]) M3 S/ j6 ^1 B3 M
ages are superposed instead of being spread out in ) K7 p. e6 S O! b; c6 d
chronological sequence. Of course, reconstruction of
% b9 I$ u5 P' L; q, d6 ^5 U3 ]plate motion from the tracks of hot-spot volcanoes
, g$ K$ x& f3 |/ o* ]assumes that hot spots are immobile, or nearly so. * P& a' A' b1 ~/ w w
Several studies support such an assumption, including
$ w( v/ [) k2 vone that has shown that prominent hot spots through- 6 D% V. r; ~8 {6 |$ H5 B' a
out the world seem not to have moved during the past
+ j% Q7 p# [; `/ r! Vten million years. |