298 PHYSICAL GEOLOGY 



ties of floating pumice have covered the neighboring waters so thickly 

 as to be a menace to navigation. During the eruption of a volcano 

 in Japan so much pumiceous material was thrown out that it was 

 possible to walk a distance of twenty-three miles upon the debris 

 floating on the sea. 



The size of the blocks of rock thrown out during eruptions varies greatly with differ- 

 ent volcanoes. A 200-ton block is said to have been hurled a distance of nine miles from 

 the volcano Cotopaxi in South America, and it is reported that a rock fragment 100 or 

 more feet in diameter was ejected from the Japanese volcano Asama. Some volcanoes, 

 however, throw out no rock fragments. 



The quantity of fragmental material ejected by volcanoes can 

 best be shown by a few examples. It has been estimated that 4.3 

 cubic miles of material were ejected from Krakatao (p. 304) in 1883, 

 and 28.6 cubic miles from Timboro in 181 5. During the eruption 

 of Sumbawa in the same year an area of nearly 1,000,000 square 

 miles was covered by an amount of fragmental material estimated to 

 be sufficient to make 185 mountains of the size of Vesuvius. 



Lava. — All molten rocks which issue from the earth and also the 

 solid rock which results when they cool are included in the term lava, 



c Lava streams issue 



^ _ C ^i^— - either from the crater 



s A\i^ ^'Sfe— — !' Ip ^ ot a volcano by over- 



C ^Jfoj^WjfiJIf ^^ flowing or breaking 



— ^tjjB — ' 7 $SP through its rim 



_ _ „ , from fissures or Open- 



ly ig. 291. — bmall craters, c, c, c, along a fissure, through . ■ a ^ 



which lava has been extruded. m S s on lts ttanks » or 



through fissures in 

 the earth's surface (Fig. 291), where there are no volcanic cones. 

 When they issue from a volcano they flow down the steepest slope ; 

 when they reach a gentle slope they spread out ; when some obstacle, 

 such as a stone wall, is encountered, their progress is at first stopped, 

 then they either overflow or overthrow it, or pass around its ends. 



Lava Streams. — The surface of a lava stream, which at first glows 

 like red-hot metal, cools quickly and blackens, but since the heat of 

 the interior is kept in by the porous crust thus formed the deeper 

 parts of the stream remain in a molten condition for a long time, oc- 

 casionally for several years. One can often walk across a lava flow 

 a few days after it ceases to move, and while the deeper portions are 

 still molten, without suffering any inconvenience. After the crust 

 has hardened, the still molten lava of the interior may continue to 



