EXTRUSIONS 229 



2. EXTRUSIONS 



When molten rock is forced to the surface it gives rise to the 

 mo>t impressive of all geological phenomena. The energies acquired 

 in tin- interior under great compression here find sudden relief. En- 

 closed gases may expand with extreme violence, hurling portions 

 of lava, to great heights and shattering them into fragments, special 

 forms of which are called bombs, cinders, ash, etc., all of which con- 

 stitute pyroclastic material. Much of the explosive violence of 

 volcanoes has been attributed to the contact of the hot rising lava 

 with ground- water. 



There are two phases of extrusion, and at their extremes they 

 are contrasted strongly. The one is explosive ejection, attended 

 in some cases with great violence; the other a quiet out-welling of the 

 lava. More or less closely related to these two phases of extrusion 

 are two classes of conduits, the one, restricted openings, such as 

 pipes, ducts, or limited fissures, from which the amount of lava ex- 

 truded is relatively small and forms cones; the other, great fissures 

 out of which the lava pours in great volume and from which it 

 spreads widely. The extent of the spreading of lava into thin sheets 

 is due more to the mass and fluidity of the lava than to the form 

 of the outlet. The stupendous outflows of certain geologic periods 

 appear to have issued mainly from extended fissures. 



Fissure Eruptions 



The chief known fissure eruptions of recent times are the vast 

 basaltic floods of Iceland; but at certain times in the past there 

 have been prodigious outpourings of lava, flow following flow, 

 making formations thousands of feet thick and covering thousands 

 of square miles. One of these occurred in Tertiary times in Idaho, 

 Oregon, and Washington (Fig. 223), where about 200,000 square 

 miles were covered with lava, aggregating in places some 2,000 feet 

 in thickness. Still earlier, in the Cretaceous period, thefe were 

 enormous flows on the Deccan, covering a like area to the depth of 

 4,000 to 6,000 feet. Still earlier, in the Keweenawan period, an 

 even more remarkable succession of lava-flows in the Lake Superior 

 region developed a series of igneous rocks of almost incredible thick- 

 ness. In these cases there is little evidence of explosive or other 

 violent action, and little pyroclastic material. For the most part 

 these wide-spreading flows are composed of basic material. Massive 

 outflows of this class are the greatest examples of extrusions, 



