Lava is a mixture of minerals from deep in the earth's crust. In the molten state, its temperature must be high enough to melt basalt and silica. The air soon cools the molten surface, but, heat is often trapped and sealed below this rocky crust. Twenty years after the eruption of a volcano in Mexico, the lava below the surface was still hot enough to light a cigar.

The roots of a volcano dip down 20 miles or more below the surface. The molten lava which spills out during an eruption comes from deep reservoirs of magma which form at these low levels. We know that the temperature rises as we dig deeper into  the earth's crust and 20 miles down it well may be as high as 1,400 degrees centigrade.

At the surface of the earth, basalt rock melts to liquid a.b about 1,250 degrees. But as we go deeper into the earth's crus;, the rocky minerals need higher temperatures to reach their melting points. Twenty miles down, basalt refuses to melt below 1,400 degrees We know, then, that the pools of deep magma which feed the rivers of molten lava must be at least this hot.

Much of this heat, however, is lost in the explosive force which shoots the molten rock to the surface. A river of lava may flc~,r down a steep slope at 60 miles an hour, so the scientist who plans to take its temperature must beware. He will be safer, of course; checking a slower flow moving at say 100 yards an hour.

The instrument for the job is complex and must be durable. Sometimes bars or plates of metal, known to melt at a certain temperature, are driven into the hot lava and their condition checked by remote control. As we would expect, there is a wide range of temperature in molten lava.

Some flows are 1,000 centigrade degrees, some 1,500 and some 2,000 degrees.

 The basalt and countless other minerals in the molten lava are now at the surface, of course, where they need less heat to reach their molten state. In the reservoirs of molten lava below, temperatures must be much higher than the rivers of flowing lava which spill over the surface.

A flow of molten lava is always hot enough to burn everything in its path to ashes. As the surface cools to a solid crust, the heat below may be sealed in for hundreds and thousands of years. The geysers and hot springs of Yellowstone National Park are caused by ancient lava flows in which the surface cooled fast and sealed heat in rocks below the ground.