Page 35 - Curriculum Visions Dynamic Book. To close the book, close the window or tab.

and 60 per cent silica, it is of intermediate nature and will probably erupt with medium violence. If the lava has less than about 45 per cent silica, then it will probably erupt quietly as a fountain of liquid rock with almost no explosive activity.
Types of eruptive material
Volcanoes eject a wide variety of types of materials and thus have an equally wide variety of ways of erupting. This affects both the area in which they erupt and the shape of the volcano that builds up.
Eruptions of liquid magma – called lava – are associated with magma that is runny and has a low gas content.
When sticky magma is full of gases, it boils explosively as it rises to the surface and blasts the magma into tiny droplets. In this case the droplets cool as they fly through the air and fall as ash. Sometimes materials do not get fragmented into ash but remain as larger pieces of semi-molten magma with a solidified crust. Some of these materials are called volcanic or lava bombs because of their size and the way in which
they are thrown up out of the volcano to
come crashing down on the surrounding countryside.
All solid (non-lava) materials ejected from a volcano are grouped together under the general term pyroclastics, where pyr means fire, and clastic means broken. Sometimes gases and pieces
of rock can act together as though they were liquid. In this, the most frightening form of eruption, the material is called a pyroclastic flow.
Volcanic rocks
No matter how sticky or runny the lava is when molten, eventually it will cool and
(Below) The material that comes from a volcano is always magma. However, whether the material flies
high into the air in tiny pieces and creates ash, or whether it remains liquid as lava depends on the chemical composition and thus on the explosiveness of the magma. In this picture the eruption is throwing magma just high enough to form fist-sized lumps called cinders.
35