tuff breccia - UNIT OF 5 STUDENT SPECIMENS of pink tuff with large clasts
Tuff is welded volcanic ash. Normally it contains welded-in fragments of rocks that were torn out of the vent and ejected with the ash. These clasts, from the Greek klastos for broken in pieces, are usually angular and help students identify a tuff, separating it from rhyolite, a lava with the same general composition and color.
A breccia is normally composed of angular broken rock fragments that have been cemented together, such as where faulting has shattered rocks. In this case, there are so many broken fragments in this tuff that it is called a tuff breccia. The angular fragments in a breccia show that the fragments were cemented where they were formed. Transportation by running water rounds the edges to form a conglomerate. Students should be able to distinguish a conglomerate from a breccia. These can also be compared to coquina, a conglomerate composed entirely of shells and shell fragments.
Silica rich rocks make up the continents. The extrusive volcanic rocks, tuff, tuff breccia, rhyolite (silica rich lava), pumice and obsidian have the same general composition as granitic rocks that have cooled far below the surface. They are generally light in color and not dense. Silica poor rocks make up the oceanic crust. Primarily basalt, they are dark, relatively heavy and rich in magnesium and iron.
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