Minerals, Magmas, and Volcanic Rocks

Lesson at a glance: Students will learn the common volcanic minerals and rocks in Hawaii and compare the composition of basalt from Kilauea to dacite from Mount St. Helens. The composition of magma controls the violence of eruptions and influences the shape of the volcanoes.

Key Concepts: Rocks are classified by the minerals they contain or by their bulk chemistry. The islands of Hawaii are made of basalt, a rock that consist of the minerals olivine, pyroxene, and plagioclase, and is relatively rich in iron and magnesium and poor in silica. Most continental volcanoes are made of andesite and dacite, rocks that consist of the minerals pyroxene, plagioclase, and hornblende and are relatively poor in iron and magnesium but rich in silica. Fluid Hawaiian lavas are associated with non-violent eruptions and the construction of shield volcanoes. More viscous lavas from continental volcanoes are associated with violent eruptions and the construction of stratovolcanoes.

Lesson Outcomes: The students will:

  1. learn the particles of which all matter is composed

  2. recognize the major elements that comprise volcanic rocks

  3. define a mineral

  4. identify rocks using mineral content or chemical composition

  5. compare and contrast rocks from Kilauea and Mount St. Helens

  6. relate magma composition to eruption violence and landform development. 
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