1. Chapter 5.1 – Igneous Rocks Magma – molten rock below Earth’s surface Lava – magma that flows out onto the surface Igneous rocks – rocks that form when magma or lava cools and crystallizes – Rocks must be heated up between 800-1200 C before they melt

2. Composition of Magma Materials in magma – Molten rock – Dissolved gases – Mineral crystals Major elements in magma – Oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, sodium – Silicon is the most abundant in magma

3. Types of Magma Basaltic – 45-52% silica Andesitic – 52-66% silica Rhyolitic – More than 66% silica

4. Magma Formation Formed in two ways – Melting of Earth’s crust – Melting within the mantle Four factors involved in formation – Temperature – Pressure – Water content – Mineral content

5. Magma Formation Temperature increases as you go deeper into the Earth crust – Change in temperature is called geothermal gradient Pressure increases with depth because of weight of rocks Water content changes melting point of rocks

6. Mineral Content Different minerals have different melting points – Basalt (made from olivine, calcium feldspar, and pyroxene) melts at higher temperatures than granite (made from quartz and potassium feldspar) – Rocks rich in iron and magnesium melt at higher temperatures

7. Mineral Content Not all parts of a rock melt at the same temperature – This is why magma is a mix of molten rock and crystals This process is called partial melting N.L. Bowen showed that as magma cools and crystallizes, minerals form in predictable patterns called Bowen’s reaction series.

8. Bowen’s Reaction Series

9. Left branch has iron-rich minerals, which undergo abrupt changes as it cools Right branch represents plagioclase feldspars, which undergo continuous change of composition

10. Fractional Crystallization When magma cools, it crystallizes in reverse order of partial melting – Fractional crystallization As elements are removed because they crystallize, silica is left behind The last elements to crystallize are potassium feldspar and quartz (from aluminum, silica, and potassium)