Section 1—What are Igneous Rocks? Objectives

Section 5.1—What are Igneous Rocks?
Objectives:

1. 
   Compare and contrast intrusive and extrusive rocks.
   
2. 
   Describe the composition of magma.
   
3. 
   Discuss the factors that affect how rocks melt and crystallize.
   

Vocabulary:

igneous rock, lava, extrusive, intrusive, partial melting, fractional crystallization, Bowen’s reaction series

Igneous rocks are formed from the crystallization of magma. The term igneous comes from the Latin word ignis, which means “fire” because early geologists often associated igneous rocks with fiery lava flows. Lava is magma that flows out onto Earth’s surface.
Types of Igneous Rocks

Fine-grained igneous rocks that cool quickly on Earth’s surface are called extrusive igneous rocks. Course-grained igneous rocks that cool slowly beneath Earth’s surface are called intrusive igneous rocks. Granite is the most common intrusive igneous rock.

Composition of Magma

Magma is often a slushy mix of molten rock, gases, and mineral crystals. The compounds that magma is composed of are the same major elements found in Earth’s crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na). Of all the compounds found in magma, silica (SiO₂) is the most abundant. Magmas are classified as basaltic, andesitic, or rhyolitic based on the amount of SiO₂ they contain. The silica content affects melting temperature and how easily magma flows.

Types of Magma
Rhyolitic	70 % SiO2
Andesitic	60% SiO2
Basaltic	50% SiO2

Origins of Magma

Rock melts at temperatures of 800^oC to 1200^oC. These temperatures are found in the upper mantle and lower crust. This heat comes from Earth’s molten formation and heat from the decay of radioactive elements.

Factors That Affect Magma Formation

The main factors that determine the type of magma are temperature, pressure, water content, and mineral composition. Temperature increases with depth in Earth’s crust. This temperature increase is called the geothermal gradient.

Differences in mineral composition cause the geothermal gradient to be higher in oceanic crust than in continental crust.

Pressure also increases with depth due to the weight of the overlying rock. As pressure on a rock increases, its melting point increases. In other words, it takes more energy to melt the same rock under higher pressure. Water content also affects how rock melts. Higher water content lowers the melting point of rock.
Mineral content also impacts how magma is formed. In general, oceanic crust is rich in iron and magnesium and therefore, melts at higher temperatures than continental crust, which contains higher levels of silicon and aluminum.
How Rocks Melts and Solidifies

Rock is a solid mixture of different components that have different melting points. Rock melts in a similar way as ice that contains bits of wax.

Rock melts in a process called partial melting. Partial melting is the process whereby some minerals melt at low temperatures while other minerals remain solid. The magma constantly changes composition as a rock melts and different groups of minerals melt and dissolve into the magma.
When magma cools it solidifies in a process called fractional crystallization. Fractional crystallization is a process wherein different mineral form at different temperatures. Fractional crystallization is the opposite of partial melting.
Bowen’s Reaction Series illustrates that as magma cools, minerals form in predictable patterns. Bowen discovered two main patterns, or branches, of crystallization.

• 
  The right side of Bowen’s reaction series represents the Non-ferro magnetic (non-iron) materials. When this magma cool slowly, the rocks that form show a continuous, gradual, change of mineral composition from calcium rich to sodium rich. When they cool quickly, the result is a zoned crystal that has sodium-rich outer layers and calcium-rich inner cores.
  
• 
  The left branch of Bowen’s reaction series represents the iron-rich minerals. These minerals undergo abrupt changes in a process called fractional crystallization. As magma cools, minerals crystallize out of solution as their melting points are reached. Olivine begins to crystallize at 1800^oC. Pyroxene begins to crystallize at 1557^oC.
  
• 
  As minerals are removed from magma due to crystallization, the liquid becomes rich in silica and oxygen, the most abundant elements in magma. The temperature eventually drops to the point where quartz crystallizes. Quartz is made of silica. Because quartz is the last liquid to solidify, it often forms in veins, or cracks or fractures in previously solidified rock.
  

Quartz, the last mineral to crystallize, often forms in rock veins when the remaining magma is squeezed into rock fractures and cools.

Crystal Separation

As crystals form, they can settle to the bottom of the magma body. This causes the liquid magma to separate from the solid crystals. As cooling continues, the rock can form crystal layers from the bottom up.

Layered Intrusions

An intrusion is a rock formation caused by a forcible entry of molten rock or magma into or between other rock formations. Crystal separation can cause a layered intrusion if the conditions of composition and cooling are right. Layered intrusions can often contain valuable sources of important elements such as gold, platinum, chromium, and nickel (see Figure 5-9 on page 106 of the textbook).