Progetto CLIL Liceo statale «James Joyce» di Ariccia A.S. 2015/2016 Dott.ssa Giulia Botticelli Dott. Mario Silvestri Geology: MINERALS AND ROCKS Lesson 3 – Intrusive Rocks

What are Igneous Rocks? Igneous rocks are formed from the solidification of molten rock material. There are two basic types. Intrusive igneous rocks crystallize below Earth's surface and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite. Extrusive igneous rocks erupt onto the surface where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff.

Igneous Rocks INTRUSIVE EXTRUSIVE MagmaLava Slow coolingFast cooling LARGER CRYSTALSSMALLER CRYSTALS

Magma Magma is a molten and semi-molten rock mixture found under the surface of the Earth. This mixture is usually made up of four parts: a hot liquid base, called the melt; minerals crystallized by the melt; solid rocks incorporated into the melt from the surrounding confines (xenoliths); and dissolved gases. When magma is ejected by a volcano or other vent, the material is called lava. Magma that has cooled into a solid is called igneous rock. Magma is extremely hot—between 700° and 1,300° Celsius (1,292° and 2,372° Fahrenheit). This heat makes magma a very fluid and dynamic substance, able to create new landforms and engage physical and chemical transformations in a variety of different environments.

How magma forms? Magma originates in the lower part of the Earth’s crust and in the upper portion of the mantle. Differences in temperature, pressure, and structural formations in the mantle and crust cause magma to form in different ways. Decompression Melting: Decompression melting often occurs at divergent boundaries, where tectonic plates separate. The rifting movement causes the buoyant magma below to rise and fill the space of lower pressure. Decompression melting also occurs at mantle plumes, columns of hot rock that rise from Earth’s high-pressure core to its lower-pressure crust. When located beneath the ocean, these plumes, also known as hot spots, push magma onto the seafloor. These volcanic mounds can grow into volcanic islands over millions of years of activity.

How magma forms? Transfer of Heat: Magma can also be created when hot, liquid rock intrudes into Earth’s cold crust. As the liquid rock solidifies, it loses its heat to the surrounding crust. Much like hot fudge being poured over cold ice cream, this transfer of heat is able to melt the surrounding rock (the “ice cream”) into magma. Transfer of heat often happens at convergent boundaries, where tectonic plates are crashing together. Flux Melting: Flux melting occurs when water or carbon dioxide are added to rock. These compounds cause the rock to melt at lower temperatures. This creates magma in places where it originally maintained a solid structure. Much like heat transfer, flux melting also occurs around subduction zones. In this case, water overlying the subducting seafloor would lower the melting temperature of the mantle, generating magma that rises to the surface.

Types of magma Mafic magma (Basaltic magma): Mafic magma has relatively low silica content, roughly 50%, and higher contents in iron and magnesium. This type of magma has a low gas content and low viscosity (resistance to flow). Mafic magma also has high mean temperatures, between 1000 o and 2000 o Celsius (1832 o and 3632 o Fahrenheit), which contributes to its lower viscosity. Low viscosity means that mafic magma is the most fluid of magma types. All magma contains gases and a mixture of simple elements. Being that oxygen and silicon are the most abundant elements in magma, geologists define magma types in terms of their silica content, expressed as SiO 2. These differences in chemical composition are directly related to differences in gas content, temperature, and viscosity.

Types of magma Felsic Magma (Rhyolitic magma): Felsic magma has the highest silica content of all magma types, between 65-70%. As a result, felsic magma also has the highest gas content and viscosity, and lowest mean temperatures, between 650 o and 800 o Celsius (1202 o and 1472 o Fahrenheit). Thick, viscous felsic magma can trap gas bubbles in a volcano’s magma chamber. Intermediate Magma (Andesitic magma): Intermediate, or andesitic, magma has higher silica content (roughly 60%) than mafic magma. This results in a higher gas content and viscosity. Its mean temperature ranges from 800 o to 1000 o Celsius (1472 o to 1832 o Fahrenheit). As a result of its higher viscosity and gas content, intermediate magma builds up pressure below the Earth’s surface before it can be released as lava.

Bowen’s reaction series

Sample Identification: the texture

C. Vesicular

Sample Identification: the texture C. Vesicular

Sample Identification: the texture C. Vesicular E

Sample Identification: the texture F. Glassy

Sample Identification: the texture G. Pyrocastic

Sample Identification: the texture H. Pegmatitic

Sample Identification

And now...Sample Identification!