Progetto CLIL
Liceo statale «James Joyce» di Ariccia
A.S. 2015/2016
Geology: MINERALS AND ROCKS
Lesson 3 – Intrusive Rocks
Dott.ssa Giulia Botticelli
Dott. Mario Silvestri
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
Magma
Lava
Slow cooling
Fast cooling
LARGER CRYSTALS
SMALLER 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
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 SiO2. These differences in chemical
composition are directly related to differences in gas content, temperature, and
viscosity.
• 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
1000o and 2000o Celsius (1832oand
3632o Fahrenheit), which contributes to its lower
viscosity. Low viscosity means that mafic magma
is the most fluid of magma types.
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 650o and
800o Celsius (1202o and 1472o 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 800o to
1000o Celsius (1472o to 1832o 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
http://digilander.libero.it/madscience/seriebowen.pdf
Sample Identification: the texture
Sample Identification: the texture
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!