Transcript Texture - StMarySES4U1 2010

By: Faisal Andrew Travis Adrian
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Geological Significance
Texture
Characteristics
Composition
Structure
Setting
Classification
Composition
•Igneous rocks make up about 90% of the upper part of the earth’s crust.
•Their minerals and chemical make-up give information about the mantle.
•Their age can be obtained using radioactive dating which can then be compared
to other aspects of the earth.
•They can give information about tectonic plates.
•Sometimes they contain important mineral deposits such as tin and uranium.
Texture
•When magma cools and hardens slowly, large, welldeveloped crystalline grains form out of the minerals in
the magma.
•The result is an intrusive igneous rock that has a
coarse-grained texture. e.g. Granite
(The core of the earth’s continental crust is made
up of granite)
•When lava cools quickly on the earth’s surface, large
crystalline grains cannot form due to the rapid loss of
heat.
•This produces a fine-grained rock. e.g. Basalt
(The earth’s oceanic crust is made up of basalt)
Texture (cont’d)
•Igneous rock that have a porphyritic texture have a
mixture of large crystals embedded in a mass of
smaller crystals.
•This happens when magma cools slowly at first, then
faster as it reaches the earth’s surface.
•When magma has a high viscosity, or is rich in
minerals, there may not be any crystals. e.g.
Obsidian (volcanic glass)
•If the magma contains large amounts of
dissolved gases, the gases may get trapped in the
rock, which results in a rock full of holes. e.g.
Pumice
Some characteristics of igneous rocks are:
•Can look glassy or frothy
•Can be porous or full of bubbles (pumice)
•Can have large, small, or a combination of large and small crystal structures
•Can be light enough to float
•Igneous rocks are formed after the cooling of molten magma. This process is
called “crystallization”.
•The three types of igneous rock that will be discussed; basalt, granite and
andesite all have different compositions.
• Granite is formed from magma high in silica.
• Andesite contains feldspar, hornblende, quartz and micas which all look
darker and formed from magma with moderate amounts of silica.
• Basalts share the same minerals as Andesites as they have feldspar, mica,
and hornblendes but they rarely have any quartz.
• The crystal structures of most Igneous rocks are very well-developed.
• Low viscosity and slow cooling helps increase the grain size of any igneous
rock.
• A microscope may be needed to see the crystal structures of Igneous rocks.
• Furthermore, various elements of Igneous rocks are formed depending on
the magma.
• All sorts of minerals fall into this magma from rocks that have been
pushed deep and melted under its fiery pressure
• Silica is added into the magma as it gets melted from underneath the
continent and mixes itself with water and carbon dioxide.
• http://videos.howstuffworks.com/hsw/10466
_forces_that_shape_the_earth_igneous_rocks
_video.htm
Setting: Igneous rocks begin as
a hot fluid material. The
word "igneous" comes from
the Latin word for fire. This
material may have been
lava erupted at the Earth's
surface, or magma at
shallow depths, or magma
in deep bodies (plutons).
Rock formed of lava is called
extrusive, rock from shallow
magma is called intrusive
and rock from deep magma
is called plutonic.
• People commonly think of lava
and magma as a liquid, like
molten metal, but geologists find
that magma is usually a mush; a
liquid carrying a load of mineral
crystals. Magma crystallizes into a
collection of minerals, and some
crystallize sooner than others.
Not just that, but when they
crystallize, they leave the
remaining liquid with a changed
chemical composition. Magma
cools and evolves as it moves
through the crust, interacting
with other rocks evolving further.
• Igneous rocks form in
three main places: where
lithospheric plates pull
apart at mid-ocean
ridges, where plates come
together at subduction
zones, and where
continental crust is
pushed together, making
it thicker and allowing it
to heat and melt.
• Igneous rocks are called
fire rocks and are formed
either underground or
above ground.
Underground, they are
formed when the melted
rock, called magma, deep
within the earth becomes
trapped in small pockets.
As these pockets of
magma slowly cool
underground, the magma
becomes igneous rocks.
• Igneous rocks are also
formed when volcanoes
erupt, causing the
magma to rise above
the earth's surface.
When magma appears
above the earth, it is
called lava. Igneous
rocks are formed as the
lava cools above
ground.
• Igneous rock formation can
also happen when
sedimentary rocks are
remelted. This takes place at
subduction zones. Subduction
zones occur when one of the
earth's plates moves under
another. Any sedimentary
rocks sitting atop the
subducting plate is pulled
down. Tremendous heat is
generated as the two plates
grind against each other. This
heat and the heat of the
underlying magma melts the
rock.
http://www.youtube.com/
watch?v=vLJ9a56FHCA
• Intrusive: Coarse-grained rocks that cool beneath the earths
surface, most common type Granite
• Extrusive: Fine-grained rocks that cool above the earths
surface , most common type is Basalt
Basalt
Granite
• Felsic: Contains high silica levels composed primarily of the
minerals Potassium feldspar, Quartz, and Plagioclase
• Intermediate: Moderate silica levels, composed of
Plagioclase, Biotite, and Amphibole
• Mafic: Low silica levels, but rich in iron and magnesium.
Composed mainly of Pyroxene, Olivine, and Plagioclase
• Ultramafic: Extremely low silica level, composed of the
minerals Pyroxene, and Olivine
Obsidian (Extrusive Felsic)
Gabbro (Intrusive Mafic)
Diorite (Intrusive Intermediate)
Peridotite (Intrusive Ultramafic)
• Cooling Rates: “When lava flows on the earths surface , it is
exposed to air and moisture. Under these conditions, the lava
cools quickly, and there is not enough time for large crystals to
form. Thus, extrusive igneous rocks such as obsidian have no
visible mineral grains”
• Texture: Geologist use texture from different igneous rocks to
classify them. By slicing the rock into thin samples geologist
can categorizes the type of crystals that have formed. During
fractional crystallization specific crystals form under certain
conditions
Works Cited
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Alden, Andrew. About.com. 10 March 2010. The New York Times Company.
<http://geology.about.com/cs/basics_roxmin/a/aa011804a.htm>.
Busbey III, Arthur B. The Nature Company Guides: Rocks & Fossils. San
Francisco:Time-Life Books, 1996.
Millage, Clayton. Earth Science: Geology, the Environment, and the Universe. New
York: Mcgraw-Hill, 2002.
“All About Gemstones.” 2009. March 2010.
<http://www.allaboutgemstones.com/rock_cycle_igneous.html>