Lab 3: Igneous Rocks
The Geology in YOUR life initiative Mount Shinmoedake erupts in Japan (Jan 26, 2010) Volcanic smoke rises from Mount Shinmoedake on 1 February, 2011. Smoke rises from Mount Shinmoedake in the Kirishima mountain range in Japan. Photo: AFP/Japan Meteorogical Agency
The Geology in YOUR life initiative Residents near Japanese volcano urged to evacuate Reuters Alert Net 3 hours ago Feb 17 (Reuters) - More than 2,500 people living near a volcano that has been spewing ash in southern Japan were advised to evacuate their homes on Thursday after heavy rain threatened mudslides of accumulated ash, a local official said. Shinmoedake... http://www.daylife.com/topic/shinmoedake
Igneous Rocks Aggregates of minerals that crystallized from molten material. FELSIC MAFIC Intrusive Extrusive The type of igneous rock formed depends on several factors including: 1. Original composition of the molten material 2. The rate of cooling 3. Reactions that occurred within the magma as cooling occurred
Why do we study igneous rocks? YELLOWSTONE CALDERA NPS 1. Learn future eruptive style of volcanoes Image by Christoph Hormann How hazardous are existing volcanoes to human life?
Rhyolite Tuff Indiana State University USGS Open-file Report 2007 1071U Photo by Jim Peaco, NPS Studying igneous texture of rocks at Yellowstone allowed us to map the extent of previous eruptions that covered the U.S., characterizing Yellowstone as a supervolcano!
2. Igneous rocks commonly form at tectonic plate boundaries. Studying igneous rocks helps us learn about the history of the Earth.
Copper = good conductor of heat and electricity Native Copper Used for: - copper wire in electronics - cooking utensils (pots and pans) 3. Hot magmas drive circulation of hot fluids which pick up and deposit important metals, creating magmatic ore deposits Copper Porphyry Deposit Open Pit Mine - Bingham Copper Mine, Utah
Molten rock goes by two names: MAGMA if it is underground cools slowly because it is insulated by the surrounding Earth LAVA if it is on the surface of the Earth cools quickly because it is exposed to the cold air
Extrusive igneous rocks cool quickly from lava on the surface Intrusive igneous rocks cool slowly from magma beneath the surface.
Texture There are two fundamental ways to classify igneous rocks: texture composition. Composition
Texture: Phaneritic rocks cool slowly, giving large visible crystals time to form. Aphanitic rocks cool quickly, so their crystals are small to microscopic. (Which do you think are normally intrusive and extrusive?)
Composition: Most familiar igneous rocks form a continuum of variation between: Felsic - rich in silica and poor in iron (Fe) and magnesium (Mg Mafic - poor in silica and rich in Fe and Mg. (Rocks of the mantle are ultramafic - very rich in Fe and Mg.) So
Mineralogy: 2 categories 4 minerals p. 78 Fig. 5.2 Top Right
p. 79 Fig. 5.3 Top A quick way to estimate the percentage of mafic minerals in a rock.
When identifying an igneous rock: Step one: Figure out its texture. Step two: Figure out its mineral content. Quartz Plagioclase Glassy Vesicular Aphanitic Phaneritic
Aphanitic Phaneritic Like this: Felsic Intermediate Mafic Ultramafic Granite Diorite Gabbro Peridotite Rhyolite Andesite Basalt Komatiite 70% 60% 50% 40% Silica content
NOTE: the following pairs of rock types look different texturally, but are made of the same material: Granite and rhyolite Diorite and andesite Gabbro and basalt Peridotite and komatiite (Note: peridotite is the primary rock of the mantle. Komatiite, its extrusive equivalent, is not formed in the modern Earth.)
Five fine points of texture: 1. Lavas of different composition behave Differently. Mafic lavas flow fluidly.
Volcanoes made of extensive lava flows are called shield volcanoes. Volcanoes made of mafic material tend to have shallow slopes formed from these lava flows.
Felsic lavas are viscous (stiff and sticky, like toothpaste.) The tend to erupt as an aerosol of pyroclasts - tiny fragments.
Mount St. Helens 1980 Eruption
As a result, felsic lavas often solidify into pyroclastic rocks like welded tuff, composed of the fused remains of this aerosol.
Volcanoes made of felsic - intermediate Material tend to have steep slopes and include both pyrocastic And lava flow layers. Thus they are called composite- or stratovolcanoes.
2. Gasses: Magma contains varying amounts of Dissolved gasses (water and CO 2 ). During eruptions, this comes out of solution, like CO 2 in a fizzing soft drink.
As a result, it can contain frozen bubbles or vesicles and has a vesicular texture. If felsic or intermediate, it is pumice. If mafic, it is scoria.
Pumice Floats!
3. Porphyries When magma slowly cools underground, different minerals crystallize at different temperatures. The magma goes through a crystal mush phase where solid crystals and liquid magma are mixed. What if this mush were to erupt. The result, would be a porphyry or a porphyritic rock - a mixture of aphanitic and phaneritic textures.
4. In any magma, there are components that remain liquid long after everything else has crystallized. These collect in fractures and slowly crystallize, forming veins of very large crystals. These are called pegmatites, and their texture pegmatitic.
5. In contrast, sometimes lava cools so quickly that no crystals can form at all. In this case its texture is glassy, as in the example of obsidian.
So, to sum up the details: Texture: 8 different choices p. 78 Fig. 5.2 Top Left
Intrusive/Extrusive equivalents p. 79 Fig. 5.3 Bottom
Mt. Saint Helens Hawaii Rhyolite APHANITIC Basalt APHANITIC FELSIC MAFIC
Cooling of Magma bodies in the subsurface PHANERITIC Granite Gabbro MAFIC or FELSIC depends on composition of magma
MAFIC igneous rocks dominate oceanic crust FELSIC igneous rocks dominate continental crust