9-7 - Igneous Rocks Igneous Rocks Solidified molten rock...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Igneous Rocks Igneous Rocks Solidified molten rock Earth is mostly igneous rock. Earth Magma Lava Magma erupts via volcanoes. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Igneous Environments Two major categories - Based on cooling site. Extrusive settings – Cool at or near the surface. Intrusive settings – Cool at depth. Intrusive Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Extrusive Characteristics Lava flows – Sheets of cooled lava. Low-viscosity mafic lava (basalt) Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Extrusive Characteristics Explosive ash eruptions. Explosive High-viscosity felsic magma builds volcanic pressure. Violent eruptions yield huge volumes of volcanic ash. Violent Ash can cover large regions. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Typical Igneous Intrusions Intrusive rocks cool at depth, they don’t surface. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Plutonic Activity Exposed Batholiths Sierra Nevada, USA Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Cooling Rates Cooling rate – How fast Cooling is heat lost? Depth Shape Ground water. Ground Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks The Petrography of igneous rocks Two primary properties of igneous rocks are the basis for their classification into different rock types: 1. the texture (size) of their grains 2. the composition of their grains Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks 1. Texture of igneous 1. Texture rocks rocks Mineral crystals require time to grow. Thus, the grain size of igneous rocks is determined by the rate of cooling of their parent magma/lava Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Slow cooling, over hundreds of thousands to millions over of years, allows time for large (i.e. visible to the eye) mineral crystals to grow: mineral • coarse grained or “phaneritic” texture (Greek phaneros ~ visible) • typical of plutonic rocks, cooled slowly at depth, Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Fast cooling, over months and years, results in the over growth of smaller mineral crystals, visible only with a lens or a microsope: lens • fine grained or “aphanitic” texture • typical of volcanic rocks: they cool much more rapidly, exposed to the cold of the atmosphere or ocean • also typical of shallow plutons, which are surrounded by cooler rocks of the upper crust Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Very fast cooling over a matter of hours and days results in little or no crystal growth: growth: • non-crystalline or amorphous or “glassy” texture • typical of volcanic rocks, and particularly submarine volcanics and the surfaces of lava flows (where they contact air/water) Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Glassy Classification Glassy igneous rocks. Obsidian – Volcanic glass from rapidly cooled lava. Pumice – Frothy felsic rock full of vesicles; it floats. Pumice Scoria – Glassy, vesicular mafic rock. Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Variable cooling histories, beginning with a phase of beginning slow cooling in a pluton and then a sudden expulsion from volcanos, results in rocks with both coarse and fine grains: fine “porphyritic” texture consisting of: • coarse phenocrysts that crystallized slowly in the pluton, and; • fine groundmass of aphanitic grains and glass particles that crystallised rapidly after volcanic eruption. • large isolated grains termed “phenocrysts” whereas a mixture is termed a “porphyry” Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks 2. Composition of igneous 2. rocks rocks Igneous rocks can contain many rare and exotic minerals. For the most part, however, they are comprised of eight minerals and mineral groups from two compositional classes: • mafic minerals: rich in Fe and Mg, and generally dark coloured • felsic minerals: rich in Si, Al, K, Na, Ca (no Fe-Mg) and often light coloured Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks • mafic minerals: rich in Fe mafic rich and Mg, and generally dark coloured coloured Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks • felsic minerals: rich in Si, felsic rich Al, K, Na, Ca (no Fe-Mg) and often light coloured and Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks Generalised Diagram Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak Chapter 6: Up from the Inferno: Magma and Igneous Rocks ...
View Full Document

This note was uploaded on 10/03/2011 for the course GEOL 1001 taught by Professor Baksi during the Fall '07 term at LSU.

Ask a homework question - tutors are online