Igneous Rocks: Terminology Igneous rocks are formed as a result of cooling and crystallization from a magma Magma is molten rock, rich in silica (SiO2), which contains dissolved volatiles such as gasses Lava is magma extruded on or very near the Earth’s surface Igneous rocks are formed as a result of cooling and crystallization from a magma Magma is molten rock (fluid), rich in silica (SiO2), which contains dissolved volatiles such as gasses Lava is magma extruded on or very near the Earth’s surface. Most lavas have been significantly degassed en route to the surface.

Igneous Rocks 2 Types Extrusive (Volcanic Rock) Cool above the Earth’s surface (fast cooling) Magma which has reached the surface is known as lava The heat from the lava goes quickly and the lava solidifies Example: basalt, rhyolites, pumice, obsidian (smooth volcanic rock) Intrusive (Plutonic Rock) Cool beneath the Earth’s surface (slow cooling) Example: granite

Type of Igneous Rock Formed Depends on three factors: Chemical composition of the magma Example: The amount of silica (SiO 2 ) found in magma can vary from 75 % to less than 45 %. Temperature of solidification The temperature of cooling determines which types of minerals are found dominating the rock's composition Low temperature rocks are rich in minerals composed of silicon, potassium, and aluminum. High temperature igneous rocks are dominated by minerals with higher quantities of calcium, sodium, iron, and magnesium Rate of cooling which influences the crystallization process Igneous rocks that form through a gradual cooling process tend to have large crystals. Relatively fast cooling of magma produces small crystals. Volcanic magma that cools very quickly on the Earth's surface can produce obsidian glass which contains no crystalline structures.

Classifying Igneous Rocks Classified based on: A. Texture The size shape and arrangement of constituent minerals Related to the cooling history Fast cooling: crystals do not grow to any great size Slow cooling: crystals grow large B. Composition The percentage of particular minerals contained in the rock Certain minerals are light in color; others dark Reflects the source of magma from which the rock was produced

Igneous Rocks: Cooling History Length of time for cooling determines the texture and degree of crystallization Extrusive Fast cooling Fine grained Example: Basalt Glassy texture Example: Obsidian (like glass) Example: Pumice (holes) Lava that cools very quickly can lock in gases as it solidifies Result: Rock with many holes Think: Pumice stone ie. Basalt

Igneous Rocks: Cooling History Intrusive Slow cooling Coarse grained and larger crystals Example: Granite Example: Diorite Example: Gabbro ie. Granite

Extrusive Rock Bodies: Volcanoes Mafic (basaltic) eruptions Character: fissures or fractures mafic magma fluid not explosive Extrusive igneous rock is formed when lava is extruded onto the earth's surface by volcanic eruptions.  As we've already discussed, basaltic eruptions (mafic ~50% silica) behave much differently, and therefore produce different structures than silicic (>65% silica),(granitic) eruptions. Basaltic eruptions Character -typically erupt from fissures or fratures -extrude mafic magma -are fluid -quiet, not explosive

Extrusive Rock Bodies: Volcanoes Mafic (basaltic) eruptions • Structures Flood basalt STRUCTURES: Lava tube: After a crust has formed on a flow, the molten interior flows away leaving a hollow structure. Spatter cone: Bits of lava cool in the air before falling to the ground and forming a cone around the lava fountain. Flood basalts: Huge volumes of highly fluid lava erupt above mantle plumes; form flat 'plains' covering 100's of square kilometers. Tephra : Includes all pyroclastic material ejected from a volcano; includes small-sized ash, larger-sized bombs; when tephra accumulates near the vent it forms a cinder cone. Pillow lava : masses of rock from lava cooled underwater. Shield volcano: A volcano with a low, broad profile; produced from the repeated superposition of thin, fluid, basaltic lava flows as they erupt from a central vent. Caldera : Summit crater; formed from the collapse of overlying rock as it looses support after magma has drained from the chamber below. Shield volcano Lava tube Tephra Caldera Pillow lava

Extrusive Rock Bodies: Volcanoes Felsic or Silicic (granitic) eruptions Character: typical at convergent plate boundaries extrude cool magma viscous so does not flow freely highly explosive Character -typically erupt at convergent plate boundaries -extrude cool magma rich in silica and water -are viscous and therefore do not flow freely, instead lava builds up -highly explosive

Extrusive Rock Bodies: Volcanoes Felsic or Silicic (granitic) eruptions • Structures Structures composite cone: Viscous lava and exploded tephra that builds up in alternating layers; form a steep-sided volcano cone. Crater: A depression at the summit of a composite cone volcano; usually the vent. Tephra: Same as what is seen at a mafic eruption, however, much more tephra is produced by silicic eruptions due to their explosive nature. Pyroclastic flow / surge: Hot 'clouds' of tephra and gasses, denser than air, which flow down the sides of the volcano; are responsible for the majority of volcano related deaths. Crater Tephra Pyroclastic flow Composite cone

Intrusive Rock Bodies: Plutons Character cools below the surface of the earth Structures formed by Igneous Rocks Pluton Batholith Stock Neck Dike Laccolith Sill Pipe Xenoliths When magma cools below the surface of the earth it forms intrusive igneous rocks.  These rocks produce the following structures: Pluton: An intrusive rock body of any size. Batholith: A pluton greater than 100 km2. Stock: A pluton less than 100 km2. Dike: Tabular intrusive rock which cuts discordantly (vertically) across strata. Laccolith: A concordant (horizontal) intrusion that generally has a horizontal floor but which arches up the strata above it; lens or spoon shaped. Sill: A concordant intrusion that is injected between layers; no upward arch. Pipe: Volcanic neck; conduit through which magma travels from chamber upwards. Xenoliths: Inclusions of surrounding rock in magma. Pluton

When magma cools below the surface of the earth it forms intrusive igneous rocks.  These rocks produce the following structures: Pluton: An intrusive rock body of any size. Batholith: A pluton greater than 100 km2. Stock: A pluton less than 100 km2. Dike: Tabular intrusive rock which cuts discordantly (vertically) across strata. Laccolith: A concordant (horizontal) intrusion that generally has a horizontal floor but which arches up the strata above it; lens or spoon shaped. Sill: A concordant intrusion that is injected between layers; no upward arch. Pipe: Volcanic neck; conduit through which magma travels from chamber upwards. Xenoliths: Inclusions of surrounding rock in magma.

Intrusive Rock Bodies: Plutons

Volcanic Neck and Dike Cartoon picture of a volcanic neck and dike. (Picture to follow)

Aerial Photo of Shiprock, New Mexico (Volcanic Neck) Shiprock, a volcanic neck in north-western New mexico, rises nearly 550 m above the surrounding plain. From: http://www.ngdc.noaa.gov/seg/hazard

Volcanic Neck and Dike A view of one of the dikes radiating from Shiprock

Edinburgh Sill From: http://www.ngdc.noaa.gov/seg/hazard

Sill - Yellowstone National Park

Sierra Nevada Batholith View of granitic rocks of the Sierra Nevada batholith in Yosemite national Park, California. The near vertical cliff is El Capitan, meaning “the chief”. It rises more than 900 m above the valley floor, making it the highest unbroken cliff in the world. Sierra Nevada Batholith

Stock – Chisos Mountains Texas Casa Grande is the remnant of a large volcanic stock on the east side of The Basin in the Chisos Mountains, Texas.

Laccolith – Black Hills, Wyoming Bear Butte is a laccolith located on the Black Hills, Wyoming - South Dakota. This laccolith is made of magma that never reached the surface to generate an eruption. The magma intruded to a shallow level and then stopped, cooled, crystallized, and solidified. Erosion then stripped the overlying layers of rock away.

Patagonia Pluton - Arizona

Xenoliths – Petersburg, Virginia Inclusion of biotite-rich schist enclosed in Petersburg Granite. Biotite-rich schist represents a fragment of older rock that was caught up in the granitic magma when the Petersburg granite intruded. Exposure along James River on the south side of Belle Island, Richmond.

Pipe – Organ Pipes National Park, Australia The Organ Pipes are a spectacular example of basaltic columns. Rising to 20 metres in height, the Pipes are up to one metre across and are hexagonal in cross section.

Igneous Rock: Metallic Mineral Deposits Rare minerals often found in Igneous rocks Place found: Often found in intrusive (plutonic rock) Think mining: where is the good stuff found Copper, gold …