Geology Chapter 5: Igneous Rocks

Three mechanisms for heat transfer

1. Conduction: thermal energy transferred by direct contact 2. Radiant Heat Transfer: heat radiates through air Convection: heat transfer by the movement of fluids

Process of Igneous Rock Formation

1. Melting 40-150 km beneath the surface. Place where melting occurs is called the source area. 2. Once magma begins to form, separate pockets of magma may accumulate to make a larger volume. Magma rises because it is less dense than the rocks around it 3. Magma accumulates to form a magma chamber. Magma may solidify in this chamber. Magma that solidifies under great depth is called plutonic rock. 4. Magma rises through the crust. A body of molten rock in the subsurface is called an intrusion. Magma that solidifies beneath the surface is called an intrusive rock 5. Magma continues to rise, may crystallize. 6. Magma that reaches the surface is called extrusive rock because it forms from magma extruded onto the surface.

How is Oceanic Crust formed:

1. Plate separate = solid mantle rock melts by decompression melting 2. Magma rises and accumulates in magma chambers 3. Magma rises further through fractures that form as plates diverge 4. older oceanic crust moves away as new oceanic crust is produced

How does magma rise through crust?

1. Rock melts = magma becomes less dense and rises over overlying rocks 2. Pressure from magma and tectonic forces help produce fractures and other weaknesses through which magma can move. FRACTURE: magma filled fissure DIKE: when magma solidifies within a fissure

Thermal Energy and Plate Tectonics

1. Solid asthenosphere rises beneath oceanic a mid-ocean ridge => brings hot rocks upward by convection, adding material to the oceanic lithosphere 2. Sea-water is drawn into hot crust of mid-ocean ridge = helps cool a bit 3. Newly hot newly created lithosphere begins to cool by conduction 4. The cooled lithosphere subducts back into the astenosphere

3 factors influencing Rock melting:

1. Temperature: different minerals melt at different points, so partial melting can occur. If pressure increases, it may be enough to prevent melting 2. Pressure: rock that is uplifted decreases pressure => may melt (decompression melting) 3. Water content: adding water can lower temperature

Most of Earth's internal temperature is due to energy released by radioactive decay of unstable atoms (alpha decay = most heat today)

1. Unstable atoms lose a proton or neutron to become more stable. In the process, they release energy in the form of electromagnetic radiation 2. In alpha decay, an unstable atom releases a speedy particle that heats surrounding material. By losing a proton or neutron, the atom becomes a different element. 3. The new atom may also undergo alpha decay which releases additional energy

Intermediate Rocks:

Coarsely Crystalline: Diorite (contains abundant plagioclase, feldspar, and amphibole and variable amounts of pyroxene and biotite) Fine-Grained: Andesite

What types of igneous rocks form along mid-ocean ridges?

All mafic. Upper oceanic crust: basaltic lava flows. Pillow basalts. Under pillow basalts, finely cystalline basalts. These thin intrusions are called dikes Sheeted dikes merge downward towards gabbro which represent magma chambers Base of gabbros is the base of the oceanic crust =ultramafic

Mafic: consists of pyroxene and other mafic materials (magnesium and iron rich)

Coarsely Crystalline: Gabbro Fine-Grained: Basalt (dark pyroxene, green olivine)

Felsic Rocks: rich in quartz and feldspar (thus has a light color)

Coarsely Crystalline: Granite Fine-Grained: Rhyolite

Ultramafic: (magnesium and iron rich)

Coarsely Crystalline: Peridotit

How are magmas generated in continental rifts?

Continental Rifts are formed where tectonic forces 1. Solid asthenopshere rises beneath rift and undergoes decompressoin melting. Partial melting of ultrmamfic mantle yeilds mafic magma. 2. Mafic magma rises into upper mantle and lower continental crust and accumulates in magma chambers. 3. Heat from the mafic magma melts adjacent continental crust, producing felsic magma. Intermediate magma forms because of mixture of mafic and felsic magma. 4. Some intermediate and felsic magma solidifes underground as granite.

Igneous Rocks:

For by solidification of magma Phaneritic: Igneous rocks with visible crystals Aphanitic: Igneous rocks with crystals not visible to the eye. Pegmatite: Igneous rocks with very large crystals (cm-m long) Coarse-Grained: most crystals are larger than several millimeters (up to cm across ) Medium-Grained: crystals that are easily visible to the unaided eye Fine-Grained: crystals are too small to see without aid Glassy: igneous rocks that consist of glass rather than crystal

How is magma generated at hot spots?

Hot Spot: Intense site of volcanic activity 1. Hot Spots and Mantle Plum: hot mantle material. Rises because it is less dense than material around it. 2. Hot Spots in Oceans: Magma from lithosphere and plum can reach the surface, creating large seafloor volcanoes 3. Hot Spots in Continents: When rising mantle plum encounters continental lithosphere, its high temperature can cause melting. Melting in lower part of lithsphere generates mafic magma while melting in upper part of lithosphere generates felsic magma.

How do large Magma Chambers form?

Magma Chamber: large underground body of molten rock. A large influx of magma is requiered to form a magma chamber which, in turn, requieres melting on a large scale.

What determines how far magma can rise?

Magma Pressure: pressure pushes magma into any avaliable space Density: different densities drive flow of magma Gass Pressure: decreasing pressure allows bubbles in magma to form Stress: tectonic stress helps magma open steep fractures that provide a pathway to the surface

Andesite to Granite

Melt andesite, partial melting => minerals welded makes up the granite If we see granite somewhere = there is water present and there is a way to get it down, indicative of a complex history

Continenal Collision:

One plate my partially slide into the other, but continental collisions result in different types of magma. 1. Increased temperature and pressure during collisons 2. Water may be released by metamorphism. Water decreases melting temperture and may produce felsic magma. 3. Magma produced by continental collisions may not reaach the surface = less volcanoes

Influences of Composition of Magma:

Partial Melting: some minerals melt before others. Felsic minerals melt at lower temperatures than mafic minerals. Crystallization: as magma cools, mafic minerals crystallize first. Assimilation and Magma Mixing: when two different magmas come into contact.

Partial-Melting of Continental Crust:

Rocks in continental crust typically include quartz, feldspar, mica, amphibole, plagioclase

How are magma chambers exposed at the surface?

Solidified magma chamber: pluton 1. Irregular Plutons: vertical cylinders 2. Sheetlike Plutons: think or thick sheets 3. Batholiths: One or more contigous plutons

How does magma form along convergent plate boundaries?

Subduction Zones: 1. When an oceanic plate converges with another oceanic plate, subductions 2. Changes in pressure and temperature convert minerals into new ones through metamorphism. The water liberated from minerals rises into overlying aesthenosphere. 3. Added water lowers melting temperature of mantle above he subduction zone. If temperature is high enough, melting occurs.

In what setting do different igneous textures form? (Depends on environment in which magma solidified)

Vesicles: gases dissolved in magma accumulate as bubbles, only near the surface under low pressure Volcanic Breccia: forms by explosive eruptions from rock and ash fragments, lava flow that breaks apart as it partially solidifies while flowing, or from volcano triggered mudflows or landslides Volcanic Glass: forms by instantaneous cooling (crystals don't have time to form) Fine-Grained: forms when magma only has enough time to grow small crystals Coarse-Grained: forms at greater depths because magma cools slower Pragmatic: may form if magma is water rich

What controls how easy magma moves?

Viscosity: controlled by 3 factors 1) Temperature: low temperature = more viscous. HIgh Temperature = less viscous 2) Composition: Abundant Silicate Chains: silicon and oxygen tetrahedra link together to create silicate chains. Silicate chains don't bend or move easily = more viscous Few Silicate Chains: less viscous Volatile: water dissolved in magma decreases viscosity 3)Percentage of crystals: Abundant crystals: crystals in flowing magma get in each other's way and cause magma to flow more slowly Few crystals: less internal obstruction and flows more easily

Laccolith:

a sheet intrusion that has been injected between two layers of sedimnetary rocks

Dike:

a sheetlike intrusion that cuts across any layer

Sill:

an intrusion that is parallel to layers in the host rock

Columnar Joints:

form when a hot but solid igneous rock contracts as it's cooled

Volcanic Necks

form when erosion wears down a volcano, exposing the harder more resistent rocks that solidified