Igneous Rocks & Intrusive Activity PDF

Preview

Summary

Everything you need to know about Igneous Rocks & Intrusive Activity from Dr. Ana Londono....

Description

Igneous Rocks & Intrusive Activity 1 IGNEOUS ROCKS & INTRUSIVE ACTIVITY (09/17/2018-09/21/2018) ● Characteristics of Magma ○ Igneous rocks form as molten rock cools & solidifies ○ General characteristics ■ Parent material of igneous rocks ■ Forms from partial melting of rocks ■ When magma reaches surface, it’s lava ■ Rocks formed from lava are classified as extrusive or volcanic rocks; rapid cooling ● Quicker it cools, the smaller the crystal size ■ Rocks formed from magma are called intrusive or plutonic rocks; slow cooling ○ Nature of magma ■ Consists of three components ● Liquid: called melt, composed of mobile ions ● Solids: silicate minerals have already crystallized from melt ● Volatiles: gases dissolved in the melt, includes water vapor, carbon dioxide, & sulfur dioxide ○ Crystallization of magma ■ Texture in igneous rocks determined by size & arrangement of mineral grains ■ Igneous rocks classified by

Igneous Rocks & Intrusive Activity 2 ● Mineral composition ● Texture ○ Nonwelded, welded, porphyritic (large crystals incorporated into fina crystal matrix), pegmatite (large crystals), coarse-to-medium grained, fine grained, glassy, vesicular (has holes in it) ● Igneous Compositions ○ Granitic Composition ■ Composed of light-colored silicates ■ Felsic (feldspar & silica) in composition ■ High amounts of silica-- highly viscous ● Moves slowly ■ Major constituents of continental crust ○ Basaltic composition ■ Composed of dark silicates & calcium-rich feldspar ■ Mafic (magnesium & iron) in composition ● Ferric iron ■ Denser than granitic rocks ■ Fluid-like behavior-- low viscosity ● Moves quickly ■ Comprise ocean floor & volcanic islands ■ Forms deeper in the mantle

Igneous Rocks & Intrusive Activity 3 ○ Andesitic (intermediate) composition ■ At least 25% dark silicates ■ Associated with explosive volcanic activity ○ Ultramafic composition ■ Rare composition high in magnesium & iron ■ Composed entirely of ferromagnesian silicates ● Naming Igneous Rocks ○ Pyroclastic rocks ■ Composed of fragments ejected during volcanic eruption ■ Varieties ● Tuff: ash-sized fragments ● Volcanic breccia: particles larger than ash ● Origin of magma ○ Highly debated ○ Generating magma from solid rock ■ Produced from partial melting of rocks in crust & upper mantle ■ Role of pressure ● Increase in confining pressure causes increase in rock’s melting point or reducing pressure lowers melting point ● When confining pressures drop, decompression melting occurs ■ Role of heat ● Temperature increases in upper crust (20o -30o C per kilometer)

Igneous Rocks & Intrusive Activity 4 ● Rocks in lower crust & upper mantle are near melting points ● Any additional heat may induce melting ■ Role of volatiles ● Volatiles (primarily water) cause rocks to melt at lower temperatures ○ CO2, SO2 ● Important where oceanic lithosphere descends into mantle ○ Ex. opening a soda ● Evolution of Magmas ○ A single volcano may extrude lavas exhibiting different compositions ○ Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on melting points ■ Bowen’s reaction series ○ Processes responsible for changing magma’s composition ■ Magmatic differentiation ● Separation of a melt from earlier-formed crystals to form different composition of magma ○ Bowen’s reaction series ■ Assimilation ● Changing a magma’s composition by incorporation of foreign matter into a magma ● Surrounding rock bodies fall into magma chamber

Igneous Rocks & Intrusive Activity 5 ■ Magma mixing ● Involves two bodies of magma intruding on one another ● Two chemically distinct magmas may produce a composition quite different from either original magma ● At a subduction zone ■ Partial melting & magma formation ● Incomplete melting of rocks ● Formation of basaltic magmas ○ Most originate from partial melting of ultramafic rock in mantle ○ Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones ○ As basaltic magmas migrate upward, confining pressure decreases which reduces melting temperature ○ Large outpourings of basaltic magma common on surface ● Formation of granitic magmas ○ Most likely form as end product of crystallization of andesitic magma ○ Granitic magmas higher in silica & more viscous than others ○ Because of viscosity, they lost their mobility before reaching surface

Igneous Rocks & Intrusive Activity 6 ■ sticky ○ Tend to produce large plutonic structures ● Intrusive igneous activity ○ Types of features ■ Dike ● Tabular, discordant pluton ■ Sill ● Tabular, concordant pluton ○ Ex. Palisades Sill in New York ■ Laccolith ● Similar to sill ● Lens or mushroom-shape ● Arches overlying strata upward...