Ch. 3: Igneous Rocks. the Origin and Evolution of Magma, and Intrusive Activity

magma

creating by melting of rock above a subduction zones or oceanic ridges - molten rock

rock

naturally formed, consolidated materials usually composed of grains of one or more minerals

rock cycle

rock and minerals are changing through time, driven by internal and external heat engines of E in a sort of equilibrium

igneous rock

less dense magma rises and cools

igneous rock exposed @ surface get weathered into

sediment

Sediments transported to low areas are buried and hardened into

sedimentary rock

Sedimentary rock heated and squeezed at depth to form

metamorphic rock

Metamorphic rock may heat up and melt to form

magma

igneous rock: intrusive

when magma solidifies underground = ex:granite

igneous rock: extrusive

when magma solidifies @ (on) E's surface (lava) = ex:basalt

granite: intrusive igneous

cool slowly deep beneath E's surface and are typically coarse-grained (most crystal >1mm)

basalt: extrusive igneous

cools quickly @ or near E's surface and are typically fine-grained (most crystals <1mm) -often w/vesicles or not shown

silica (SiO2) contents

determines mineral contents and general color of igneous rocks

mafic rock

- about 50% silica

-contains dark-colored minerals high in Mg, Fe, and C

-ex: intrusive/extrusive = gabbro/basalt

felsic (silicic)

- >65% silica

-contains light-colored minerals high in silica, Al, Na, K

-ex: intrusive/extrusive = granite/rhyolite

intermediate rock

- have silica contents btwn mafic and felsic roc

-ex: intrusive/extrusive = diorite/andesite

ultramafic

- <45% silica by weight

-composed of almost entirely of dark-colored ferromagnesian materials

- most common, peridotite (intrusive)

intrusive rocks exist in bodies or structures that penetrate or cut through pre-existing

country rock

two types of intrusive rock bodies

shallow and deep intrusive

intrusive bodies

are given names based on their size, shape and relationship to country rock

Shallow intrusions

-Form <2 km beneath Earth's surface

-Chill and solidify fairly quickly in cool country rock

-Generally composed of fine-grained rocks

Deep intrusions: Plutons

-Form at considerable depth bneath E's surface when rising blobs of magma (diapirs?) get trapped w/in the crust

-Crystallize slowly in warm country rock

-Generally composed of coarse-grained rocks

three types of shallow intrusives

volcanic necks, dikes, and sills

shallow intrusive: volcanic necks

formed when magma solidifies in throat of volcano

shallow intrusive: dikes

tabular intrusive structure that cuts across any layering in country rock (not parallel to any layering) - are discordant

shallow intrusive: sills

Tabular intrusive structure that parallels layering in country rock - are concordant

two types of deep intrusives

plutonic rocks and plutons

plutonic rock

igneous rocks that crystallize at great depths >several kilometers, coarse grained, slow cooling

two types of plutons

stocks and batholiths

plutons

large, irregular shaped discordant igneous bodies. Commonly granitic

plutons: stocks

small plutons with <100 sq. km of exposed area

plutons: batholiths

large plutons or group of plutons w/outcrop area >100 sq. km.

how magma forms

from minerals being melted

how magma forms :two type of minerals being melted

heat for melting rocks and geothermal gradient & partial melting

heat for melting rocks

heat moves upward (by conduction and convection) from the very hot (>5000°C) core through the mantle and crust

geothermal gradient & partial melting

This is the rate @ which temperature increases w/increasing depth bneath the surface (about 3⁰C per 100 meters = 30⁰C/km).

how magma forms: Factors that Control Melting Temperatures (2)

1. Pressure (decompression melting...in mantle @ spreading ridges - partial melting of mantle peridotite)

2. Hot water under pressure (flux melting) at subduction zone

Pressure (decompression melting...in mantle @ spreading ridges - partial melting of mantle peridotite)

Melting point of minerals increases w/increasing pressure

Hot water under pressure (flux melting) @ subduction zones

Water bcomes increasingly reactive @ higher temps. @ sufficient pressures and temps., highly reactive water vapor can reduce the melting point of rocks by over 200°C

Bowen's Reaction Series

sequence in which minerals crystallize from a cooling basaltic magma

How Magmas of Different Compositions Evolve by the processes

differentiation, assimilation, magma mixing, and partial melting

differentiation

the changing of magma composition by the removal of denser early-formed ferromagnesian minerals by crystal settling

assimilation

occurs when a hot magma melts and incorporates surrounding country rock = changing the chemical composition of the magma

magma mixing

-involves mixing of silica and mafic magma to make intermediate composition

-bcuz of their significant temp. differences, two magma not mixed throughly -> end up w/blobs of finer grained gabbro included in the felsic magma

-but the intrusion has an overall intermediate composition

partial melting

Only part of the rock melts... Under increasing temp. a rock will bgin to melt in a sequence progressing upward through Bowen's reaction series

igneous activity

occurs primarily @ or near tectonic plate boudaries

mafic igneous rocks formed

-commonly @ divergent

-decompression melting: increased heat flow and decreased overburden pressure produce mafic magmas from partial melting of the asthenosphere @ 50km depth

intermediate igneous rock formed

-commonly @ convergent

-partial melting of basaltic oceanic crust produces intermediate magmas

flux melting

happens @ convergent and the subducted oceanic crust releases water into the overlying asthenosphere, lowers its melting temp.