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

magma consist of

liquid (ions of elements), solid

(crystals of silicate), and gaseous components (water

vapor, carbon dioxide, sulfur dioxide)

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

What Makes the cycle move

The cycle begins with magma

» Magma forms from melting Earth’s crust and upper

mantle

» Less dense magma rises toward the surface

» Over time magma cools and solidifies – crystallization

» This can be beneath the surface (extrusive) or following a

volcanic eruption, at the surface (intrusive igneous rock)

igneous rock

less dense magma rises and cools

What is weathering

The process that breaks down rocks and minerals into smaller pieces through physical, chemical, or biological means.

igneous rock exposed @ surface get weathered into

sediment

lithification

the process of converting sediment into solid rock through compaction and cementation.

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

Convergent Plate Boundary

where two tectonic plates collide, often leading to subduction and volcanic activity. This boundary is characterized by the descent of one plate beneath another, resulting in the formation of mountains, trenches, and associated igneous activity.

Rock cycle at a convergent plate boundary

Magma forms in melting zone above

subduction zone

Magma migrate upwards to surface

Eruption at surface, solidified into igneous

Exposed rock subjected to weathering

Sediment transported, deposited, lithify into

sedimentary rock

Tectonic forces push it deeper, and deeper

Heat and pressure causes sedimentary rock to

recrystallize to for metamorphic rock

As metamorphic rock is push deeper, may get

to the zone of melting – magma produced

The magma produced rises, erupt as a to the

surface, completing the cycle

Rate of Cooling SLOW

results in larger crystal sizes in igneous rocks. This occurs during the slow solidification of magma beneath the Earth's surface, allowing time for crystals to grow. (>1mm)

Rate of Cooling FAST

results in smaller crystal sizes in igneous rocks. This occurs when lava cools quickly on the Earth's surface, not allowing sufficient time for crystal growth. (<1mm)**

randomly distributed atoms

in solidified lava. This structure forms when cooling occurs so rapidly that there is no opportunity for atomic arrangement, resulting in a glassy texture.

chemical composition 

refers to the specific elements and compounds that make up an igneous rock. It influences the rock's properties, behavior during crystallization, and overall classification.

 Texture

refers to the size, shape, and arrangement of crystals in an igneous rock, which can vary based on the rate of cooling and the environment of formation.

 Crystalline Rocks

made up of interlocking

crystals (e.g. quartz and feldspar). Size of grain or

crystal determine by rate of cooling & viscosity

Glassy Rocks

composed primarily of glass and

contain few, if any, crystals

Fragmental Rocks

composed of fragments of igneous material

 Crystalline Rocks Types

Aphanitic and Phaneritic

Aphanitic

describes a type of igneous rock with a fine-grained texture, where individual crystals are not visible to the naked eye, typically formed from rapid cooling of lava.

Phaneritic

A type of igneous rock characterized by a coarse-grained texture, where mineral crystals are large enough to be seen with the naked eye.

 Crystalline Rocks Textures

Pegmatitic and Porphyritic

Pegmatitic

describes an igneous rock texture that features exceptionally large crystals, typically formed from slow cooling in a magma chamber.

Porphyritic

A type of igneous rock texture characterized by large crystals, called phenocrysts, embedded in a finer-grained or glassy matrix, indicating varied cooling rates during formation.

phenocrysts 

 with the larger —————— having formed first during slow

cooling underground

groundmass

the smaller —————— forming during more rapid cooling at the Earth’s surface

Glassy Textures

Glassy and Vesicular texture 

Glassy

contains no crystals at all and is formed by extremely rapid cooling of the magma. ex Obsidian

Vesicular texture

contains cavities (vesicles) in extrusive rocks resulting from gas bubbles that were in the lava. Scoria and pumice are examples. ex Pumice

Fragmental Texture

Pyroclastic Texture

Pyroclastic Texture

Debris such as ash, pumice, or crystalline rock fragments consolidate(accumulate and are cemented) to form new rock. Tuff (small) and Volcanic Breccia (large) are examples of rocks formed from explosive volcanic eruptions.

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

felsic (silicic)

- >65% silica

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

-ex: intrusive/extrusive = granite (coarse) /rhyolite (fine)

intermediate rock or andesitic

- 55% and 65 % silica

-have silica contents between mafic and felsic rock

-ex: intrusive/extrusive = diorite (coarse) /andesite (fine)

mafic rock

- 45% and 55% silica

-contains dark-colored minerals high in (magnesium, iron, and calcium)

- gabbor (coarse) /basalt (fine)

ultramafic

- <45% silica by weight

-composed of almost entirely of dark-colored ferromagnesian materials

-ex: extrusive = peridotite (coarse) /komatiite (fine)

3 ways to identify Igneous Rocks

Extrusive Intrusive Composition and Texture

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

conduction

the transfer of heat through direct contact between materials.

convection 

the transfer of heat through fluid motion, causing magma to rise and cool.

Decompression Melting

is the process where a decrease in pressure allows magma to form from solid rock without the addition of heat.

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.

Crystal Settling 

is a process where denser crystals formed from cooling magma sink to the bottom of the magma chamber, which can lead to the differentiation of the magma composition.

Mafic magmas

are typically hotter than felsic magmas.

Felsic magmas

are typically formed when mafic (basaltic) magma undergoes

magmatic differentiation

Mantle Plume

is a column of hot rock rising from deep within the mantle, leading to volcanic activity at the surface.

diapir

A type of rock formation where magma rises through the Earth's crust, causing the overlying rock layers to rise and fold.

phenocryst

a large crystal embedded in a finer-grained matrix of an igneous rock, indicating slow cooling.

porphyritic

describing an igneous rock texture characterized by large crystals (phenocrysts) surrounded by a finer-grained groundmass, indicating a complex cooling history.

Chill Zone

the area of an igneous rock where the temperature drops rapidly, typically around the edge of a magma body, resulting in finer-grained texture.