Earth and Environment chapter 6 - Magma and Igneous Rocks

magma

melt that is found underground

lava

melt that has emerged at the Earth’s surface

volcano

a vent from which lava emerges

volcanic eruption

an event during which melt flows from or explodes out of a volcano

lava flow

lava moves in a stream down a slope

pyroclastic debris

when clouds of shattered pre-existing igneous rock as well as droplets or blobs of lava are sent skyward as a result of explosive volcanic eruptions

extrusive igneous rock

rock that forms either by the freezing of lava above ground after it spills out onto the surface of the Earth and comes into contact with air or water, or by the cementing or welding together of pyroclastic debris

intrusive igneous rock

formed from magma that pushed its way into preexisting rock and solidified out of view underground (a body of such a rock is called igneous rock intrusion)

magma chamber

why is it hot inside the Earth?

some heat was left over from the Earth’s early days, as according to the nebula theory the planet was formed from the collision and merging of countless planetesimals, and every time a collision occurred, its kinetic energy transformed into heat energy; as the Earth grew, gravity pulled matter inward until eventually the weight of overlying material squeezed the matter inside tightly together; this compression made the Earth’s insides even hotter, and eventually the Earth became hot enough for the iron inside it to melt, and the dense iron sank to the center, forming the core; even more heat was generated via friction caused by the sinking iron and its surroundings; when a Mars-sized object collided with the Earth vast amounts of heat was generated, but even after the Earth had grown into a planet intense bombardment continued to add heat energy; since then the Earth has cooled by radiating heat into space, and eventually the sea of lava on its surface solidified and formed igneous rock; thanks to the heat of radioactive decay produced by radioactive elements in the Earth the Earth has not cooled down too much to become inhabitable

melting due to a decrease in pressure

by plotting a geotherm, i.e. a curving line on a graph that plots temperature on the horizontal axis and pressure on the vertical axis we can portray how temperature changes with increasing depth in the Earth; this graph emphasizes that temperature always increases with depth but that the rate of increase varies with depth; from the graph we can for example see that temperatures comparable to those of lava exist in the upper mantle, but while the upper mantle has a very high temperature, its rock stays solid because of the great pressure exerted on it by overlying rock, so pressure squeezes atoms together so that they can’t easily break free from solid mineral crystals; this means that a decrease in pressure can permit melting, so if the pressure affecting hot mantle rock decreases while the rock’s temperature remains nearly unchanged, the rock may melt, and this kind of melting (decompression melting) takes place in locations where hot mantle rock rises to shallower depths in the Earth; such movement occurs in mantle plumes, beneath rifts, and beneath mid-ocean ridges

melting as a result of the addition of volatiles

magma can also form in places where chemicals called volatiles have the opportunity to mix with hot mantle rock; volatiles are substances e.g. water and carbon dioxide that evaporate easily and can exist in gaseous forms at the Earth’s surface; when volatiles mix with hot, dry rock, they help break chemical bonds and the rock begins to melt; adding volatiles decreases a rock’s melting temperature

flux melting

melting due to addition of volatiles

melting as a result of heat transfer

when very hot magma from the mantle rises up into the crust, it brings substantial amounts of heat with it; this heat can be conducted into the wall rock surrounding an intrusion and can raise the temperature of the wall rock; in some cases the added heat may be sufficient to cause the wall rock to begin melting (called heat-transfer melting) as it results from the movement of thermal energy from a hotter material to a cooler one

what is in molten rock?

oxides, which bond together in clusters or short chains that can move with respect to one another; dry melts contain no volatiles, whereas wet melts do; since they contain volatiles, magma and lava provide not only the molecules that make up rocks, but also the molecules that comprise the Earth’s water and air

based on what do geologists distinguish among four major compositional types of molten rock?

based on the proportion of silica relative to the combination of magnesium oxide and iron oxide that a melt contains

mafic melts

contain a relatively high proportion of magnesium oxide and iron oxide relative to silica (ma stands for magnesium and -fic comes from Latin word for iron) 46-52% silica

ultramafic melts

have an even higher proportion of magnesium oxide and iron oxide relative to silica, 38-45% silica

felsic melts

relatively high proportion of silica relative to magnesium oxide and iron oxide; 67-76% silica

intermediate melts

composition lies partway between those of mafic and felsic melts; 53-66% silica

why do melts of so many compositions form in the Earth?

source-rock composition; the composition of a melt reflect the composition of the solid from which it was derived, and not all melts form from the same source rock, so not all melts have the same composition

partial melting: under the temperature and pressure conditions that occur in the Earth, only 2-30% of a source rock can melt to produce magma at a given location, since the temperatures at the sites of magma production simply never get high enough to melt the entire source rock, and magma tends to migrate away from the site of melting before all of the original rock has melted; the process by which only part of an original rock melts to produce magma is called partial melting; magmas formed by partial melting are more felsic than the source rock from which they were derived because more silica enters the liquid as melting begins, than remains behind in the still-solid source; partial melting of an ultramafic rock produces a mafic magma

assimilation: as magma sits in a magma chamber before completely solidifying, it can incorporate chemicals dissolved from the wall rock of the chamber or from blocks that detach from the wall and sink into the magma; this process is called assimilation and changes a melt’s composition

magma mixing: different magmas formed in different locations from different source rock may enter the same magma chamber; in some cases the originally distinct magmas mix or mutually dissolve in eachother to produce new, different magma, thoroughly mixing a felsic magma with a mafic magma in equal proportions produces an intermediate magma

why does magma in the Earth rise?

1. magma is less dense than surrounding rock, so a buoyancy force acts on it to drive it upward

2. the weight of rock produces pressure at depth, and this pressure squeezes magma upward

what controls the speed at which molten rock flows?

the viscosity (resistance to flow), which depends on temperature, volatile content and silica content (hotter melt tends to be less viscous than cooler melt because thermal energy breaks bonds and allows atoms or molecules move more easily); a melt containing more volatiles has lower viscosity than does a dry (volatile-free) melt because volatiles also tend to break apart silicate molecules; mafic melt is less viscous than felsic melt because relatively more silicon-oxygen tetrahedra occur in felsic melts and the tetrahedra tend to link together to create long chains that can’t move past one another easily

which factors control the cooling time of magma trapped in an intrusive realm?

1. the depth of intrusion: magma intruded deep in the crust, where hot wall rock surrounds it, cools more slowly than does magma intruded into cool wall rock near the ground surface

2. shape and size of magma body: heat escapes from magma at an intrusion’s surface, so the greater the surface area for a given volume of intrusion, the faster it cools

3. The presence of circulating groundwater: water passing through wall rock carries away heat; magma that interacts with circulating groundwater cools faster than does magma that intrudes dry rock

fractional crystallization

sequential crystal formation; e.g. with mafic magma mafic minerals crystallize first and these solid crystals are denser than the remaining liquid, so they start becoming isolated from the magma, such that the remaining magma becomes more felsic

lapilli

volcanic eruption which is like a cataclysmic explosion, where turbulent clouds of pyroclastic debris is forcefully ejected; lapilli are pea- to golf ball-sized fragments that fall like hail on or near the volcano

pyroclastic flow

caused by an explosive eruption, is a scalding avalanche of ash and other debris that races down the surface of the volcano destroying everything in its path

tabular intrusion

roughly planar, of fairly uniform thickness; e.g. dike, sill

dike

tabular intrusion that cuts across pre-existing layering

sill

tabular intrusion that injects between layers

laccolith

bluster-shaped intrusion that pushes overlying strata upward into a dome, occurs in places where magma injecting between layers gets blocked and cannot spread very far, so the magma accumulates

plutons

irregular or blob-shaped intrusions that range in size

batholith

intrusion of numerous plutons in a region that yields a vast composite igneous body

stoping

pluton intrusions can also involve this, it’s a process during which magma assimilates wall rock, and blocks of wall rock break off and sink into the magma

crystalline igneous rocks

consist of mineral crystals that intergrow when the melt solidifies, so that they fit together like pieces of a jigsaw puzzle

fine-grained rocks probably cooled quickly in lava flows or near-surface dikes and sills, coarse-grained rocks probably cooled more slowly in plutons

the color of an igneous rock provides a rough guide to its composition: mafic rocks tend to be black or dark grey and intermediates tend to be lighter grey or greenish grey, whereas felsic rocks tend to be pink or maroon

fragmental igneous rocks

form from pyroclastic debris and consist of igneous chunks, grains or flakes that are packed together, welded together, or cemented together after they have solidified

glassy igneous rocks

rocks made of a solid mass of glass or of glass surrounding isolated small crystals

glassy texture develops most commonly in felsic igneous rocks because the high concentration of silica inhibits diffusion and therefore the growth of crystals

basaltic and intermediate lavas can also form glass if they cool rapidly enough

extrusive environment

in an extrusive environment, melt may cool quickly so extrusive rocks tend to be fine grained or may even have a glassy texture

lava flow cools and solidifies quickly

intrusive environment

magma can cool slowly, so larger crystals can grow

what clues does a rock’s texture provide us with?

the rate at which it cooled and therefore to the environment in which it formed; a rock’s composition tells us about the original source of the magma and about the way in which the magma evolved before finally solidifying

vesicles

sometimes a rapidly cooling lava freezes while it still contains gas bubbles, these bubbles remain as open holes known as vesicles

volcanic arc

a chain of volcanoes, develops along all convergent boundaries

how does subduction trigger melting?

most of the melt at convergent boundaries comes from flux melting of the asthenosphere in the region just above the downgoing plate; this melting happens because some of the minerals in oceanic-crust rocks contain volatile compounds; at shallow depths the volatiles chemically bond to the minerals in the crust, but when subduction brings oceanic crust down into the hot asthenosphere, the wet crystal rocks start to form and at a certain depth they become so hot that the volatiles separate from the minerals and diffuse upward into the overlying asthenosphere, the addition of volatiles triggers partial melting of the hot ultramafic rock in the asthenosphere and this yields mafic magma

formation of igneous rocks at mid-ocean ridges

most of the igneous activity at the Earth’s surface happens at the mid-ocean ridges of divergent boundaries

magmas form at mid-ocean ridges because of decompression melting; as seafloor spreading takes place and oceanic lithosphere moves away from the ridge, hot asthenosphere rises beneath the ridge axis, and as this asthenosphere rises, the weight of the overlying rock and therefore the pressure of the asthenosphere progressively decreases which leads to partial melting and the generation of basaltic magma

some of this magma rises into the crust and collects in a shallow magma chamber forming a mush of liquid and crystals; slow cooling produces gabbro i

igneous rocks at rifts

rifts occur at places where continental lithosphere undergoes horizontal stretching and therefore vertical thinning; when this happens, pressure on the asthenosphere decreases and decompression melting takes place, producing basaltic magma

some of this magma intrudes the crust to form sills or dikes and some makes it to the Earth’s surface and erupts as basaltic lava

part of the magma produced in the asthenosphere beneath the rifts may become trapped at the base of the continental crust or even in the crust itself; this magma transfers enough heat to the continental crust to cause partial melting which in turn yields felsic magmas that erupt mostly as rhyolitic ash

products of hot spots

not a consequence of plate-boundary interactions; most oceanic hot-spot volcanoes erupt in the interior of an oceanic plate, away from the convergent or divergent boundaries

hot-spot igneous activity is usually associated with mantle plumes - i.e. columns of hot rock rising from deeper in the mantle; according to the plume hypothesis, the plume itself does not consist of magma, but of solid rock that is hot and soft enough to flow plastically

when the hot rock of a plume reaches the base of the lithosphere, decompression causes partial melting, which is a process that generates mafic magma; at oceanic hot spots much of the mafic magma erupts as basalt

difference between magma and lava

magma is liquid rock (melt) under the Earth’s surface, whereas lava is melt that has erupted from a volcano at the Earth’s surface

how does magma form?

when hot rock partially melts, occurs when pressure decreases, when volatiles diffuse into hot rock, and when heat transfers from hot magma into adjacent rock

during partial melting, what is the composition of magma?

mainly felsic, as only part of the source rock melts

viscosity of felsic vs mafic magma

felsic magma is more viscous than mafic

the origin of igneous rocks

can be understood in the context of plate tectonics, magma forms at continental or island volcanic arcs along convergent boundaries due to flux melting. Igneous rocks form at hot spots due to decompression melting. Igneous rocks form at rifts as a result of decompression melting or due to heat transfer into crustal rocks. Igneous rocks form along mid-ocean ridges because of decompression ­melting

potassium feldspar

pink, in felsic rocks, e.g. granite

with increasing Si…

density decreases

with increasing Fe, Mg…

increasing density

Bowen’s reaction series

Bowen found that as the melt cools, its composition progressively changes

Bowen described the specific sequence of mineral-producing reactions that take place in a cooling, initially mafic magma

the sequence in more detail: in a cooling melt, olivine and calcium rich plagioclase form first, but as the melt cools, the plagioclase that forms contains more sodium

• this Na rich plagioclase can encase earlier-formed crystals or grow as new crystals

  • meanwhile, some olivine crystals react with the remaining melt to produce pyroxene, which can encase olivine crystals or replace them

  • some of the olivine and Ca-plagioclase crystals can become isolated from the melt and take iron, magnesium and calcium atoms with them, by this process the remaining melt becomes enriched with silica

• as the melt continues to cool, pyroxene crystals react with the melt to form amphibole and then some amphibole reacts with the remaining melt to form biotite

  • the entire time crystals continue to become isolated so the remaining melt continues to become more felsic

the minerals that crystallize later in the discontinuous series have more Si-O-Si bonds and smaller O/Si ratios than those that crystallize later

continuous vs discontinuous series (Bowen)

continuous: the progressive change from calcium-rich to sodium rich plagioclase because the steps yield different versions of the same mineral

discontinuous: refers to the sequence (olivine, pyroxine etc); each step yields a different class of silicate mineral

hot spots on continents

mantle plumes sometimes intersect continental crust, melting rocks by heat transfer

assimilation of crystal rocks leads to lavas that contain high Si

more felsic than oceanic hotspots and way more explosive