---
title: "Igneous Rocks"
book: "High School Earth and Space Sciences"
subject: earth
language: en
chapter: 17
exercises: 15
source: https://one-course.com/books/earth/2/en/chapter/17-igneous-rocks
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 17 — Igneous Rocks

On the north coast of Ireland, stone columns packed side by side step down into the sea: the Giant’s Causeway, a basalt that cooled from a lava. In Yosemite, in California, the granite wall of El Capitan rises some $900\,\mathrm{m}$ above the valley floor, a single body of coarse-grained rock. Both crystallised from magma. The basalt cooled at the surface, its crystals too small to see; the granite cooled deep below ground, its crystals large enough to see, and it is visible today only because the rocks above it have been worn away. This chapter reads in a rock’s grains how fast, and how deep, it cooled.

![Left: basalt columns of the Giant’s Causeway, Northern Ireland. Photograph Adam Bishop (CC BY-SA 3.0). Right: El Capitan, Yosemite, California, a wall of granite about 900\, m high. Photograph U.S. Geological Survey (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-4efd2c9d0738.jpg)

![Left: basalt columns of the Giant’s Causeway, Northern Ireland. Photograph Adam Bishop (CC BY-SA 3.0). Right: El Capitan, Yosemite, California, a wall of granite about 900\, m high. Photograph U.S. Geological Survey (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-84a0c311b331.jpg)

*Left: basalt columns of the Giant’s Causeway, Northern Ireland. Photograph Adam Bishop (CC BY-SA 3.0). Right: El Capitan, Yosemite, California, a wall of granite about $900\,\mathrm{m}$ high. Photograph U.S. Geological Survey (CC0).*

**You already know.**

Magma is molten rock beneath the surface; it erupts as lava, gently or explosively (Book 1, grade 7). An igneous rock is one formed when molten rock cools and becomes solid (Book 1, grade 6). Basalt is a dark [volcanic rock](#def-g11-igneous-rocks-volcanic-plutonic), granite a light rock made of feldspar, quartz and mica (Book 1, grade 6). Mantle [peridotite](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-peridotite) melts partly as it rises beneath a ridge, giving the basalt and [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro) of the [oceanic crust](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-oceanic-lithosphere) ([Chapter 14](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#ch-g11-ocean-ridges)). The [rock-forming silicates](https://one-course.com/books/earth/2/en/chapter/16-minerals#def-g11-minerals-rock-forming) are olivine, pyroxenes, amphiboles, micas, feldspars and quartz; the feldspars are potassium feldspar and plagioclase, a series from calcium-rich to sodium-rich ([Chapter 16](https://one-course.com/books/earth/2/en/chapter/16-minerals#ch-g11-minerals)). Heat spreads by conduction over a distance of about $\sqrt{\kappa t}$ in a time $t$, where $\kappa$ is the thermal diffusivity ([Chapter 15](https://one-course.com/books/earth/2/en/chapter/15-subduction-zones#ch-g11-subduction-zones)).

## 17.1 Volcanic and plutonic rocks

**Definition 17.1 (Volcanic rock; plutonic rock).**

An igneous rock, formed by the crystallisation of a magma, is *volcanic* (or extrusive) when the magma solidified at the Earth’s surface, as a lava, and *plutonic* (or intrusive) when it crystallised at depth, inside other rocks.

The difference shows in the size of the crystals. A crystal grows by adding atoms taken from the liquid around it, and that takes time. At the surface, a lava loses its heat to the air or water fast: material erupted into air or water cools in seconds, a lava flow in days to weeks. Deep down the surrounding rocks are hot and the magma loses its heat slowly: a shallow [intrusion](#def-g11-igneous-rocks-intrusion) cools in months to years, a large deep one may take millions of years. Fast cooling gives many small crystals, or none at all; slow cooling gives fewer, much larger ones.

**Definition 17.2 (Texture).**

The *texture* of a rock is the size, shape and arrangement of its grains. An igneous rock is *glassy* when it has no crystals, *fine-grained* when its crystals are too small to be told apart with the naked eye, and *coarse-grained* when they can all be seen.

A lava quenched before any crystal can grow becomes a glass: obsidian, black and glassy, breaks with the conchoidal fracture of a bottle. Basalt is fine-grained: in the hand its minerals cannot be identified, and even in [thin section](https://one-course.com/books/earth/2/en/chapter/16-minerals#def-g11-minerals-thin-section) the crystals are small (the basalt of [Chapter 16](https://one-course.com/books/earth/2/en/chapter/16-minerals#ch-g11-minerals)). Granite, [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro) and [diorite](#def-g11-igneous-rocks-rock-names) are coarse-grained: every grain shows.

**Definition 17.3 (Phenocryst; porphyritic).**

A *phenocryst* is a large crystal set in a mass of much smaller crystals or glass, the *groundmass*. A rock with phenocrysts is *porphyritic*.

A [porphyritic](#def-g11-igneous-rocks-porphyritic) [texture](#def-g11-igneous-rocks-texture) records two stages of cooling: first slow, at depth, while the [phenocrysts](#def-g11-igneous-rocks-porphyritic) grew in a magma that was still mostly liquid; then fast, when the magma rose and erupted, or was injected into cold rock, and the rest of the liquid froze into the groundmass.

![From left to right: obsidian, a volcanic glass, with its curved conchoidal fracture (9.5\, cm across); a porphyritic rock, blocky plagioclase phenocrysts in a finer groundmass (9\, cm); then two thin sections between crossed polars: a granite, all grains large and interlocked — brown biotite, a brightly coloured muscovite, striped plagioclase, grey and white quartz; an andesite, phenocrysts of plagioclase and amphibole in a fine-grained groundmass. Photographs Twyla Baker, Mike Davis and Darla Sondrol, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-4ee1179f99aa.jpg)

![From left to right: obsidian, a volcanic glass, with its curved conchoidal fracture (9.5\, cm across); a porphyritic rock, blocky plagioclase phenocrysts in a finer groundmass (9\, cm); then two thin sections between crossed polars: a granite, all grains large and interlocked — brown biotite, a brightly coloured muscovite, striped plagioclase, grey and white quartz; an andesite, phenocrysts of plagioclase and amphibole in a fine-grained groundmass. Photographs Twyla Baker, Mike Davis and Darla Sondrol, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-992e3185c904.jpg)

![From left to right: obsidian, a volcanic glass, with its curved conchoidal fracture (9.5\, cm across); a porphyritic rock, blocky plagioclase phenocrysts in a finer groundmass (9\, cm); then two thin sections between crossed polars: a granite, all grains large and interlocked — brown biotite, a brightly coloured muscovite, striped plagioclase, grey and white quartz; an andesite, phenocrysts of plagioclase and amphibole in a fine-grained groundmass. Photographs Twyla Baker, Mike Davis and Darla Sondrol, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-6064d5f1cc1f.jpg)

![From left to right: obsidian, a volcanic glass, with its curved conchoidal fracture (9.5\, cm across); a porphyritic rock, blocky plagioclase phenocrysts in a finer groundmass (9\, cm); then two thin sections between crossed polars: a granite, all grains large and interlocked — brown biotite, a brightly coloured muscovite, striped plagioclase, grey and white quartz; an andesite, phenocrysts of plagioclase and amphibole in a fine-grained groundmass. Photographs Twyla Baker, Mike Davis and Darla Sondrol, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-d2608673a8c9.jpg)

*From left to right: obsidian, a [volcanic](#def-g11-igneous-rocks-volcanic-plutonic) glass, with its curved conchoidal fracture ($9.5\,\mathrm{cm}$ across); a [porphyritic](#def-g11-igneous-rocks-porphyritic) rock, blocky plagioclase [phenocrysts](#def-g11-igneous-rocks-porphyritic) in a finer groundmass ($9\,\mathrm{cm}$); then two [thin sections](https://one-course.com/books/earth/2/en/chapter/16-minerals#def-g11-minerals-thin-section) between crossed polars: a granite, all grains large and interlocked — brown biotite, a brightly coloured muscovite, striped plagioclase, grey and white quartz; an [andesite](#def-g11-igneous-rocks-rock-names), [phenocrysts](#def-g11-igneous-rocks-porphyritic) of plagioclase and amphibole in a fine-grained groundmass. Photographs Twyla Baker, Mike Davis and Darla Sondrol, Geology Digital Image Library (CC0).*

## 17.2 Naming igneous rocks

The second key to an igneous rock is its chemistry, and above all its content of silica, $\ce{SiO2}$, which ranges in common magmas from about $45\,\%$ to about $75\,\%$ of the mass.

**Definition 17.4 (Mafic; intermediate; felsic).**

A magma, or the rock it forms, is *mafic* when it holds about $45\,\%$ to $55\,\%$ $\ce{SiO2}$ and is rich in iron, magnesium and calcium; *intermediate* between $55\,\%$ and $65\,\%$; *felsic* when it holds about $65\,\%$ to $75\,\%$ $\ce{SiO2}$ and is poor in iron, magnesium and calcium but rich in sodium and potassium.

Each kind of magma gives two rocks, one [volcanic](#def-g11-igneous-rocks-volcanic-plutonic) and fine-grained, one [plutonic](#def-g11-igneous-rocks-volcanic-plutonic) and coarse-grained, with the same chemistry. Basalt and [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro) are the [mafic](#def-g11-igneous-rocks-felsic-mafic) pair, met with the [oceanic crust](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-oceanic-lithosphere); granite is the [felsic](#def-g11-igneous-rocks-felsic-mafic) [plutonic rock](#def-g11-igneous-rocks-volcanic-plutonic). Three names complete the set.

**Definition 17.5 (Andesite; diorite; rhyolite).**

*Andesite* and *diorite* are the [volcanic](#def-g11-igneous-rocks-volcanic-plutonic) and the [plutonic rock](#def-g11-igneous-rocks-volcanic-plutonic) of [intermediate](#def-g11-igneous-rocks-felsic-mafic) composition, made mostly of plagioclase with amphibole or pyroxene. *Rhyolite* is the [volcanic rock](#def-g11-igneous-rocks-volcanic-plutonic) of [felsic](#def-g11-igneous-rocks-felsic-mafic) composition, the fine-grained equivalent of granite.

![Naming the common igneous rocks (a drawing). Columns: the silica content of the magma; rows: fine-grained (volcanic) and coarse-grained (plutonic) rocks. Below, where each rock-forming mineral is common: olivine and pyroxene in mafic rocks with calcium-rich plagioclase; amphibole and plagioclase in intermediate rocks; potassium feldspar, quartz, sodium-rich plagioclase and micas in felsic rocks. The boundaries are gradual.](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/fig-1139a1c56764.svg)

*Naming the common igneous rocks (a drawing). Columns: the silica content of the magma; rows: fine-grained ([volcanic](#def-g11-igneous-rocks-volcanic-plutonic)) and coarse-grained ([plutonic](#def-g11-igneous-rocks-volcanic-plutonic)) rocks. Below, where each rock-forming mineral is common: olivine and pyroxene in [mafic](#def-g11-igneous-rocks-felsic-mafic) rocks with calcium-rich plagioclase; amphibole and plagioclase in [intermediate](#def-g11-igneous-rocks-felsic-mafic) rocks; potassium feldspar, quartz, sodium-rich plagioclase and micas in [felsic](#def-g11-igneous-rocks-felsic-mafic) rocks. The boundaries are gradual.*

**Method 17.6 (Naming an igneous rock).**

1. Look at the [texture](#def-g11-igneous-rocks-texture) : glassy, fine-grained or coarse-grained; [porphyritic](#def-g11-igneous-rocks-porphyritic) or not. Fine-grained or glassy means [volcanic](#def-g11-igneous-rocks-volcanic-plutonic) (or a small, quickly cooled [intrusion](#def-g11-igneous-rocks-intrusion) ); coarse-grained means [plutonic](#def-g11-igneous-rocks-volcanic-plutonic) .
2. Identify the minerals that can be seen ( [Method 16.4](https://one-course.com/books/earth/2/en/chapter/16-minerals#met-g11-minerals-identify) ): quartz and potassium feldspar point to [felsic](#def-g11-igneous-rocks-felsic-mafic) , amphibole with plagioclase to [intermediate](#def-g11-igneous-rocks-felsic-mafic) , olivine and pyroxene to [mafic](#def-g11-igneous-rocks-felsic-mafic) . In a fine-grained rock, use the [phenocrysts](#def-g11-igneous-rocks-porphyritic) , the colour of the groundmass — dark for [mafic](#def-g11-igneous-rocks-felsic-mafic) , light for [felsic](#def-g11-igneous-rocks-felsic-mafic) — as a hint, or a chemical analysis.
3. Read the name at the crossing of the row and the column of the chart.

**Example 17.7 (Two basalts).**

A dark rock from a lava flow is fine-grained throughout; another, from the same volcano, shows green grains of olivine a few millimetres across in a dark groundmass. Both are [mafic](#def-g11-igneous-rocks-felsic-mafic) and fine-grained: both are basalts. The second is a [porphyritic](#def-g11-igneous-rocks-porphyritic) basalt: its olivine grew first, slowly, in the magma below the volcano, before the lava erupted and froze around it.

![The three pairs of igneous rocks, mafic to felsic from left to right: volcanic above, plutonic below. Top: a fine-grained basalt; an andesite, small amphibole crystals in a fine-grained groundmass (about 10\, cm); a pink rhyolite. Bottom: gabbro (9\, cm); diorite, black hornblende and glassy plagioclase (10\, cm); a coarse granite with large pale pink potassium feldspars, grey plagioclase, quartz, biotite and hornblende (10\, cm). Photographs Nessa Eull, Twyla Baker, Joseph H. Hartman, Darla Sondrol and Dexter Perkins, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-e0e1e2dd64cb.jpg)

![The three pairs of igneous rocks, mafic to felsic from left to right: volcanic above, plutonic below. Top: a fine-grained basalt; an andesite, small amphibole crystals in a fine-grained groundmass (about 10\, cm); a pink rhyolite. Bottom: gabbro (9\, cm); diorite, black hornblende and glassy plagioclase (10\, cm); a coarse granite with large pale pink potassium feldspars, grey plagioclase, quartz, biotite and hornblende (10\, cm). Photographs Nessa Eull, Twyla Baker, Joseph H. Hartman, Darla Sondrol and Dexter Perkins, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-2a9cb1f80250.jpg)

![The three pairs of igneous rocks, mafic to felsic from left to right: volcanic above, plutonic below. Top: a fine-grained basalt; an andesite, small amphibole crystals in a fine-grained groundmass (about 10\, cm); a pink rhyolite. Bottom: gabbro (9\, cm); diorite, black hornblende and glassy plagioclase (10\, cm); a coarse granite with large pale pink potassium feldspars, grey plagioclase, quartz, biotite and hornblende (10\, cm). Photographs Nessa Eull, Twyla Baker, Joseph H. Hartman, Darla Sondrol and Dexter Perkins, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-44da9b8458dd.jpg)

![The three pairs of igneous rocks, mafic to felsic from left to right: volcanic above, plutonic below. Top: a fine-grained basalt; an andesite, small amphibole crystals in a fine-grained groundmass (about 10\, cm); a pink rhyolite. Bottom: gabbro (9\, cm); diorite, black hornblende and glassy plagioclase (10\, cm); a coarse granite with large pale pink potassium feldspars, grey plagioclase, quartz, biotite and hornblende (10\, cm). Photographs Nessa Eull, Twyla Baker, Joseph H. Hartman, Darla Sondrol and Dexter Perkins, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-494bd48ac81b.jpg)

![The three pairs of igneous rocks, mafic to felsic from left to right: volcanic above, plutonic below. Top: a fine-grained basalt; an andesite, small amphibole crystals in a fine-grained groundmass (about 10\, cm); a pink rhyolite. Bottom: gabbro (9\, cm); diorite, black hornblende and glassy plagioclase (10\, cm); a coarse granite with large pale pink potassium feldspars, grey plagioclase, quartz, biotite and hornblende (10\, cm). Photographs Nessa Eull, Twyla Baker, Joseph H. Hartman, Darla Sondrol and Dexter Perkins, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-784e1516f210.jpg)

![The three pairs of igneous rocks, mafic to felsic from left to right: volcanic above, plutonic below. Top: a fine-grained basalt; an andesite, small amphibole crystals in a fine-grained groundmass (about 10\, cm); a pink rhyolite. Bottom: gabbro (9\, cm); diorite, black hornblende and glassy plagioclase (10\, cm); a coarse granite with large pale pink potassium feldspars, grey plagioclase, quartz, biotite and hornblende (10\, cm). Photographs Nessa Eull, Twyla Baker, Joseph H. Hartman, Darla Sondrol and Dexter Perkins, Geology Digital Image Library (CC0).](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/img-9e4f370f62cf.jpg)

*The three pairs of igneous rocks, [mafic](#def-g11-igneous-rocks-felsic-mafic) to [felsic](#def-g11-igneous-rocks-felsic-mafic) from left to right: [volcanic](#def-g11-igneous-rocks-volcanic-plutonic) above, [plutonic](#def-g11-igneous-rocks-volcanic-plutonic) below. Top: a fine-grained basalt; an [andesite](#def-g11-igneous-rocks-rock-names), small amphibole crystals in a fine-grained groundmass (about $10\,\mathrm{cm}$); a pink [rhyolite](#def-g11-igneous-rocks-rock-names). Bottom: [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro) ($9\,\mathrm{cm}$); [diorite](#def-g11-igneous-rocks-rock-names), black hornblende and glassy plagioclase ($10\,\mathrm{cm}$); a coarse granite with large pale pink potassium feldspars, grey plagioclase, quartz, biotite and hornblende ($10\,\mathrm{cm}$). Photographs Nessa Eull, Twyla Baker, Joseph H. Hartman, Darla Sondrol and Dexter Perkins, Geology Digital Image Library (CC0).*

## 17.3 The order of crystallisation

A magma does not freeze at one temperature, as water does. Its minerals appear one after another as it cools, each in its own range of temperature, so that a cooling magma can hold crystals and still be mostly liquid.

**History — Bowen’s experiments.**

Norman L. Bowen (1887–1956) joined the Geophysical Laboratory of the Carnegie Institution in Washington in 1912. Working mostly with basaltic magmas, he melted a rock in a furnace, let it cool slowly to a chosen temperature so that some minerals formed, then quenched it, cooling it so fast that the remaining liquid became a glass. Under the microscope, the crystals showed which minerals had formed above that temperature. Repeated at lower and lower temperatures, the experiment gave the order in which the minerals appear. Bowen published this order as a reaction series in 1922 and his book *The Evolution of the Igneous Rocks* in 1928.

![The order in which minerals crystallise from a cooling magma (Bowen’s reaction series, a drawing). Olivine forms first; plagioclase starts at about the point where pyroxene does, rich in calcium at first, then richer and richer in sodium. Potassium feldspar, quartz and sometimes muscovite form last, from what remains of a silica-rich liquid.](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/fig-924c8b353dcd.svg)

*The order in which minerals crystallise from a cooling magma (Bowen’s reaction series, a drawing). Olivine forms first; plagioclase starts at about the point where pyroxene does, rich in calcium at first, then richer and richer in sodium. Potassium feldspar, quartz and sometimes muscovite form last, from what remains of a silica-rich liquid.*

The word *reaction* matters. Olivine is stable at high temperature only: as the magma cools, olivine crystals react with silica in the liquid and turn into pyroxene,

$$
\ce{Mg2SiO4 + SiO2 -> 2 MgSiO3},
$$

and so on down the branch, as long as silica remains in the liquid. Along the other branch, plagioclase crystals keep growing, but each new layer is richer in sodium than the last: a plagioclase crystal can be zoned, from a calcium-rich core to a sodium-rich rim, like the plagioclase [phenocrysts](#def-g11-igneous-rocks-porphyritic) of the [andesite](#def-g11-igneous-rocks-rock-names) above.

The series explains the chart. A [mafic](#def-g11-igneous-rocks-felsic-mafic) magma runs out of silica early: it crystallises olivine, pyroxene and calcium-rich plagioclase, and gives a [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro) if it cools slowly underground, a basalt if it cools fast at the surface. Only a magma rich in silica goes on to the end of the series and forms potassium feldspar and quartz: the minerals of granite and [rhyolite](#def-g11-igneous-rocks-rock-names). How a magma changes its composition on the way, when its early crystals are separated from the liquid, is the subject of the second university year.

## 17.4 Intrusions

Magma rises because it is less dense than the rocks around it, forcing its way into fractures and widening them. Much of it never reaches the surface: it crystallises in the crust as [intrusions](#def-g11-igneous-rocks-intrusion), which are seen only once erosion has removed the rocks above.

**Definition 17.8 (Dyke; sill; pluton).**

An *intrusion* is a body of igneous rock that crystallised inside other rocks. A *dyke* is a sheet of igneous rock that cuts across the layers or structures of the rocks it intrudes; a *sill* is a sheet that lies parallel to the layers, between them. A *pluton* is a large intrusion of any shape, formed at depth; many plutons joined together form a batholith ([Chapter 22](https://one-course.com/books/earth/2/en/chapter/22-magmatism-at-subduction-zones#ch-g12-subduction-magmatism)).

[Dykes](#def-g11-igneous-rocks-intrusion) are commonly less than $20\,\mathrm{m}$ wide and [sills](#def-g11-igneous-rocks-intrusion) less than $50\,\mathrm{m}$ thick; a [sill](#def-g11-igneous-rocks-intrusion) is usually fed by a [dyke](#def-g11-igneous-rocks-intrusion). Shallow [dykes](#def-g11-igneous-rocks-intrusion) and [sills](#def-g11-igneous-rocks-intrusion) cool fast: they are fine-grained, sometimes [porphyritic](#def-g11-igneous-rocks-porphyritic). Against the cold rock they intrude, their edges cool fastest of all and are even finer grained than their centre: a *chilled margin*, the mark of a body that was hot when it came into place. The Sierra Nevada of California, where Yosemite lies, is a batholith: the granitic rocks whose density was measured in [Chapter 16](https://one-course.com/books/earth/2/en/chapter/16-minerals#ch-g11-minerals) come from about two hundred of its [plutons](#def-g11-igneous-rocks-intrusion).

![Intrusions (a drawing, not to scale). A pluton crystallises at depth; dykes cut across the layers of the rocks above it, one of them feeding a volcano; a sill spreads between two layers. Only erosion of the rocks above lays these bodies bare at the surface.](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/fig-53ce95a8b3c2.svg)

*[Intrusions](#def-g11-igneous-rocks-intrusion) (a drawing, not to scale). A [pluton](#def-g11-igneous-rocks-intrusion) crystallises at depth; [dykes](#def-g11-igneous-rocks-intrusion) cut across the layers of the rocks above it, one of them feeding a volcano; a [sill](#def-g11-igneous-rocks-intrusion) spreads between two layers. Only erosion of the rocks above lays these bodies bare at the surface.*

How long does an [intrusion](#def-g11-igneous-rocks-intrusion) take to crystallise? Consider a sheet of magma — a [dyke](#def-g11-igneous-rocks-intrusion) or a [sill](#def-g11-igneous-rocks-intrusion) — of thickness $b$, losing its heat by conduction into the rock on both sides. The only quantities that set the pace are the thickness $b$, in metres, and the thermal diffusivity $\kappa$, in $\mathrm{m}^{2}/\mathrm{s}$; the only time that can be built from them is $b^2/\kappa$.

**Proposition 17.9 (The cooling time of a sheet of magma).**

A sheet of magma of thickness $b$ that cools by conduction solidifies in a time

$$
t = C\,\frac{b^2}{\kappa},
$$

where $\kappa$ is the thermal diffusivity of the rocks and $C$ a number set by the temperatures of the magma and of the surrounding rock and by the heat released as the magma crystallises. Doubling the thickness multiplies the time by four. (Admitted; the value of $C$ comes from a computed model.)

For a basaltic magma intruded at $1150{}^{\circ}\mathrm{C}$ into rock at $50{}^{\circ}\mathrm{C}$, which crystallises completely at $950{}^{\circ}\mathrm{C}$ and releases $900\,\mathrm{MJ}$ of heat for each cubic metre that crystallises, with $\kappa = 1 \times 10^{-6}\,\mathrm{m}^{2}/\mathrm{s}$, the model gives $C \approx 0.21$. A sheet $2\,\mathrm{m}$ thick then solidifies in about ten days, a [sill](#def-g11-igneous-rocks-intrusion) $20\,\mathrm{m}$ thick in about two and a half years, one $100\,\mathrm{m}$ thick in about sixty-five years. Without the heat of crystallisation, the times would be three times shorter: crystallising magma keeps itself warm.

![A sheet of basaltic magma cooling by conduction (computed model). Left: the time it takes to solidify, against its thickness, on logarithmic axes: a straight line of slope 2, the time growing as the square of the thickness. Right: temperatures across a sill 20\, m thick (shaded), intruded at 1150 C into rock at 50 C. The edges fall below the solidus at once, the centre only after about two and a half years.](https://one-course.com/images/onecourse/chapters/earth-2/g11-igneous-rocks/fig-8bac5a9778af.svg)

*A sheet of basaltic magma cooling by conduction (computed model). Left: the time it takes to solidify, against its thickness, on logarithmic axes: a straight line of slope 2, the time growing as the square of the thickness. Right: temperatures across a [sill](#def-g11-igneous-rocks-intrusion) $20\,\mathrm{m}$ thick (shaded), intruded at $1150{}^{\circ}\mathrm{C}$ into rock at $50{}^{\circ}\mathrm{C}$. The edges fall below the solidus at once, the centre only after about two and a half years.*

The model also shows the chilled margin forming. The rock at the contact jumps at once to about $690{}^{\circ}\mathrm{C}$, far below the solidus: the first centimetres of the [sill](#def-g11-igneous-rocks-intrusion) freeze within a day, while its centre stays liquid for years. Real [intrusions](#def-g11-igneous-rocks-intrusion) are more complicated — the properties of rocks change with temperature, water circulates, a deep [pluton](#def-g11-igneous-rocks-intrusion) sits in rock that is already hot and may be fed again and again — which is why large deep [intrusions](#def-g11-igneous-rocks-intrusion) take far longer to cool than the law, with a cold host, would suggest.

## 17.5 Exercises

**Exercise 17.1 ★.**

Define a [volcanic rock](#def-g11-igneous-rocks-volcanic-plutonic) and a [plutonic rock](#def-g11-igneous-rocks-volcanic-plutonic). Give the three pairs of [volcanic](#def-g11-igneous-rocks-volcanic-plutonic) and [plutonic rocks](#def-g11-igneous-rocks-volcanic-plutonic) of the same composition, from [mafic](#def-g11-igneous-rocks-felsic-mafic) to [felsic](#def-g11-igneous-rocks-felsic-mafic).

**Solution of Exercise 17.1.**

A [volcanic rock](#def-g11-igneous-rocks-volcanic-plutonic) formed from a magma that solidified at the surface, as a lava; a [plutonic rock](#def-g11-igneous-rocks-volcanic-plutonic) crystallised at depth, inside other rocks. Pairs: basalt and [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro) ([mafic](#def-g11-igneous-rocks-felsic-mafic)), [andesite](#def-g11-igneous-rocks-rock-names) and [diorite](#def-g11-igneous-rocks-rock-names) ([intermediate](#def-g11-igneous-rocks-felsic-mafic)), [rhyolite](#def-g11-igneous-rocks-rock-names) and granite ([felsic](#def-g11-igneous-rocks-felsic-mafic)).

**Exercise 17.2 ★.**

Match each rock to its [texture](#def-g11-igneous-rocks-texture) — glassy, fine-grained, coarse-grained, [porphyritic](#def-g11-igneous-rocks-porphyritic): (1) obsidian; (2) granite; (3) a basalt with green olivine crystals a few millimetres across in a dark groundmass; (4) a basalt in which no mineral can be seen.

**Solution of Exercise 17.2.**

(1) Glassy. (2) Coarse-grained. (3) [Porphyritic](#def-g11-igneous-rocks-porphyritic) (olivine [phenocrysts](#def-g11-igneous-rocks-porphyritic) in a fine groundmass). (4) Fine-grained.

**Exercise 17.3 ★.**

Three magmas contain $48\,\%$, $60\,\%$ and $72\,\%$ $\ce{SiO2}$. Classify each, and name the rock each gives when it cools slowly at depth.

**Solution of Exercise 17.3.**

$48\,\%$: [mafic](#def-g11-igneous-rocks-felsic-mafic), [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro). $60\,\%$: [intermediate](#def-g11-igneous-rocks-felsic-mafic), [diorite](#def-g11-igneous-rocks-rock-names). $72\,\%$: [felsic](#def-g11-igneous-rocks-felsic-mafic), granite.

**Exercise 17.4 ★.**

Define a [dyke](#def-g11-igneous-rocks-intrusion), a [sill](#def-g11-igneous-rocks-intrusion) and a [pluton](#def-g11-igneous-rocks-intrusion). On the drawing of [intrusions](#def-g11-igneous-rocks-intrusion) in this chapter, which body fed the volcano?

**Solution of Exercise 17.4.**

A [dyke](#def-g11-igneous-rocks-intrusion) is a sheet of igneous rock that cuts across the layers of the rocks it intrudes; a [sill](#def-g11-igneous-rocks-intrusion) is a sheet parallel to the layers, between them; a [pluton](#def-g11-igneous-rocks-intrusion) is a large [intrusion](#def-g11-igneous-rocks-intrusion) of any shape formed at depth. The vertical [dyke](#def-g11-igneous-rocks-intrusion) that rises from the [pluton](#def-g11-igneous-rocks-intrusion) fed the volcano.

**Exercise 17.5 ★.**

In Bowen’s reaction series, which mineral forms first, and which minerals form last? How does plagioclase change as the magma cools?

**Solution of Exercise 17.5.**

Olivine forms first; potassium feldspar, quartz and sometimes muscovite last. Plagioclase starts rich in calcium and becomes richer and richer in sodium as the magma cools.

**Exercise 17.6 ★★.**

Two rocks are analysed (mass percentages). Rock A: $\ce{SiO2}$ 74, $\ce{Al2O3}$ 14, $\ce{FeO}$ 3, $\ce{CaO}$ 3, $\ce{MgO}$ 0.5, $\ce{Na2O}$ 5, $\ce{K2O}$ 4. Rock B: $\ce{SiO2}$ 47, $\ce{Al2O3}$ 14, $\ce{FeO}$ 8, $\ce{CaO}$ 10, $\ce{MgO}$ 8, $\ce{Na2O}$ 1, $\ce{K2O}$ 2. (1) Classify each. (2) Check your answer with the other oxides. (3) Rock A is fine-grained: name it. Rock B is coarse-grained: name it.

**Solution of Exercise 17.6.**

(1) A: $74\,\%$ $\ce{SiO2}$, [felsic](#def-g11-igneous-rocks-felsic-mafic); B: $47\,\%$, [mafic](#def-g11-igneous-rocks-felsic-mafic). (2) A is poor in $\ce{FeO}$, $\ce{MgO}$ and $\ce{CaO}$ and rich in $\ce{Na2O}$ and $\ce{K2O}$, as a [felsic](#def-g11-igneous-rocks-felsic-mafic) magma is; B is rich in $\ce{FeO}$, $\ce{MgO}$ and $\ce{CaO}$, poor in $\ce{Na2O}$, as a [mafic](#def-g11-igneous-rocks-felsic-mafic) magma is. (3) A is a [rhyolite](#def-g11-igneous-rocks-rock-names); B a [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro).

**Exercise 17.7 ★★.**

A [dyke](#def-g11-igneous-rocks-intrusion) $2\,\mathrm{m}$ thick solidifies in about ten days. Using the proposition on the cooling time, how long does a [dyke](#def-g11-igneous-rocks-intrusion) of the same magma take if it is $1\,\mathrm{m}$ thick? A [sill](#def-g11-igneous-rocks-intrusion) $20\,\mathrm{m}$ thick? One $200\,\mathrm{m}$ thick?

**Solution of Exercise 17.7.**

The time grows as the square of the thickness. $1\,\mathrm{m}$: $10/4 = 2.5$ days. $20\,\mathrm{m}$: $10 \times 10^2 = 1000$ days, about 2.7 years. $200\,\mathrm{m}$: $10 \times 100^2 = 100\,000$ days, about 270 years.

**Exercise 17.8 ★★.**

(1) Show that $b^2/\kappa$ is a time. (2) Compute it for $b = 20\,\mathrm{m}$ and $\kappa = 1 \times 10^{-6}\,\mathrm{m}^{2}/\mathrm{s}$, in seconds and in years ($1\text{ year} \approx 3.16 \times 10^{7}\,\mathrm{s}$). (3) With $C = 0.21$, how long does the [sill](#def-g11-igneous-rocks-intrusion) take to solidify?

**Solution of Exercise 17.8.**

(1) $\mathrm{m}^{2}$ divided by $\mathrm{m}^{2}/\mathrm{s}$ gives seconds. (2) $400/10^{-6} =
4 \times 10^{8}\,\mathrm{s}$, that is $4 \times 10^8 / 3.16 \times 10^7 \approx 12.7$ years. (3) $0.21 \times 12.7 \approx 2.7$ years.

**Exercise 17.9 ★★.**

A rock shows blocky plagioclase crystals about a centimetre long in a groundmass whose grains cannot be seen. Describe its [texture](#def-g11-igneous-rocks-texture), and tell the story of its cooling in two stages.

**Solution of Exercise 17.9.**

It is [porphyritic](#def-g11-igneous-rocks-porphyritic): plagioclase [phenocrysts](#def-g11-igneous-rocks-porphyritic) in a fine-grained groundmass. First the magma cooled slowly at depth, and the plagioclase crystals grew large while most of the magma stayed liquid; then it rose and cooled fast, and the remaining liquid crystallised as the fine groundmass.

**Exercise 17.10 ★★.**

Olivine reacts with silica in the liquid: $\ce{Mg2SiO4 + SiO2 -> 2
MgSiO3}$. (1) Check that the equation is balanced. (2) With the molar masses $140.7\,\mathrm{g}/\mathrm{mol}$ ($\ce{Mg2SiO4}$) and $60.1\,\mathrm{g}/\mathrm{mol}$ ($\ce{SiO2}$), what mass of silica does $1\,\mathrm{kg}$ of olivine take from the liquid? (3) Why does a magma poor in silica keep its olivine?

**Solution of Exercise 17.10.**

(1) Left: 2 Mg, 2 Si, 6 O; right: 2 Mg, 2 Si, 6 O. (2) $1\,\mathrm{kg}$ of olivine is $1000/140.7 \approx 7.11$ mol, which takes as many moles of silica: $7.11 \times 60.1 \approx 427\,\mathrm{g}$. (3) With no silica left in the liquid, the reaction cannot go on: the olivine stays.

**Exercise 17.11 ★★.**

A [sill](#def-g11-igneous-rocks-intrusion) $20\,\mathrm{m}$ thick and $1\,\mathrm{m}^{2}$ in area cools from $1150{}^{\circ}\mathrm{C}$ to $50{}^{\circ}\mathrm{C}$. (1) Its heat capacity is $2.5\,\mathrm{MJ}$ per cubic metre and per degree: what heat does it lose by cooling? (2) It also releases $900\,\mathrm{MJ}$ per cubic metre as it crystallises: what heat is that? (3) Compare the two.

**Solution of Exercise 17.11.**

(1) $2.5 \times 20 \times 1100 = 55\,000\,\mathrm{MJ}$, $5.5 \times 10^{10}\,\mathrm{J}$. (2) $900 \times 20 = 18\,000\,\mathrm{MJ}$, $1.8 \times 10^{10}\,\mathrm{J}$. (3) The heat of crystallisation is about a third of the heat lost by cooling, a quarter of the total: enough to slow the freezing a great deal, because it is all released above the solidus.

**Exercise 17.12 ★★★.**

In the computed model of a [sill](#def-g11-igneous-rocks-intrusion) $20\,\mathrm{m}$ thick, the rock $12.5\,\mathrm{cm}$ from the contact falls below the solidus after about $0.6$ days, the rock $5\,\mathrm{m}$ from the contact after about 545 days, and the centre after about 950 days. (1) Where in the [sill](#def-g11-igneous-rocks-intrusion) do you expect the smallest crystals, and where the largest? (2) Is the rate at which the solid layer grows constant? (3) Why is the [sill](#def-g11-igneous-rocks-intrusion) fine-grained even at its centre, compared with a granite?

**Solution of Exercise 17.12.**

(1) The smallest crystals at the contact, which froze within a day; the largest at the centre, which stayed liquid for about 950 days. (2) No: the first $5\,\mathrm{m}$ took about 545 days, the last $5\,\mathrm{m}$ about 400 days; the front slows down at first, then speeds up near the centre, where the heat leaving through both sides meets. (3) Even at its centre, the [sill](#def-g11-igneous-rocks-intrusion) froze in under three years; a granite with large crystals needs a far longer time.

**Exercise 17.13 ★★★.**

A [pluton](#def-g11-igneous-rocks-intrusion) is $2\,\mathrm{km}$ thick. (1) Using the proposition and the $20\,\mathrm{m}$ [sill](#def-g11-igneous-rocks-intrusion) (about 2.6 years), estimate its cooling time. (2) Large deep [intrusions](#def-g11-igneous-rocks-intrusion) are known to take up to millions of years to cool. Give two reasons why the simple law underestimates the time.

**Solution of Exercise 17.13.**

(1) The [pluton](#def-g11-igneous-rocks-intrusion) is 100 times thicker: $2.6 \times 100^2 = 26\,000$ years. (2) For example: a deep [pluton](#def-g11-igneous-rocks-intrusion) sits in rock that is already hot, so it loses heat more slowly than into rock at $50{}^{\circ}\mathrm{C}$; it may be fed by new magma again and again; a [pluton](#def-g11-igneous-rocks-intrusion) is not an infinite sheet; the properties of rock change with temperature.

**Exercise 17.14 ★★★.**

[Felsic](#def-g11-igneous-rocks-felsic-mafic) magmas erupt at about $650\text{ to }800{}^{\circ}\mathrm{C}$, [mafic](#def-g11-igneous-rocks-felsic-mafic) magmas at about $1000\text{ to }1200{}^{\circ}\mathrm{C}$. Using Bowen’s reaction series, explain why a granite contains potassium feldspar, quartz and sodium-rich plagioclase, and no olivine, while a [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro) contains olivine, pyroxene and calcium-rich plagioclase, and no quartz.

**Solution of Exercise 17.14.**

A [mafic](#def-g11-igneous-rocks-felsic-mafic) magma is hot and poor in silica: it crystallises the first minerals of the series — olivine, pyroxene, calcium-rich plagioclase — and runs out of silica before quartz can form. A [felsic](#def-g11-igneous-rocks-felsic-mafic) magma is cooler and rich in silica: olivine is not stable in it, and it goes on to the end of the series, forming sodium-rich plagioclase, potassium feldspar and quartz.

**Exercise 17.15 ★★★.**

Material erupted into air cools in seconds, lava flows in days to weeks, shallow [intrusions](#def-g11-igneous-rocks-intrusion) in months to years. (1) Using the proposition, what thickness of sheet solidifies in about a day? (2) A lava flow $5\,\mathrm{m}$ thick solidifies in about a month: is that consistent with the law for a sheet cooled on both sides? (3) Which [texture](#def-g11-igneous-rocks-texture) do you expect in each case?

**Solution of Exercise 17.15.**

(1) $b = \sqrt{t\kappa/C} = \sqrt{86400 \times 10^{-6}/0.21} \approx
0.64\,\mathrm{m}$. (2) The law gives $0.21 \times 25/10^{-6} \approx
5.3 \times 10^{6}\,\mathrm{s}$, about two months: the same order as a month (a lava flow also loses heat to the air at its top). (3) Glassy for material erupted into air or water; fine-grained for lava flows and thin sheets; coarser, but still small, for thick shallow [intrusions](#def-g11-igneous-rocks-intrusion).

## 17.6 Problem: A Sill and its Crystals

**Problem 17.1.**

Weekend problem — how long a sill takes to freeze

In the wall of a valley, a dark sheet of igneous rock $20\,\mathrm{m}$ thick lies between two beds of sandstone, parallel to them. Samples are taken from its edge to its centre. At the contact the rock is very fine-grained, almost glassy; $1\,\mathrm{m}$ inside, its crystals are about $0.1\,\mathrm{mm}$; at the centre, about $1\,\mathrm{mm}$, and plagioclase and pyroxene can just be told apart with a hand lens.

**Part I — The textures.**

1. Is the sheet a [dyke](#def-g11-igneous-rocks-intrusion) or a [sill](#def-g11-igneous-rocks-intrusion) ? Why?
2. Describe the [texture](#def-g11-igneous-rocks-texture) at the contact and at the centre.
3. Why is the rock finest grained at the contact?
4. What is this fine-grained edge called, and what does it prove about the sheet’s temperature when it came into place?
5. The sheet is fed from below by a vertical sheet of the same rock, $2\,\mathrm{m}$ thick, cutting across the sandstone. What is it?

**Part II — Naming the rock.**

6. An analysis gives $49\,\%$ $\ce{SiO2}$ , $10\,\%$ $\ce{FeO}$ , $7\,\%$ $\ce{MgO}$ and $10\,\%$ $\ce{CaO}$ . Is the magma [mafic](#def-g11-igneous-rocks-felsic-mafic) , [intermediate](#def-g11-igneous-rocks-felsic-mafic) or [felsic](#def-g11-igneous-rocks-felsic-mafic) ?
7. Which minerals do you expect, by Bowen’s series?
8. Which name fits the rock of the contact? Which [plutonic rock](#def-g11-igneous-rocks-volcanic-plutonic) has the same composition?
9. Why can one body of magma give rocks with different [textures](#def-g11-igneous-rocks-texture) ?
10. In which order did olivine, pyroxene and plagioclase begin to crystallise?

**Part III — The cooling time.**

11. Compute $b^2/\kappa$ for the [sill](#def-g11-igneous-rocks-intrusion) , with $\kappa = 1 \times 10^{-6}\,\mathrm{m}^{2}/\mathrm{s}$ , in seconds.
12. The model gives $C = 0.206$ . Compute the time the [sill](#def-g11-igneous-rocks-intrusion) took to solidify, in seconds, then in years ( $1\text{ year} \approx 3.16 \times 10^{7}\,\mathrm{s}$ ).
13. How long did the feeding sheet, $2\,\mathrm{m}$ thick, take?
14. Without the heat of crystallisation, $C$ would be about $0.07$ . How long would the [sill](#def-g11-igneous-rocks-intrusion) take? Why does crystallisation slow the cooling?
15. The model gives about $0.6$ days for the rock $12.5\,\mathrm{cm}$ from the contact. How many times faster did it freeze than the centre?

**Part IV — What the crystals tell.**

16. Lava flows cool in days to weeks, shallow [intrusions](#def-g11-igneous-rocks-intrusion) in months to years. Where does the [sill](#def-g11-igneous-rocks-intrusion) fit?
17. A granite with crystals of a centimetre or more cooled far more slowly. Estimate, with the law, the thickness of a sheet that would take a thousand times longer than the [sill](#def-g11-igneous-rocks-intrusion) .
18. Why must such a body have formed deep in the crust?
19. The [sill](#def-g11-igneous-rocks-intrusion) is seen at the surface today. What must have happened to the rocks that lay above it?
20. Conclude: how long did the [sill](#def-g11-igneous-rocks-intrusion) take to solidify?

**Solution of Problem 17.1.**

**1.** A [sill](#def-g11-igneous-rocks-intrusion): it lies parallel to the beds, between them.

**2.** At the contact, very fine-grained, almost glassy; at the centre, fine- to medium-grained, the minerals just visible with a lens.

**3.** The contact lost its heat fastest, into the cold sandstone: the crystals had no time to grow.

**4.** A chilled margin. It proves that the magma was hot when it came into place, and the sandstone cold.

**5.** A [dyke](#def-g11-igneous-rocks-intrusion), the feeder of the [sill](#def-g11-igneous-rocks-intrusion).

**6.** [Mafic](#def-g11-igneous-rocks-felsic-mafic): $49\,\%$ $\ce{SiO2}$, rich in iron, magnesium and calcium.

**7.** Olivine, pyroxene and calcium-rich plagioclase.

**8.** A basalt; the [plutonic rock](#def-g11-igneous-rocks-volcanic-plutonic) of the same composition is [gabbro](https://one-course.com/books/earth/2/en/chapter/14-ocean-ridges-and-the-oceanic-lithosphere#def-g11-ocean-ridges-gabbro).

**9.** [Texture](#def-g11-igneous-rocks-texture) depends on the cooling rate, which differs from the contact to the centre, not on the composition.

**10.** Olivine first; plagioclase starting at about the same point as pyroxene.

**11.** $b^2/\kappa = 400/10^{-6} = 4 \times 10^{8}\,\mathrm{s}$.

**12.** $0.206 \times 4 \times 10^8 \approx 8.2 \times 10^{7}\,\mathrm{s}$, that is $8.2 \times 10^7 / 3.16 \times 10^7 \approx 2.6$ years.

**13.** $0.206 \times 4/10^{-6} \approx 8.2 \times 10^{5}\,\mathrm{s}$, about 9.5 days.

**14.** $0.07 \times 4 \times 10^8 = 2.8 \times 10^{7}\,\mathrm{s}$, about 0.9 year. Crystallising magma releases heat, which must also be carried away before the magma becomes solid.

**15.** About 950 days against 0.6 days: about 1600 times faster.

**16.** About 950 days, some 2.6 years: far longer than a lava flow, within the months to years of shallow [intrusions](#def-g11-igneous-rocks-intrusion).

**17.** A thousand times longer needs a thickness $\sqrt{1000} \approx
32$ times larger: about $630\,\mathrm{m}$.

**18.** Near the surface, a body of magma would erupt, or cool too fast; a large body that cools for a very long time must be deep, in rock that is already hot.

**19.** They have been worn away by erosion.

**20.** The [sill](#def-g11-igneous-rocks-intrusion) took about **2.6 years** to solidify.