<p>Magmas have a wide range of chemical (and mineralogical) compositions. Basalts come from <em>mafic</em> magmas (they used to be called &quot;basic&quot; magmas), while granites come from <em>felsic</em> magmas (used to be &quot;acid&quot;). (The reasons for such diversity is out of scope of the question ; you can look for &quot;magmatic differentiation&quot;.) It is this difference in chemistry that creates the difference in density. Felsic magmas are enriched in silica, which is light, while mafic magmas contain more heavy oxides such as <span class="math-container">$\ce{FeO}$</span>, <span class="math-container">$\ce{Fe2O3}$</span>, <span class="math-container">$\ce{CaO}$</span>... Here are two geochemical analyses (in wt%) from the USGS <a href="https://www.usgs.gov/centers/gggsc/science/usgs-geochemical-and-microanalytical-reference-materials-distribution" rel="noreferrer">Geology, Geophysics, and Geochemistry Science Center</a>. BHVO-2 is a basalt from Kilauea while G-3 is a granite from Rhode Island. Both are used worldwide as rock standards for analytic calibration.</p>
<div class="s-table-container">
<table class="s-table">
<thead>
<tr>
<th>Oxide</th>
<th style="text-align: right;">BHVO-2</th>
<th style="text-align: right;">G-3</th>
</tr>
</thead>
<tbody>
<tr>
<td><span class="math-container">$\ce{SiO2}$</span></td>
<td style="text-align: right;">49.60</td>
<td style="text-align: right;">68.66</td>
</tr>
<tr>
<td><span class="math-container">$\ce{Al2O3}$</span></td>
<td style="text-align: right;">13.44</td>
<td style="text-align: right;">15.92</td>
</tr>
<tr>
<td><span class="math-container">$\ce{CaO}$</span></td>
<td style="text-align: right;">11.40</td>
<td style="text-align: right;">1.830</td>
</tr>
<tr>
<td><span class="math-container">$\ce{Fe2O3T}$</span></td>
<td style="text-align: right;">12.39</td>
<td style="text-align: right;">2.866</td>
</tr>
<tr>
<td><span class="math-container">$\ce{K2O}$</span></td>
<td style="text-align: right;">0.5130</td>
<td style="text-align: right;">4.57</td>
</tr>
<tr>
<td><span class="math-container">$\ce{MgO}$</span></td>
<td style="text-align: right;">7.257</td>
<td style="text-align: right;">0.75</td>
</tr>
<tr>
<td><span class="math-container">$\ce{MnO}$</span></td>
<td style="text-align: right;">0.1690</td>
<td style="text-align: right;">0.0363</td>
</tr>
<tr>
<td><span class="math-container">$\ce{Na2O}$</span></td>
<td style="text-align: right;">2.219</td>
<td style="text-align: right;">3.968</td>
</tr>
<tr>
<td><span class="math-container">$\ce{P2O5}$</span></td>
<td style="text-align: right;">0.2685</td>
<td style="text-align: right;">0.136</td>
</tr>
<tr>
<td><span class="math-container">$\ce{TiO2}$</span></td>
<td style="text-align: right;">2.731</td>
<td style="text-align: right;">0.476</td>
</tr>
</tbody>
</table>
</div>
<p>Of course these are just examples. There are basalts containing more potassium, or granites containing less aluminum, and these main rock types can be further divided depending on these criteria (e.g., a basalt can be alkali or subalkali; high-K, medium-K or low-K, etc., see <a href="https://doi.org/10.1017/CBO9780511535581" rel="noreferrer">Le Maitre et al. 2002</a> for a full classification). But overall the difference in silica content is always huge.</p>
<p>Note that mafic magmas can also stay in the crust and crystalize, like granite, yielding a rock called gabbro. Conversely, felsic magmas can erupt at the surface, yielding a rock called rhyolite.</p>

CC BY-SA 4.0
2023-07-30T18:08:05.100Z
2023-07-31T12:49:59.270Z
2023-07-31T12:49:59.270Z
Jean-Marie Prival
<p>If lava is nothing but magma breaking through the earth's surface, then why is basalt more dense than granite?</p>
<p>I understand that something happens during the cooling process, but cannot find any comprehensive answer.</p>

<geology><plate-tectonics><volcanology><rocks>
Why is basalt denser than granite?
Koushik Sarkar
# Why is basalt denser than granite?

(Asked by: username_0 on 2023-07-30)

If lava is nothing but magma breaking through the earth's surface, then why is basalt more dense than granite?

I understand that something happens during the cooling process, but cannot find any comprehensive answer.

**Tags:** geology, plate-tectonics, volcanology, rocks


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## 1 answers

### Answer by username_1 on 2023-07-30. Score: 19

Magmas have a wide range of chemical (and mineralogical) compositions. Basalts come from *mafic* magmas (they used to be called "basic" magmas), while granites come from *felsic* magmas (used to be "acid"). (The reasons for such diversity is out of scope of the question ; you can look for "magmatic differentiation".) It is this difference in chemistry that creates the difference in density. Felsic magmas are enriched in silica, which is light, while mafic magmas contain more heavy oxides such as $\ce{FeO}$, $\ce{Fe2O3}$, $\ce{CaO}$... Here are two geochemical analyses (in wt%) from the USGS [Geology, Geophysics, and Geochemistry Science Center](https://www.usgs.gov/centers/gggsc/science/usgs-geochemical-and-microanalytical-reference-materials-distribution). BHVO-2 is a basalt from Kilauea while G-3 is a granite from Rhode Island. Both are used worldwide as rock standards for analytic calibration.

| Oxide | BHVO-2 | G-3 |
| --- | --- | --- |
| $\ce{SiO2}$ | 49.60 | 68.66 |
| $\ce{Al2O3}$ | 13.44 | 15.92 |
| $\ce{CaO}$ | 11.40 | 1.830 |
| $\ce{Fe2O3T}$ | 12.39 | 2.866 |
| $\ce{K2O}$ | 0.5130 | 4.57 |
| $\ce{MgO}$ | 7.257 | 0.75 |
| $\ce{MnO}$ | 0.1690 | 0.0363 |
| $\ce{Na2O}$ | 2.219 | 3.968 |
| $\ce{P2O5}$ | 0.2685 | 0.136 |
| $\ce{TiO2}$ | 2.731 | 0.476 |

Of course these are just examples. There are basalts containing more potassium, or granites containing less aluminum, and these main rock types can be further divided depending on these criteria (e.g., a basalt can be alkali or subalkali; high-K, medium-K or low-K, etc., see [Le Maitre et al. 2002](https://doi.org/10.1017/CBO9780511535581) for a full classification). But overall the difference in silica content is always huge.

Note that mafic magmas can also stay in the crust and crystalize, like granite, yielding a rock called gabbro. Conversely, felsic magmas can erupt at the surface, yielding a rock called rhyolite.


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