<p>Quartz monzonite from the Cretaceous of Montana, USA. (6.7 cm across at its widest)
</p><p>Igneous rocks form by the cooling &amp; crystallization of hot, molten rock (magma &amp; lava).  If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks.  If this happens deep underground, they are intrusive igneous rocks.  Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.
</p><p>Quartz monzonites have the same general characteristics and compositions as true granites (= coarsely crystalline/phaneritic-textured, intrusive igneous rocks composed of quartz, K-feldspar, plagioclase feldspar, and usually one or more mafic minerals).  Quartz monzonites, in contrast, have less quartz than true granites.  The quartz component of quartz monzonites is 5% to 20%, whereas true granites have 20% to 60% quartz.
</p><p>The Cretaceous-aged quartz monzonite shown above is dominated by orthoclase feldspar, plagioclase feldspar, quartz, and biotite mica.
</p><p>Geologic unit &amp; age: Butte Quartz Monzonite, Boulder Batholith, Campanian Stage to Maastrichtian Stage, late Late Cretaceous, 68-78 Ma
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Locality: outcrop along the southern side of I-90, just east of Homestake, southeast of Butte, southwestern Jefferson County, southwestern Montana, USA (~45° 54’ 53” North, ~112° 22’ 34” West)
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/16168713613/">Quartz monzonite (Butte Quartz Monzonite, Late Cretaceous, 68-78 Ma; Interstate 90 outcrop, southeast of Butte, Montana, USA)</a>