noun[mass noun] Geology
A dark, fine-grained, brown or greyish intermediate volcanic rock which is a common constituent of lavas in some areas.
- The clasts range from angular to subrounded and show a weak preferred orientation when elongate; they include micaceous siltstone, fine-grained sandstone, tuffaceous rock, andesite and medium-grained diorite.
- They are marked by eruptions of cooler, stickier lavas such as andesite, dacite and rhyolite.
- Some rocks have basaltic compositions similar to the well-studied meteorites thought to have come from Mars, but other rocks are more like the volcanic rock andesite.
- Example sentences
- The sequence includes tuffaceous sandstone, andesitic sandstone and radiolarian chert.
- The Longcroft Tuffs, a poorly exposed succession at least 140 m thick, are dominated by lapilli tuffs of andesitic composition.
- Previous work also has documented the presence of two distinct magma compositions beneath Nevado del Ruiz, one andesitic and the other dacitic, with mixing and mingling between the two during the November 1985 eruption.
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