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Dacite Tools
Many specimens of dacite are fine-grained and of relatively uniform composition. Ancient people have knapped them into sharp tools and worked them into utilitarian objects. When knapped into projectile points, scrapers, and knife blades, they have an edge that is not as sharp as obsidian but is much more durable.
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Dacite Aggregate
Dacite is sometimes used to produce crushed stone. It performs well as fill and as a loose aggregate in a wide variety of construction projects. It does not perform well as a concrete aggregate because its high silica content reacts with the cement.
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Dacite Magma
Dacite magma generally develops in subduction zones were a relatively young oceanic plate is subducting under a continental plate. As the oceanic plate descends into the mantle, it undergoes partial melting with liberated water facilitating the melting of surrounding rocks. The subduction zone where the Juan de Fuca plate subducts under the North America plate…
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Composition of Dacite
A generalized mineral composition for dacite is intermediate between rhyolite and andesite. It usually contains more quartz than andesite and more plagioclase than rhyolite. The plagioclase feldspars are often oligoclase, andesine or labradorite. Dacite can be considered the fine-grained equivalent of granodiorite. Plagioclase is the most abundant mineral in many dacites. Other minerals that might be found in dacite include quartz, biotite, hornblende, augite, and enstatite. Dacites consisting mostly of…
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What Is Dacite?
Dacite is a fine-grained igneous rock that is normally light in color. It is often porphyritic. Dacite is found in lava flows, lava domes, dikes, sills, and pyroclastic debris. It is a rock type usually found on continental crust above subduction zones, where a relatively young oceanic plate has melted below.
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Uses of Basalt
Basalt is used for a wide variety of purposes. It is most commonly crushed for use as an aggregate in construction projects. Crushed basalt is used for road base, concrete aggregate, asphalt pavement aggregate, railroad ballast, filter stone in drain fields, and may other purposes. Basalt is also cut into dimension stone. Thin slabs of…
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Plumes & Hotspots Below Continents
The third basalt-forming environment is a continental environment where a mantle plume or hotspot delivers enormous amounts of basaltic lava through the continental crust and up to Earth’s surface. These eruptions can be from either vents or fissures. They have produced the largest basalt flows on the continents. The eruptions can occur repeatedly over millions of…
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Oceanic Hotspots
Another location where significant amounts of basalt are produced is above oceanic hotspots. These are locations (see map above) where a small plume of hot rock rises up through the mantle from a hotspot on Earth’s core. The Hawaiian Islands are an example of where basaltic volcanoes have been built above an oceanic hotspot. Basalt production at…
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Basalts at Oceanic Divergent Boundaries
Most of Earth’s basalt is produced at divergent plate boundaries on the mid-ocean ridge system (see map). Here convection currents deliver hot rock from deep in the mantle. This hot rock melts as the divergent boundary pulls apart, and the molten rock erupts onto the sea floor. These submarine fissure eruptions often produce pillow basalts…
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Basalt-Forming Environments
Most of the basalt found on Earth was produced in just three rock-forming environments: 1) oceanic divergent boundaries, 2) oceanic hotspots, and 3) mantle plumes and hotspots beneath continents. The images on this page feature some of these basalt-forming environments.