Basalt is which type of rock

Yosemite Valley in the Sierra Nevada and Mt. Rushmore are two notable examples of granitic rocks. But granitic "basement rock" can be found just about everywhere east of the Rockies if you're willing to dig through the dirt and sedimentary rocks at the surface. Granite is intrusive, which means that the magma was trapped deep in the crust , and probably took a very long time to cool down enough to crystallize into solid rock. This allows the minerals which form plenty of time to grow, and results in a coarse-textured rock in which individual mineral grains are easily visible.

Granite is the ultimate silicate rock. Relatively small amounts of iron and magnesium occur, but since they have generally higher densities it's not surprising that there isn't very much in the granite. Due to the process of differentiation , most of the heavier elements are moving towards the core of the earth, allowing the silicon and oxygen to accumulate on the surface.

And accumulate it has. We call this purified material felsic because of the relatively high percentage of silica and oxygen. Basalt is extrusive. The magma from which it cools breaks through the crust of the earth and erupts on the surface. We call these types of events volcanic eruptions , and there are several main types. The volcanoes that make basalt are very common, and tend to form long and persistent zones of rifting in nearly all of the ocean basins. We now believe that these undersea volcanic areas represent huge spreading ridges where the earth's crust is separating.

It's a lot like a cut on your arm, which will bleed until a scab forms. Basaltic magma is like the blood of the earth - it's what comes out when the earth's skin is cut the whole way through.

As an eruption ends, the basalt "scab" heals the wound in the crust, and the earth adds some new seafloor crust. Because the magma comes out of the earth and often into water it cools very quickly, and the minerals have very little opportunity to grow. Basalt is commonly very fine grained , and it is nearly impossible to see individual minerals without magnification. Basalt is considered a mafic silicate rock. Among other characteristics, mafic minerals and rocks are generally dark in color and high in specific gravity.

The active mid-ocean ridges host repeated fissure eruptions. Most of this activity is unnoticed because these boundaries are under great depths of water. At these deep locations, any steam, ash, or gas produced is absorbed by the water column and does not reach the surface.

Earthquake activity is the only signal to humans that many of these deep ocean ridge eruptions provide. However, Iceland is a location where a mid-ocean ridge has been lifted above sea level. There, people can directly observe this volcanic activity. Thermal image of a hot basalt flow on the flank of Hawaii's Kilauea volcano. Hot lava at the front of the flow is revealed in yellow, orange and red colors.

The channel that it flowed through on the previous day appears as a purple and blue track. United States Geological Survey image. 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 these locations begins with an eruption on the ocean floor.

If the hotspot is sustained, repeated eruptions can build the volcanic cone larger and larger until it becomes high enough to become an island. All of the islands in the Hawaiian Island chain were built up from basalt eruptions on the sea floor. The island that we know today as "Hawaii" is thought to be between , and , years old. It began as an eruption on the floor of the Pacific Ocean. The volcanic cone grew as recurrent eruptions built up layer after layer of basalt flows.

About , years ago it is thought to have grown tall enough to emerge from the ocean as an island. Today it consists of five overlapping volcanoes. Kilauea is the most active of these volcanoes. It has been in amost continuous eruption since January, Basalt flows from Kilauea have extruded over one cubic mile of lava, which currently covers about 48 square miles of land.

These flows have travelled over seven miles to reach the ocean, covering highways, homes and entire subdivisions that were in their path. Columbia River Flood Basalts: The Columbia River Flood Basalts are an extensive sequence of stacked lava flows that reach a cumulative thickness of up to feet. The outcrops in the foreground and in the distance of this photo are all made up of layered basalt flows.

Although basalt is typically a dark black rock, it often weathers to a yellow-brown color similar to the rocks shown here. Public domain image by Williamborg. The area shown is what has not yet been eroded away - the original extent of these basalt flows was much greater. Over individual flows have been identified, and several hundred meters of basalt underlies much of the area shown in the map above.

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 years, producing layer after layer of basalt stacked in a vertical sequence see outcrop photo. The word "traps" is derived from the Swedish word for "stairs," which describes the outcrop profile of these layered basalt deposits, as shown in the outcrop photo.

The best way to learn about rocks is to have specimens available for testing and examination. The Roman theatre: left in Bosra, Syria. The dark building stone is basalt. Basalt paving stones: right on a city street in Rome, Italy.

Basalt pavers were often used in areas close to volcanoes. 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 basalt are cut and sometimes polished for use as floor tiles, building veneer, monuments, and other stone objects.

Article by: Hobart M. Find Other Topics on Geology. Maps Volcanoes World Maps. Hardness Picks.