The Scoria

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The bodies that solidify in the air and become scoria produces ground cover all around the volcano vent with the heaviest deposits on the downwind side. Scoria has an angle of repose of 30 to 40 degrees, which is very steep due to the small volcanoes produced by brief eruptions with a vertical relief less than a few thousand feet. Sometimes newly erupted lava flows contain abundant dissolved gas, and once lava starts to solidify, the trapped gas bubbles are the vesicles, and if the upper portion has an abundant number of vesicles, it is often called scoria.

Pumice is very similar to scoria that is also a vesicular igneous rock, but there are a few differences that can distinguish them. Scoria is usually always black or dark gray to reddish brown and pumice is white to light gray or light tan, due to their composition differences. Pumice has a higher concentration of trapped bubbles allowing it to float, but thick walls of scoria make it heavy enough to sink.

Using a hand lens, a person can often see very tiny mineral crystals in scoria. The production of lightweight aggregate is one of the main uses of scoria. It is crushed to the specific sizes and sold for a variety of uses. Using concrete with scoria weighs about pounds per cubic foot, but with typical sand and gravel it would weigh about pounds per cubic foot.

The lighter scoria allows buildings to be constructed with less structural steel, and the air trapped in the scoria makes the concrete a better insulator resulting in lower heating and cooling costs. Crushed scoria is used for ground cover in landscape projects, as a substrate in hydroponic gardening, and as roofing granules. As the molten rock emerges from the Earth, the pressure upon it is reduced and the dissolved gas starts to escape in the form of bubbles.

If the molten rock solidifies before the gas has escaped, the bubbles become small rounded or elongated cavities in the rock.

Scoria Facts

This dark-colored igneous rock with the trapped bubbles is known as scoria. When some volcanoes erupt, a rush of gas blows out of the vent.

Scoria rock

This gas was once dissolved in the magma below. The gas often blows out small bodies of magma that solidify as they fly through the air. This action can produce a ground cover of scoria all around the volcanic vent, with the heaviest deposits on the downwind side. Small particles of scoria that litter the landscape around the volcano are known as "lapilli" if they are between 2 millimeters and 64 millimeters in size. Larger particles are known as "blocks. Artistic drawing illustrating the subsurface magma source and layer-by-layer build-up of scoria in a cinder cone eruption.

Sunset Crater cinder cone: Photograph of the Sunset Crater cinder cone that was formed by eruptions that occurred about years ago. It is located near Flagstaff, Arizona and is about feet tall. It is one of over cinder cones in the San Francisco Volcanic Field. Most of the scoria falls to the ground near the vent to build up a cone-shaped hill called a "cinder cone. They are usually very steep because scoria has an angle of repose of 30 to 40 degrees.

In some parts of the world, cinder cones occur in clusters of a few to hundreds of individual cones. These areas are called "volcano fields. Magma being blown from the vent at Stromboli Volcano. This type of eruption would produce the small scoria cinders known as "lapilli.

More Topics

Some newly erupted lava flows contain abundant dissolved gas. The gas bubbles in the flow move upwards towards the surface in an attempt to escape while the lava is still molten. However, once the lava starts to solidify, the bubbles are trapped in the rock. These trapped gas bubbles are known as vesicles.


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If the upper portion of a lava flow contains a large concentration of vesicles, it is often called "scoria" or "vesicular basalt. This image shows a field on Mars that is strewn with pieces of scoria, erupted from a Martian volcano. The piece of Martian scoria in the foreground is about 18 inches across and was found on the surface of Mars by the Spirit Rover.

Have you ever slowly opened a bottle that contains a carbonated beverage and watched the gas bubbles form on the walls of the bottle? Then as the seal on the bottle is broken, the bubbles grow larger and a hiss of gas escapes from the bottle, followed by a rush of foam. The depressurization and the escape of gas from a beverage is the same process that occurs when magma is depressurized as it emerges from a volcanic vent.

The foam is equivalent to what will become scoria on solidification. Mauna Kea cinder cone: A red cinder cone and a cinder-covered landscape at Mauna Kea, Hawaii.

Pumice and Scoria

A vesicular igneous rock that is very similar to scoria is pumice. There are a few differences that can be used to distinguish them.

First is their color. Scoria is almost always black or dark gray to reddish brown, while pumice is almost always white to light gray to light tan. This color difference is a result of their composition. Scoria forms from basaltic magmas, while pumice forms from rhyolitic magmas - which usually contain more gas. Pumice has a much higher concentration of trapped bubbles - so many that the walls between them are very thin. The vesicles in pumice contain enough air that the rock will float on water.

The thick walls of scoria make it heavy enough to sink. Finally, when observed closely with a hand lens, you can often see tiny mineral crystals in scoria. However, close observation of pumice reveals a "glassy" texture similar to obsidian.


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