Magma or hot molten rock beneath the earth’s surface has an average temperature of around 1300 degrees F to 2400 degrees F or 700 degrees C to 1300 degrees C. Komatiite magma, in particular, can reach the temperature of 1600 degrees Celsius.
Magma formation depends on the type of environment it is formed. Magma is typically found several kilometers from the surface of the earth or in areas with low pressure and high temperature. Geologists identify the types of magma by their silica content which affects their gas content, temperature, and viscosity.
Table of Contents
- Basic Types of Magma
- What is the Difference Between Lava and Magma?
- Types of Volcanoes
- Types of Lava
- Glossary of Terms
Basic Types of Magma
When it comes to its features, heat, fluidity, and gas content, andesitic magma is considered to be in the middle. It has a silica content of roughly 55% to 65%, and its sodium, potassium, iron, calcium, and magnesium contents are considered average.
After andesitic magma cools it becomes andesite rock. The magma is slightly explosive, although not as vicious as other types of volcanoes, and it is found in small- to medium-sized earthquakes because of the faults that result from its explosive eruptions. A good example of andesitic magma is the eruption of Oregon’s Mount St. Helens. Andesitic magma has a mean temperature that is between 1471ᵒ and 1832ᵒ Fahrenheit.
With a 45-55% accumulation of silica, basaltic magma is high in minerals such as magnesium, iron, and calcium, while its potassium and sodium contents are fairly low. Basaltic magma has a mean temperature that is between 1832ᵒ and 2192ᵒ Fahrenheit, although this estimate is based mostly on limited field operations and measurements within the lab.
It is very difficult to get close to basaltic magma, which is why exact measures of this type of magma are not possible. Basaltic magma is also more fluid than any other type of magma, though it is nowhere near the fluidity of water. In fact, basaltic magma is roughly 10,000 to 100,000 times less fluid than water.
It also has a very low amount of gas and is found in “hot spots” throughout the earth, including Hawaii. Basaltic magma becomes basalt rock once it cools.
Of all the types of magma that exist, felsic magma has the most silica in it. The percentage of silica in felsic magma is between 65% and 70%, and, because of this, it also has the highest viscosity (gooiness) and gas content. Of course, it also has low mean temperatures compared to other types of magma, with numbers that range from 1202ᵒ and 1472ᵒ Fahrenheit.
Felsic magma is very thick and gooey, which means that it sometimes gets gas bubbles in the magma chamber of the volcano. These bubbles can cause extremely dangerous eruptions that cause lava to spew into the air and eventually turn into rhyolite and dacite rock. Felsic magma is similar to intermediate magma, as both have sticky, thick lava that spews out in a rough manner and causes rock formations later on.
Felsic magma tends to form when seawater and the earth’s crust meet, causing the crust to melt. Its magnesium, calcium, and iron contents are low, but it is high in both sodium and potassium. The magma also has a lot of unstable gases and melted rock, and it can form very large calderas, which are depressions or craters found on larger volcanoes. You can see a large caldera in Yellowstone Park, and these craters are usually formed when the magma chamber of a volcano becomes empty.
Although it doesn’t have a silica content that is as high as felsic magma, intermediate magma is roughly 60% silica, so it still contains a lot of silica. Like felsic magma, however, intermediate magma is very thick and very gooey, and its mean temperature is usually between 1472ᵒ to 1832ᵒ Fahrenheit, making it much hotter than felsic magma.
Intermediate magma tends to build up pressure when it is below the surface of the earth, and, afterwards, it is released as lava. Intermediate magma often transforms into andesite because of its heat content, and these are rocks that get their name from the Andes Mountains in South America.
Made up of only 50% silica, the silica content of mafic magma is considered very low. However, it does contain a lot of magnesium and iron, along with a low gas content and not very much gooiness. Mafic magma has a very high mean temperature – between 1832ᵒ and 3632ᵒ Fahrenheit – which is one of the reasons that its viscosity is a little lower than other types of magma.
Of course, this factor also allows it to be one of the most fluid types of magma, and it both moves very quickly and erupts in a non-aggressive manner. Its lava eventually cools down into a rock called basalt, which is dark-colored and very heavy. The color is due to the amount of magnesium and iron in the magma, because both of these minerals are very dark in color.
In addition, basalt is a very common rock found in the earth’s crust, and it is also found in the volcanic islands developed by hot spots. The islands found in Hawaii are a perfect example of the results of eruptions made up of mafic magma.
Rhyolitic magma is formed by a combination of basaltic magma and another material that is siliceous. It can also be formed by melting parts of the earth’s sialic layer. The magma is made up of rhyolite, which has a high content of silica and is very gelatinous.
Its flow is similar to that of toothpaste when it is coming out of its tube, and it often piles up and forms domes of lava. Rhyolitic magma is sometimes rich in gas, and when this is so, it can spew viciously into the air. In these instances, pumice is formed, which is solid magma that is lightweight and light in color.
In other instances, rhyolite lava is very porous and can form a glass-like material that is dark in color, called obsidian. In fact, rhyolite is similar to granite when it comes to volcanoes.
Also known as komatiite magma, this type of magma is not found anywhere today because of the planet’s cooler temperatures. When this type of magma was still around, it reached temperatures of approximately 2900ᵒ Fahrenheit. Ultramafic magma burned very hot but flowed very fast; in fact, it would flow down a volcano almost as quickly as water would.
What is the Difference Between Lava and Magma?
- Lava is a mixture of hot gases and molten rocks which comes out of a volcano.
- Once magma comes out of a volcano, it is called lava.
- Magma is the name for the material made up of the molten rock and found deep inside of the earth’s crust.
- Magma is found in chambers that have passages up to the volcanoes. Magma spews out of volcanoes.
Types of Volcanoes
Cinder Cone Volcanoes
The simplest form of volcano, cinder cone volcanoes happen when lava blobs and particles spew forth from a volcanic tent. The lava then blows into the air aggressively, causing the pieces to rain down around the vent. After a certain amount of time, it builds up into a cone shaped like a circle or oval-shaped, with a bowl-shaped crater at the top. These types of volcanoes usually grow no more than 1,000 feet above their surroundings.
Also called stratovolcanoes, these are volcanoes that have a conduit system inside of them from which magma channels through to the surface of the earth. Lava breaks through walls or issues from fissures to form clusters of vents, and it flows down the sides of the mountain. Examples of composite volcanoes include Mount Fuji, Mount Rainier, and Mount Cotopaxi.
These are domes created by small amounts of lava that are too thick to go very far. Unlike shield volcanoes, which are not as thick, lava domes have magma that piles up, around, and over the vent. The dome grows when the lava within it expands, and the mountain forms when materials spill off the sides of the expanding dome. These domes can also explode very aggressively and release large amounts of hot ash and rock.
These volcanoes are quite large and broad, and from above, they look a lot like shields, which is where the name comes from. Lava pouring out of these volcanoes is thin and therefore, it can travel down the volcano’s shallow slopes for very long distances. They build up slowly over time and have hundreds of eruptions that form many different layers. When they do explode, it is not likely to be a catastrophic event, and examples include Mauna Kea and Mauna Loa.
Types of Lava
This word is pronounced “ah-ah” and refers to a basaltic type of lava; it doesn’t flow very quickly. In fact, it has a look which is similar to hot Jell-O and a rough, cool surface. After it hardens, this type of lava is sharp and spiny, making it difficult to walk across. A’a lava generally erupts when it gets to temperatures that are higher than 1800ᵒ Fahrenheit.
Pronounced “pa-ho-ho,” this is a type of lava that is much thinner and a lot less gooey than the a’a type of lava. It often flows down the slopes of volcanoes located in very large rivers. The surface of this lava congeals, or sets, and produces a smooth, thin crust. Pahoehoe lava also forms lava tubes, which involves rock that hardens around a liquid core that moves quickly. When that core spews out of the tube, it results in the formation of a long tunnel. Pahoehoe lava usually erupts when it gets to over 2100ᵒ Fahrenheit.
This lava is usually formed from volcano vents found underwater that start to erupt. Once the lava makes contact with the water, it cools down then forms a hard shell. Then, if more lava spews from the vent, this shell cracks and allows more pillows to come out of those cracks.
Glossary of Terms
Active Volcano: An active volcano is one that is either currently erupting or has erupted during some time in recorded history.
Accessory Fragment: Coming from mafic eruptions that are extremely fluid, these fragments are made of country rock that exploded into the air from the volcano.
Andesite: Andesite is a volcanic rock that is grey or grey-green in color and is composed of approximately 60% silica.
Andesite Magma: This magma has a chemical composition of roughly 60% and it forms an andesite rock when it crystallizes.
Ash: Volcanic fragments that are typically found every time a volcano erupts.
Ash-Flow Tuff: A deposit made up of a volatile mixture of things such as gas, pumice, and ash, and which travels quickly from a volcanic vent during an aggressive eruption.
Basalt: A volcanic rock that has a low silica content and is dark green, dark grey, or even black in color.
Basaltic Magma: This is a type of magma that has a low silica content and forms a rock called basalt when it crystalizes.
Bimodal: A bimodal compound is one that is made of two components that vary in their texture and composition. The term is often used to describe volcanic terrains of almost equal parts of mafic and felsic rocks.
Block-and-Ash Flow: This refers to the flow of angular rock fragments and ash, with the fragments being at least 10 inches in diameter.
Caldera: A caldera is a depression that is either circular or elongated in shape and which is formed from different types of magma that have extruded out in rapid fashion.
Composite Volcano: These are volcanoes that have steep sides and slopes that increase towards their summits, in part because of the sedimentary deposits and lava flows found near their base and the pyroclastic deposits towards the top.
Crust: This is the outermost layer of Earth, and it ranges from six to 40 miles in thickness worldwide. The uppermost part is the part which is brittle enough to produce earthquakes.
Dacite: Volcanic rock that is rich in silica and light in color. When it is a part of magma, that magma produces eruptions that are thick and muffin-shaped.
Decompressive Melting: This melting occurs whenever rocks experience a decrease in pressure, normally close to hot spots such as Hawaii.
Dormant Volcano: A volcano not experiencing any eruption but which might in the future.
Down-Sag Caldera: This is a type of caldera that slopes inward and has wall rocks that tilt inward. It is believed they are the result of a smaller-sized eruption.
Effusive Eruption: This eruption results from an outpouring of lava that spews onto the ground. It is different from aggressive or violent eruptions, and it includes lava flows which vary in length, shape, thickness, and width. These factors vary depending on the discharge, type of lava which erupted, and the slope of the ground which allows the lava to travel, not to mention the duration of the eruption.
Eruption: This happens when volcanic materials such as gases and magma explode from a fissure at the earth’s surface.
Extinct Volcano: This type of volcano is not currently erupting and is not expected to erupt in the future.
Fuel-Coolant Interaction: This happens when magma and external water interact, and it sometimes results in thermal explosions.
Hot Spot: An area where the temperatures and other factors make it conducive to the formation of volcanoes; Hawaii is a great example of a hot spot.
Hydrovolcanic Eruptions: Put simply, these are eruptions that are the result of mixing water and magma.
Igneous: Any process that involves magma or the rocks formed when magma solidifies.
Lapilli: These are fragments found in volcanic rocks and deposits, and they can range from less than an inch to 2.5 inches in diameter.
Lateral Blast: This is the eruption of a volcano that happens horizontally instead of vertically. An example is the eruption of Mt. St. Helens in 1980.
Lava: This is the term used when magma has erupted on the earth’s surface.
Mesa Lava: Made mostly of rhyolitic rock, this lava flow forms a round, biscuit-shaped body.
Monogenetic Volcano: This is a volcano known to have erupted only one time.
Nested Caldera: A nested caldera is found within an older and larger caldera structure.
Pahoehoe Lava: This term is a Hawaiian term, and it means lava that is flowing with smooth and continuous surfaces.
Petrology: The study of rocks and the composition, origin, and occurrence of those rocks.
Piecemeal Caldera: This is a caldera with an internal structure made up of two or more blocks bounded by faults.
Perlite: This is obsidian rock that is hydrated and light grey in color. It usually has fractures that are round in nature and resemble onion skin.
Ponded Flow: A flow of lava that sets inside of a depression or hole of some type.
Pumice: Magma or lava fragments that have fast-expanding gas found with magma that is cooling rapidly.
Pyroclastic: This term is used to describe magma or lava which has exploded into fragments, but it can also mean deposits formed by very aggressive volcanic activity.
Quenching: This term refers to magma that cools quickly and, therefore, forms a type of glass.
Rhyolite: This is a rock with more than 68% silica. They are composed mostly of quartz and alkali feldspars, but they can also contain minerals such as zircon, tourmaline, and biotite. Because of their high silica content, rhyolite lava usually forms lava domes and mesa lavas.
Rhyolite Magma: This type of magma has a high silica content, more than 68% total.
Rift: A rift is formed by an extension of the earth’s crusts and are associated with volcanoes.
Shield Volcano: A very broad volcano with fairly fluid lava and flank slopes of < 5°.
Silica: A chemical compound known as silicon dioxide and demonstrated by the symbol SiO2.
Silicic Lava: This is any lava that has more than 62% silica. The term is usually used interchangeably with the term “felsic lava.”
Stratovolcano: A volcano with steep sides and which has alternating layers of lava flows, volcaniclastic sedimentary deposits, and pyroclastic deposits. Their slopes increase towards their summits, and they can also be called a composite volcano.
Strombolian Eruption: These eruptions consist of basaltic magma and are a little more violent than Hawaiian eruptions. They form a lot of ash and scoria that merge to form a cone, and the eruptions are usually pulsating. Their deposits are made of vesicular bombs, mafic ash, lava spatter, and scoria lapilli.
Tectonic: This is a generalized term describing the force involved when the earth’s crust becomes deformed. It also is used to describe the geological features or structures that are produced by this deformation.
Trap-Door Caldera: This type of caldera is formed when a part of its floor sinks to a depth that is larger than the other side of the floor. They usually have either completely collapsed or have formed because of magma chambers which are shallow and asymmetrical in shape.
Tremor: This is a continuous vibration felt in the ground located around active volcanoes. They can be seen on a seismograph.
Tuya: A volcano with steep sides and a flat top and which erupted into a lake that was previously thawed into a glacier due to the heat from a volcano.
Unconformity: The surface of an erosion which separated older rocks and younger strata.
VEI Index: This describes the Volcanic Explosivity Index, which measures the size of an eruption and is based on the eruption’s intensity, magnitude, and destructive power. It consists of an eight-point scale, with 8 being the most dangerous and aggressive type of eruption.
Vent: This is an opening in the surface, or a hole in a planet, which spews out volcanic products such as ash and magma, among others.
Vesicle: Vesicle are frozen bubbles found inside volcanic rocks. They are formed when magma has crystallized around a gas bubble.
Vesicular: A term used to describe volcanic rocks with frozen gas bubbles, or vesicles, inside of them.
Volcanic Cycle: A term used to describe a period consisting of an increase in volcanic activity.
Volcano: A hill, mountain, or mound made when lava and other materials form under the ground and when lava spews into the air through a vent in the earth’s crust.
Vulcanian Eruption: An eruption that is aggressive and reaches heights up to 12 miles, but which only lasts a few minutes.