The odds are quite good that you have never seen magma – apart from photographs or perhaps on a nature show on television. However, depending on where you live or have traveled you may very well have interacted with its byproducts without even knowing it (for example, when you walk on a volcanic landscape, or on a granite floor).
Simply put, magma is liquefied, molten (which, in this case, is synonymous with melted) rock which exists below the surface of the planet. With only a few exceptions (lightning, which can actually be hotter than the surface of the sun, being the most well known), magma is the hottest naturally occurring thing that ever reaches the surface of the earth in any form. Depending on its type and composition, magma can range from 700 degrees Celsius (1300 degrees Fahrenheit) up to 1250 degrees Celsius (almost 2300 degrees Fahrenheit). In the earth’s distant past, it is believed that magma reached a temperature of over 1600 degrees Celsius (over 3000 degrees Fahrenheit).
Magma is only found beneath the earth’s surface – and in recent years evidence has been found which suggests that it may exist, or have existed in the distant past, on other planets – and only in specific areas beneath the surface. When magma comes to the earth’s surface – usually, but not always, as the result of a volcanic eruption – it is called lava. When lava cools it forms volcanic, or extrusive, igneous rock which accounts for about 15% of the surface area of the earth. It should be noted that not all magma makes it to the surface of the earth as lava; a great deal of magma cools before it reaches the surface, in the upper layers of the earth’s crust – in which case it forms what is called plutonic, or intrusive, igneous rock.
While people have certainly known about (and even, in some cases, worshiped) volcanoes for thousands of years, and had at least a general idea about lava for several centuries, a true understanding of magma – both what it is and where it actually comes from – is a relatively recent thing in human history. Even less than a century ago the technology did not exist to actually measure the heat and composition of magma, much less understand how and where it was formed. As technology continues to advance more is understood about magma – but there is still a lot to learn.
Where Does Magma Come From?
Contrary to what many people think, magma does not come from the center of the earth. In order to understand where magma actually does come from – and how it is formed – it is necessary to understand a bit about the basic structure of the earth itself (the stuff most of us daydreamed through when it was being taught to us in high school).
The earth has four basic levels: the inner core, the outer core, the mantle, and the crust. The inner core is believed to be comprised primarily of solid iron and nickel and is the hottest of all the levels, at over 5500 degrees C (about 10,000 F). The outer core is made up of mostly liquefied iron and nickel and is a bit ‘cooler’, at around 4400 degrees C (8000 F). The mantle is divided into an upper and lower part (called the lithosphere and the asthenosphere respectively); the lower part of the mantle is comprised of much the same materials as the outer core, has a plastic-like liquid consistency and is only slightly cooler, while the upper mantle is made of very dense solid rock and is considerably cooler – usually between 300 and 500 degrees C (about 600 to 900 F). The crust is the top layer of the earth; the lower portions of the crust will range from 200 to 400 degrees C (about 350 to 750 F) while the upper part of the crust is the surface of the planet where all known life – including humans – exist.
Simply put, magma is formed when solid rock and other constituents in the upper mantle or lower crust melts as the result of heat and pressure exerted upon them over a considerable length of time by a number of factors including ‘hotspots’, which are areas where the much hotter lower mantle effectively superheats the cooler material of the upper mantle and lower crust above and melts them. There is no evidence to suggest that magma is produced in any part of the earth other than the lower crust and upper mantle. While the lower mantle and outer core are both comprised of extremely hot liquefied elements, their chemical composition is different from the substance that we commonly call magma.
Magma is not evenly or generally distributed at any level within the earth, and is not found ‘everywhere’ at a certain depth; specific conditions must be present for the production of magma to occur. Once magma has been formed, it is forced upward into and through the crust by pressure from below – traveling through cracks in the existing solid rock – and will often melt some of the rock it passes, creating more magma. Eventually, it will either cool beneath the earth’s surface and turn back into rock or be stored in its liquefied form in ‘magma chambers’ (discussed below) which are found at varying depths beneath the surface of the crust, often directly beneath volcanoes.
What is Magma Made Of?
Generally speaking, magma has a high silica content – in most cases over 50 percent – and so silica can be said to form the base of magma. Silica (silicon dioxide) is one of the most common naturally occurring elements and is a major component of sand, as well as quartz. The basic types of magma (discussed in depth below) are classified based in large part on their silica content.
Other components of magma will normally include oxygen (as a constituent of silica), iron, aluminum, magnesium, calcium, potassium, phosphorus, and sodium. Most magma will also include relatively small amounts of other gasses that have dissolved which can include water vapor, sulfur, and carbon dioxide among others.
The silica and other mineral content in magma will generally determine both the temperature and the viscosity of the liquid, and therefore both its thickness and the rate at which it flows. Magma with a very high silica content will normally be cooler and have a higher viscosity (in other words, be thicker) and therefore slower moving than magma with low silica content.
Basic Types of Magma
While they all have a number of things in common (for example, they’re hot!), not all magmas are the same, and there are different classifications used by geologists and other scientists to classify them. The different types of magma are generally determined by the chemical composition, which is based on what materials have been melted or otherwise incorporated into the magma. This chemical composition – and particularly the mineral and gas content – will determine the temperature range of the magma, its viscosity, and rate of flow, and its general ‘behavior’.
The exact temperature of the different types of magma is extremely difficult to determine, in large part due to the danger and difficulty involved in taking exact measurements. As can be seen below, the temperatures of the various magmas listed are normally expressed as ranges and are determined in large part by estimates based on the magma’s behavior as well as their ‘eruption temperatures’. It is not uncommon for the basic types of magma discussed below to overlap or to switch back and forth between ‘types’ due to a number of possible factors (temperature, mineral content, etc.) during its existence.
Basaltic Magma
Also often referred to as Malfic, Basaltic magmas are the hottest of all the magma types currently in existence, with temperatures ranging from 1000 to 1200 degrees C (about 1800 to 2200 F). The silica content of Basaltic magmas is between 45 and 55 percent, which is lower than any other magma type. Because of the relatively low silica content and high temperature, Basaltic magmas have the lowest viscosity of all magma types, meaning that they are ‘thinner’ and more free-flowing – although it is still usually between 15,000 and 100,000 times less fluid (or free-flowing) than water.
Basaltic magmas have relatively high iron, calcium, and magnesium content (in some cases as much as 19 percent) but are generally low in potassium and sodium. Due to their high temperature (and the corresponding relatively low amount of trapped gasses), Basaltic magmas are considered to be ‘gentle’ when forced to the surface in the form of a volcanic eruption. Although in its lava-form it can be quite fast-moving and cover a considerable amount of territory in a relatively short time, eruptions of Basaltic magmas/lava are less explosive than the other types of magmas. When Basaltic magmas cool and harden, they form basaltic and malfic igneous rock.
Basaltic magmas generally occur at or near divergent or convergent tectonic plate boundaries and hot spots, and a significant percentage of Basaltic magmas/lava eruptions take place beneath the surface of the oceans. Volcanic eruptions in the US State of Hawaii are an example of Basaltic eruptions on land. It is believed in most scientific circles that Basaltic magmas – and periodic eruptions (in the past or present) – are present on the moon, Venus, Mars, the asteroid Vesta, and Io (the third largest of Jupiter’s moons).
Andesitic Magma
Andesitic magmas are also sometimes referred to as Intermediate, due to the fact that they fall in between Basaltic and Felsic (discussed below) magmas in terms of temperature and viscosity. Andesitic magmas will usually have temperatures ranging from 800 to 1000 degrees C (about 1500 to 1800 F) and a silica content that will range from 55 to 65 percent. As a result of the lower temperature and higher silica content, Andesitic magmas have a higher viscosity than Basaltic and are, therefore ‘thicker’ and slower moving.
Andesitic magmas have what has generally referred to as intermediate (hence the nickname) levels of magnesium, iron, potassium, calcium, and sodium – meaning that, again, they fall in between Basaltic and Felsic. As they are cooler and slower moving than Basaltic, Andesitic magmas have a higher volume of gasses – particularly sulfur and carbon dioxide – trapped within them which, in turn, help to make them significantly more volatile. They are generally considered to be moderately explosive types of magma. Andesitic magmas often pool in magma chambers in the earth’s crust, where they will either cool and form rock or build up pressure.
Andesitic magmas are generally found around convergent plate boundaries and near island arcs including the Aleutians off the Alaskan coast and the Philippine Islands in the Western Pacific Ocean. Andesitic magmas can be released to the surface of the earth by earthquakes (many of which take place beneath the surface of the ocean in what are known as ‘reverse fault’ areas), as well as via volcanic eruptions such as the May 18th, 1980 eruption of Mount St. Helens in the US State of Washington. When it cools, Andesitic magmas form andesite igneous rock, which is named after the Andes Mountain range in South America.
Felsic Magma
Felsic magmas are the ‘coolest’ of all magma types, normally ranging from about 650 to 800 degrees C (roughly 1200 to 1500 F) and also have the highest silica content, coming in at anywhere between 65 and 75 percent. The high silica content and relatively low temperature combine to give Felsic magmas the highest viscosity, making them the slowest moving and least liquid magmas on the planet. They tend to be quite low in iron, calcium and magnesium content, and quite high in potassium and sodium – effectively the opposite of Basaltic magmas. They are also high in gas content – particularly water vapor (H2O) and carbon dioxide (C02); sulfur, chlorine and fluorine gas may also be present.
There are two main types of Felsic magma: Rhyolitic and Dacitic. Dacitic magma is at the upper end when it comes to both silica content and temperature, and is closer in temperature and composition to Andesitic magmas, while Rhyolitic magma tends to be the cooler and thicker of the two. Both magmas are extremely volatile; volcanic eruptions of these types of magma can be highly explosive due in large part to the high gas content and can rip solids from the sides of the volcano. When these magmas cool, they form felsic igneous rocks, including pumice, ryolite and granite.
Felsic magmas seem to be formed when parts of the earth’s crust melt in conjunction with seawater – which serves to lower the temperature of the magma immediately following the melt – usually near continental rifts and hotspots in the continental crust. Felsic magmas can often be found in huge calderas, which are cauldron-shaped hollows that form at or near the earth’s surface, usually following the emptying of a magma chamber or reservoir via a volcanic eruption. The most famous caldera in the continental United States can be found at Yellowstone National Park.
Ultramafic Magma
Ultramafic (also sometimes called komatiite or picritic) magmas no longer exist today, probably becoming extinct right around the time the earth’s crust had cooled sufficiently following its initial creation to allow the beginnings of life to develop on the planet – somewhere in the vicinity of 2.5 to 3.5 billion years ago. Based on the evidence found in ultramafic rock samples, Ultramafic magmas were very low in silica content (probably around 40 percent or lower) and very high in iron, magnesium, and calcium (perhaps running as high as 32 percent). They were also quite hot – often reaching a temperature of 1600 degrees C (almost 3000 F). Due to its chemical composition and extreme heat, Ultramafic magmas had an extremely low viscosity – far lower than any of the magmas found on earth today – meaning that they were probably able to move very quickly.
A considerable percentage of the earth’s upper mantle is believed to be composed of ultramafic rock, leading some experts to believe that this was, at one time, the most common type of magma on the planet. Today, both the upper mantle and the lower crust have cooled to a point where conditions are such that it is impossible for Ultramafic magmas to develop. It is believed that Ultramafic magma may currently be present on Io – the third largest of Jupiter’s moons – and on the planet Mercury.
Magma Chambers
Magma chambers are large pools that exist beneath the surface of the earth and which – not surprisingly – hold magma. Magma travels up through and across the earth’s mantle and crust through cracks in the existing rock; when it can no longer find a way up or across, over time it will pool and create a magma chamber.
While a magma chamber can exist almost anywhere in the earth’s upper mantle or crust, they are very difficult to detect at great depths even with the most modern equipment, and so most of the currently known magma chambers are between 1 and 10 kilometers (a little over half a mile to roughly six miles) beneath the earth’s surface. Many of the world’s volcanoes – both beneath the oceans and on land – are situated near or directly above a magma chamber. In some cases, a large magma chamber may exist below a smaller chamber or magma reservoir which, in turn, might exist beneath a volcano.
Any of the existing types of magma can pool in a magma chamber and once it has pooled, it comes under great pressure and will usually begin to cool. Depending on the rate of cooling, the number of trapped gasses in the magma, and several other factors, the magma will either form intrusive igneous rock – such as granite and diorite – or it will fracture the rock around it and continue its journey upwards, often resulting in a volcanic eruption. During an eruption, the rock surrounding the magma chamber will usually collapse and, in some cases, will form a depression at the surface which can result in the formation of a caldera (discussed above).
A Few Words About Lava
As has been stated before, lava is basically what magma becomes when (and if) it reaches the surface of the earth. While lava will sometimes pick up a few extra components such as some sold rock or sediment along the way, the composition of lava will usually be substantially the same as the magma which forms its base.
There are four basic types of lava (and their names, not surprisingly, correspond with the names of the major magma types): Basaltic, Andesitic, Dacitic and Rhyolitic (the last two being the names of the major Felsic magmas). The type of lava will determine both the explosiveness of its entry to the surface (force of eruption) and its flow. Once it has cooled and hardened, it will also help to determine the make-up of the volcano – both inside and outside – as well as the surrounding land area.
In most cases, Dacitic and Rhyolitic lavas will produce the most explosive eruptions – in some rare instances literally blowing the tops off of volcanic mountains – and the slowest flow; in some cases so slow that they will actually seal off the passageways and the opening out of which it is flowing, cooling faster than it can flow. Andesitic lava will normally be thinner and less explosive – and therefore flow further – but an eruption can still pack one hell of a wallop. An andesitic lava flow is most commonly found in the Andes Mountains in South America (after which it is named) and in eruptions near island arcs.
Basaltic lava is by far the most common type of lava found on earth today. It is both the hottest of all lava types and produces the fastest and most sustained flows – due primarily to the fact that, as the hottest, it takes the longest to cool and so will flow the farthest. Basaltic lava eruptions occur both below the surface of the oceans and on dry land. Eruptions that occur on the ocean floor are often the result of ‘hot spots’ in the earth’s crust; over the course of millions of years and countless lava flows, islands formed primarily of cooled lava can eventually rise above the surface of the ocean. The US State of Hawaii was created in this way over the course of an estimated 70 million years, and today the islands are home to some of the most active (and spectacular) Basaltic volcanoes in the world. As a result of a continuing lava flow, several of the Hawaiian Islands continue to grow in landmass with the passing years.
There are three basic types of Basaltic lava: Pillow, Pahoehoe, and A’a.
Pillow Lava
Pillow lava is generally believed to be the most common type of lava. Pillow lava is formed as a result of the subaqueous extrusion of Basaltic lava; in other words, when a volcanic or fissure eruption occurs under water or ice. The temperature difference between the water and lava causes the outer layer of the lava to cool quite quickly, and form rounded tubular masses of lava that will resemble a pillow after it cools rather than a continuous lava flow. As more lava erupts, more pillows are formed. Pillow lava will usually be anywhere from 3 to 5 feet in diameter and can be found in many places on the ocean floor, as well as beneath glaciers and in ‘pillow formations’ on dry land. Pillow lava that is found on land is usually a good indication that the area where it is located was at one time beneath the surface of the ocean.
Pahoehoe Lava
Generally considered to be the second most common type of Basaltic lava, Pahoehoe lava flows are what most people tend to associate with volcanic eruptions. Pahoehoe lava is both the hottest and most liquid (and, consequently, the fastest moving) type of lava. Pahoehoe lava will normally have an eruptive temperature of around 1100 degrees C (about 2000 F) and will generally have a smooth, continuous flow at the beginning that will form landscapes that can range from mostly smooth and level near the mouth of the volcano to beautiful and bazaar designs that have been said to resemble rock sculptures further away. In some cases, Pahoehoe lava will form tunnels as it flows that will carry the lava all the way to the sea where – in the case of the Hawaiian and some other island chains – will cause the island to increase in size over time. Once the lava flow ceases, the tunnels will sometimes become lava caves.
A’a
Pronounced ‘ah-ah’ and meaning ‘stony with rough lava’ in Hawaiian, A’a lava is very closely related to Pahoehoe in its composition; in fact, Pahoehoe that travels over land for a particularly long distance will sometimes become A’a lava. It is cooler and more viscous (thicker) than Pahoehoe and consequently flows at a slower rate. When it cools (which can take weeks or even years), A’a lava tends to form a very rough, jagged, and ‘spiny’ landscape of rock that can be extremely difficult to walk across. Examples of cooled A’a lava can be found in abundance on the Big Island of Hawaii.
Lisa has a Bachelor’s of Science in Communication Arts. She is an experienced blogger who enjoys researching interesting facts, ideas, products, and other compelling concepts. In addition to writing, she likes photography and Photoshop.