Basaltic lavas, the most abundant lava type on earth, are the focus of this lesson. By the end, you should understand where basaltic lava comes from and some of its common forms.
What Is Basaltic Lava?
When you envision lava in your mind, you likely think about rivers of molten rock flowing over a landscape much like Hawaii. This idealized vision of lava is actually basaltic lava.
Basaltic lava is the most abundant lava on earth and is the lava most commonly depicted in pictures and media. In this lesson, we’ll learn what defines basaltic lava and some of the different forms it takes.
Basaltic lava is another term for mafic lava. Mafic lava is molten rock that is enriched in iron and magnesium and low in silica. When mafic lava cools on the earth’s surface, it forms basalt, which is why mafic lava is commonly called ‘basaltic lava.’
The Origin of Basaltic Lava
You may be wondering, why should any one type of lava be more prevalent than another? To understand the answer to this question, we must first understand the origin of basaltic lava.
The mantle of the earth is constantly convecting due to the heat from the planet’s interior. Mafic magma is formed within the earth when a portion of the earth’s mantle is melted. Remember that magma is the term for underground lava.
The earth’s mantle can melt by three methods: adding heat to the mantle, changing its composition, or decreasing its pressure. The melting point of a rock is the temperature and pressure at which a rock will begin to melt. Adding heat to the mantle is the most obvious way to exceed the melting point of the rock.
Changing the composition will alter the location of the melting point and will often make a rock melt at a lower temperature. This is the same reason we add salt to icy roads — the salt acts to lower the melting point of the ice and hopefully will cause the ice to melt from the roads.The most common method of making basaltic magma is by decreasing the pressure of the mantle rocks. Mantle rocks are extremely hot, and the only reason they do not melt is that they are being held at high pressures within the earth. When these rocks ascend toward the surface, the pressure decreases and allows the rocks to begin melting. This process is responsible for the volcanism at mid-ocean ridges, which are the most volcanically active features on earth.
Basaltic lavas are so abundant because they are the product of melting the mantle, which makes up the majority of the earth’s volume. Almost all volcanism starts with melts that begin in the mantle, which is why basaltic melts are so plentiful.
Basaltic Lava Morphologies
Since much of our understanding and knowledge about basaltic lavas has come from in-depth study of the Hawaiian Islands, the two most common basaltic morphologies on land have Hawaiian names. In geology, morphology means the physical shape of a landform or feature.Pahoehoe is basaltic lava that exhibits a smooth, sometimes ropey, texture.
‘A’a is chunky and blocky basaltic lava. There is no usually no compositional difference between pahoehoe and ‘a’a. Instead, the difference in morphology is due to their flow rate and cooling rate. Pahoehoe experiences a slower cooling rate and velocity.
On the other hand, ‘a’a cools so quickly that the flow is literally breaking apart as it’s rapidly cooling and moving.
Pillow basalts are the basaltic lava morphology formed when lava erupts underwater.
They form when the exterior of the molten rock cools immediately when it comes in contact with the cold water. The glassy exterior then inflates with molten lava into the pillowy shape, much like inflating a balloon.
A feature unique to basaltic lava are lava lakes. Lava lakes are volcanic vents filled with basaltic lava and open to the air.
These features are usually long-lived, lasting several years and fluctuate in height based on the activity below the surface.
Another rare and unique basaltic lava feature is the lava fountain.
These occur during eruptions when lava can be shot over 1,000 feet into the sky before falling back down to form a lava flow. These have been beautifully captured at both Hawaiian and Icelandic eruptions.
Basaltic lava, or mafic lava, is molten rock enriched in iron and magnesium and depleted in silica.
Basaltic magmas are formed by exceeding the melting point of the mantle either by adding heat, changing its composition, or decreasing its pressure. The majority of volcanism on earth can be traced back to beginning in the mantle, which is why basaltic lavas are so abundant. The lava can take on many different morphologies. When on land, basaltic lavas are commonly seen as pahoehoe or ‘a’a flows. Underwater, basaltic lavas are erupted as pillow basalts. More rare basaltic lava features include lava lakes and lava fountains.
- basaltic lava/mafic lava: molten rock that is enriched in iron and magnesium and low in silica
- magma: underground lava
- melting point: the temperature and pressure at which a rock will begin to melt
- morphology: in geology, the physical shape of a landform or feature
- pahoehoe: basaltic lava that exhibits a smooth, sometimes ropey, texture
- ‘a’a: chunky and blocky basaltic lava
- pillow basalts: basaltic lava morphology formed when lava erupts underwater
- lava lakes: volcanic vents filled with basaltic lava and open to the air
- lava fountain: eruptions when lava can be shot over 1,000 feet into the sky before falling back down to form a lava flow
Upon completing this lesson, you should be able to define and describe the different types of basaltic lava.