Shield volcano

A shield volcano is a massive volcanic structure defined by its low profile and broad shape. This distinct form resembles a warrior's shield lying flat on the ground.

Unlike other volcanic types, shield volcanoes are built by the eruption of highly fluid lava. This lava has low viscosity, which means it is thin and runny. Because it flows so easily, it can travel great distances from the eruption site. Over many repeated eruptions, these thin layers of lava accumulate to create a wide, gently sloping mountain.

The construction of a shield volcano is a steady, long-term process. Most eruptions are described as effusive, meaning they are gentle rather than explosive. These eruptions involve the emission of basaltic lava with low gas content. This lava often flows out of central vents or along decentralized fissure vents. A fissure is a long crack in the ground that can create a "curtain of fire" or a "wall of fire."

As the lava travels, it can move through lava tubes. These are cave-like structures formed when the top layer of lava hardens. The walls of these tubes insulate the molten rock inside, allowing it to travel much further. In fact, about 58% of the lava at Kīlauea moves through these tubes.

Scientists categorize different types of lava flows based on their texture and movement. One common type is pāhoehoe, which is relatively smooth and has a ropey appearance. The other type is ʻaʻā, which is denser and more viscous, meaning it is thicker and moves more slowly. ʻAʻā lava often forms into blocky shapes. These flows move through a specific pressure mechanism. The partially solidified front of the flow steepens because of the weight of the lava behind it. This causes the front to break off and move forward. While pāhoehoe can turn into ʻaʻā if the lava becomes more viscous, the reverse never happens.

While most shield volcanoes are made of basaltic lava, there are rare exceptions. Some are called pyroclastic shields, which are formed by the accumulation of fragmented material from powerful explosive eruptions. Examples include the Billy Mitchell volcano in Papua New Guinea. There are also rarer felsic lava shields formed by unusually fluid felsic magmas. The Ilgachuz Range in British Columbia is an example of a felsic shield. Most mature shields also feature cinder cones on their flanks. These are smaller, secondary volcanic cones formed by the ejection of tephra during activity.

The history of the term "shield volcano" began with the Austrian geologist Eduard Suess. In 1888, he coined the German term "Schildvulkan." By 1910, the term had been translated into English. Much of our modern scientific understanding comes from studying the Hawaiian Islands. These islands are formed by a hotspot, which is a place where magma wells up from deep within the Earth. As the oceanic plate moves over this hotspot, it creates a long trail of volcanoes across the seafloor.

Shield volcanoes can reach staggering dimensions. Mauna Loa in Hawaii is one of the largest active volcanoes on Earth. Its mass is so great that it has caused the Earth's crust to slump downward. When you measure from its base on the sea floor to its summit, Mauna Loa is about 30,000 feet tall. This makes it much taller than Mount Everest.

Even larger structures exist in our solar system. Olympus Mons on Mars is a giant shield volcano that dwarfs any volcanic feature found on Earth.

These volcanoes are found in many different environments across the globe. They often form over hotspots, such as the Galápagos Islands, or over continental rift zones, like those in East Africa. They can also appear in ocean basins or near subduction zones, such as in California and Oregon. Some lava flows from these volcanoes create vast plateaus. In places like Idaho and Washington, these basaltic plateaus can reach thicknesses of over 1,000 feet. Whether in the ocean or on land, shield volcanoes are powerful builders of planetary surfaces.