Volcanic Landforms

The landscapes created from volcanic activity seem to fascinate us. The volcanic activity at Yellowstone National Park alone draws 2 to 3 million eager visitors each year from across the country and around the world. Devils Tower (pictured here) is a volcanic feature of legend to many Native American tribes, and even a popular movie.

Explore a feature to learn more about it and the national parks where you can experience it.

 

Craters

A crater is the circular surface depression caused by volcanic activity, usually on the summit or flank of a volcano. Craters are built by the collapse of the summit, accumulation of lava and pyroclastic material around an open vent or pipe, or explosion of gas, lava, and pyroclastics from a volcano. Fissures may erupt from the walls or base of craters. During periods of volcanic activity, the crater floor can be a molten, seething lake of liquid lava. Mark Twain described the Kilauea crater in this way in the late 1800s.

Craters may naturally fill with water, creating a crater lake. Some crater lakes can have high temperatures and be acidic, due to continued volcanic activity.

Hawaii Volcanoes National Park, Hawaii
Katmai National Park and Preserve, Alaska

 

Calderas

After an eruption, the pressure inside of the volcano is decreased, because large volumes of magma and gas have been released. The empty system of conduits and shallow reservoirs cannot support the weight of the mountain above it, so the volcano collapses on itself. They can be steep-sided and are often circular.

Calderas can be more than 25 kilometers in diameter and several kilometers deep. There can be tectonic activity in calderas, such as earthquakes, uplifts, or ground subsidence. Hot springs, geysers, boiling mud pits, and other signs of geothermal activity are also common in calderas. Caldera activity can be monitored and researched to predict future volcanic eruptions.

Crater Lake National Park, Oregon
Aniakchak National Monument and Preserve, Alaska
Katmai National Park and Preserve, Alaska

 

Diatremes and Maars

Diatreme is a general term used to describe a volcanic vent or pipe that formed when magma was forced through flat-lying sedimentary rock. The explosive energy of magmas with high dissolved gas contents allowed the magma to be forced through the rocks to form an expanded vent. The sedimentary rock around the vent is lithified (melted together) and small amounts of magma intrude into the rock.

Phreatic eruptions are also associated with diatremes. These steam-driven explosions occur when groundwater is heated by nearby magma, lava or volcanic deposits. The intense and high temperatures of these materials cause water to boil or flash to steam, leading to explosive eruptions of steam, ash, and other volcanic material.

Maars are shallow, low-relief craters with flat floors. They often fill with water, forming natural lakes. It is typical for maars to form over diatremes. The walls of a maar are a combination of fragmented volcanic rock, and fragments of the underlying diatreme.

It is not unusual to find minerals and ores mined around diatremes. For example, the kimberlite pipes in South Africa are diatremes that have abundant diamonds. Also, it is not uncommon to find gold and silver mines around diatremes. Click on the link below to learn about diatremes and maars in national parks.

Katmai National Park and Preserve, Alaska

 

Igneous Rock Features

Magma is stored below the surface in reservoirs called magma chambers. It creates and follows paths called conduits to the surface. This network is often referred to as the volcano's plumbing system. These networks can cover vast areas. When magma cools and solidifies in these spaces, intrusive features like stocks, laccoliths, sills, and dikes are formed. If the conduits are emptied after an eruption, they can collapse in the formation of a caldera, or remain as lava tubes and caves.

Devils Tower is believed to be an ancient intrusive igneous rock body, created 60 million years ago during the Tertiary period. Learn more about Devils Tower by opening the case study.

Devils Tower National Monument, Wyoming

 

Lava Flows

When magma is erupted from a volcano, it moves down the slope of the volcano and surrounding topography as it cools. Lava can flow for distances of over 100km, and at speeds near 30 miles per hour. Depending on the chemistry, viscosity, and eruption style, lava flows can have very different and distinct appearances.

Pahoehoe is a smooth, ropy lava, common on the Hawaiian islands. 'A'a is a more chunky and thick flow, that is most commonly associated with composite volcano eruptions. 'A'a lava flows are also called clinkers, because of their rough and jagged appearance.

The eruptions of submarine volcanoes result in pillow lavas, which form as the lava cools in seawater. Plateau basalts occur as a result of fissure eruptions of basaltic lava. Fissure eruptions occur along elongated cracks in Earth's surface rather than from a single vent or pipe. Magma wells up through the crack, and pours out on either side in effusive eruptions. These eruptions form extensive sheets of basaltic lava. Columnar basalts are another feature that can form on basaltic lava flows when they cool uniformly. The lava cracks into vertical columns, typically hexagonal in shape.

‘A’a

`A`a (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough surface. The top layers of these flows are composed of broken lava blocks. The sharp and rugged surface of a hardened `a`a flow makes for treacherous and slow walking conditions.

The most active area of the flow is the core, which is much denser than the rough surfaces at the top of the flow, and underneath it. The dense, pasty lava in the core of the flow travels downslope, carrying the jagged chunks along the top of it. The cooler chunks can fall down the leading edge of the flow and be covered with advancing lava. This creates the rough layer below the core of an 'a'a flow.

It is possible for a pahoehoe flow to become and 'a'a flow, but 'a'a flows never become pahoehoe flows.

Pahoehoe

"Pahoehoe" is a Hawaiian word used to describe a lava flow with a smooth, ropy surface. Pahoehoe flows advance slowly, with small amounts of lava squeezing out of a cooler crust. Pahoehoe flows can exhibit all kinds of different shapes as they form and cool. These are sometimes called "lava sculpture."

 

Lava Tubes

During long eruptions, lava flows can be channelized into a few main streams. Flows that persist for many hours can solidify on the top and sides of the stream, leaving a tube for the liquid lava to flow through. The solidified lava is a good thermal insulator, so the lava flowing inside remains liquid much longer than an exposed flow. Lava tubes can transport lava several miles from an eruption site.

After the lava has drained from the tube, a lava tube cave is left. These caves are often big enough for people to walk through, and can cover long distances.

Hawaii Volcanoes National Park, Hawaii
Pu’uhonua o Honaunau National Historical Park, Hawaii
Lava Beds National Monument, California

 

Fumaroles

These occur in areas where a magma conduit passes through the water table. The magma can be liquid or recently solidified, but still hot. The heat from the magma causes water to become steam. As the steam rises it carries volcanic gases such as hydrogen sulfide (H2S) to the surface. This mixture of steam and gas is erupted from vents and fissures in the ground.

Due to this chemical activity, fumaroles can be very dangerous. Associated chemical reactions can color the surrounding rocks.

These features are sometimes called "dying volcanoes" because they occur near the end stages of volcanic activity as the magma deep underground solidifies and cools.

Yellowstone National Park, Wyoming, Idaho, and Montana
Hawaii Volcanoes National Park, Hawaii

 

Geothermal Features

Geothermal features can be observed in areas of active volcanism, or areas that have inactive volcanoes. Subsurface magma heats groundwater, creating steam and hot water. The hot, less dense water rises through fissures and cracks in the ground. When it reaches the surface, features such as geysers, fumaroles, hot springs, and mud pits are created.

Geysers are the most well known geothermal feature. Scientists do not completely understand how geysers work. They think that large amounts of groundwater fill underground cavities. The water in these is heated by nearby magma. Suddenly, some of the water flashes into steam, and the rest is forced violently from the vent in an explosion of hot water and steam. This cycle can be repeated regularly. Old Faithful is a geyser in Yellowstone National Park that has erupted once every 65 minutes for hundreds of years! <a href="03_Fumarole.htm">Fumaroles</a> are also geothermal features that depend on the interactions of released volcanic gases and the local groundwater system.

Geothermal features have great benefits. Geothermal processes create heat and electricity that provide power and hot water to cities in Iceland, New Zealand, Italy and Northern California. As well as being used for energy sources, geothermal waters can also contain minerals and elements such as sulfur, gold, silver, and mercury that can be recovered and used.

Yellowstone National Park, Wyoming, Idaho, and Montana

 

Links

Introduction to Volcanism

Eruptions and Hazards

Landforms and Features

Monitoring and Forecasting

Volcanisim in National Parks

Challenge Your Understanding

 

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