Lava landforms

Volcanic eruptions have the ability to create many different types of landforms and come in many shapes and sizes.  Landforms that form from lava and ash include shield volcanoes, cinder cone volcanoes, composite volcanoes, lava plateaus, and calderas.  A shield volcano occurs when lava flows out and gradually builds a wide, sloping mountain.  It has a wide base and a flat top.  Shield volcanoes are very large, and their eruptions are nonexplosive.  A cinder cone volcano is the smallest and most common volcano found.  

When lava has high viscosity, it produces ash, cinders, and bombs, which all build up around the vent in a steep, cone-shaped hill or small mountain.  A composite volcano, or a stratovolcano, is a tall, cone-shaped mountain in which layers of lava alternate with layers of ash.  They typically have a large crater at the top.  A lava plateau is a high, level area that has been built up over time from lava seeping out of several cracks then traveling a distance before cooling and solidifying.  A caldera is a huge hole that is left by the collapse of a volcanic mountain.

Landforms that form from magma include volcanic necks, dikes, sills, batholiths, and dome mountains.  A volcanic neck forms when magma hardens in a volcano’s pipe.  Softer rock around the pipe wears away, which exposes hard rock.  Dikes form when magma forces itself across rock layers and hardens.  Sills form when magma squeezes between horizontal layers of rock.  Batholiths are masses of rocks formed when a large body of magma cools inside the crust.  A dome mountain forms when an uplift pushes a batholith or smaller body of hardened magma toward the surface.

When water is heated by magma, it can provide an energy source called geothermal energy, which can heat homes and make electricity.  Geothermal activity occurs when magma, a few kilometers beneath Earth’s surface, heats underground water and can form hot springs or geysers.  Hot springs form when groundwater is heated by a nearby body of magma or hot rock and eventually rises to the surface to collect in a natural pool.  A geyser is a fountain of water and steam that erupts from the ground when buildup of pressure is released.

Extrusive igneous landforms are the result of magma coming from deep within the earth to the surface, where it cools as lava. This can happen explosively or slowly, depending on the chemical composition of the lava and whether there is an easy path for it to take to the surface. If there is not a pathway, pressure builds up over time (like a shaken soda) until the magma forcibly explodes outward. Volcanic processes are constantly changing the Earth. Eruptions can create new islands, build and destroy mountains, and alter landscapes. 

The most obvious landforms created by lava are volcanoes, most commonly as cinder cones, composite volcanoes, and shield volcanoes. Eruptions also take place through other types of vents, commonly from fissures. The eruptions that created the entire ocean floor are essentially fissure eruptions.

A fissure eruption on Mauna Loa in Hawaii travels toward Mauna Kea on the Big Island.

Viscous lava flows slowly. If there is not enough magma or enough pressure to create an explosive eruption, the magma may form a lava dome. Because it is so thick, the lava does not flow far from the vent. 

Lava domes are large, round landforms created by thick lava 

A lava plateau forms when large amounts of fluid lava flow over an extensive area. When the lava solidifies, it creates a large, flat surface of igneous rock. Much larger, hundreds to thousands of square kilometer sized lava plateaus can form from a succession of fissure eruptions related to crustal rifting. In these plateaus, individual volcanic cones are sometimes present, but often not. The largest lava plateaus are on the floors of the oceans, and we cannot visit them. On continents, lava plateaus contain flood basalts that are many tens of meters thick and that extend hundreds of kilometers. Flood basalt terranes develop over long times and are products of many eruptions in the same area. The eruptions produce multiple flows that stack one on top of another.

Layer upon layer of basalt have created the Columbia Plateau

Some basaltic eruptions occur without creating a volcanic peak. Instead, lavas emanate from fissures and spread laterally. Multiple basaltic flows in the same area can produce large lava fields, sometimes called lava plains, that may extend up to several to hundreds of kilometers in long dimension. These fields, typically next to stratovolcanoes, comprise mostly horizontal flows but may include cinder cones, maars, and pyroclastic deposits. A figure shows a lava field at Hell’s Half Acre on the Snake River Plain 40 kilometers west of Idaho Falls, Idaho. This volcanic field is actually larger than a half-acre, it covers 390 square kilometers.

Hell’s Half Acre lava field in Idaho

Cooling flood basalts often shrink and develop columnar fractures, and so become columnar basalts such as the ones shown in the photograph of the Columbia River basalts, above (Figure 6.18). Erosion has uncovered the vertical columns to leave steep cliffs called palisades. Perhaps the world’s most famous columnar basalts are those of the Giant’s Causeway. 

Giant’s Causeway, Northern Ireland

The Causeway is part of a lava plateau on the north coast of Northern Ireland. The basalt is part of a large volcanic plateau, the Thulean Plateau, that formed about 60 million years ago. In this view, the tops of columns can be seen, contrasting with the side view of the Columbia River basalts shown in this figure.

Most of the lava was produced from two or more fissure vents, cracks in the ground from which lava flowed. In Saudi Arabia, the Harrat Rahat was created in 1256, a lava field that traveled almost 15 miles, stopping about 2 miles outside the holy city of Medina. Canada’s Precambrian Shield is an area of arctic northern Canada that produced a basaltic lava plateau, a lava field of 66,000 square miles, approximately 1.2 billion year ago.