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Reading: Basics-Volcanoes


The Pacific Northwest is volcanically active. Nearly every type of volcano exists or has existed in the Pacific Northwest, and every type of volcanic eruption has occurred. The Cascade volcanic arc has been catastrophically active during recent human memory. In the last 10,000 years volcanic eruptions have also occurred in such places as eastern Idaho and central Oregon. In some landscape regions of the Northwest, the entire upper crust is almost entirely volcanic rock.

To understand volcanoes, you have to start with magma, the molten rock that comes up from the interior of the earth to form volcanoes. The composition, gas content, and crystal content of the magma determines the type of eruption that occurs, the kind of volcanic rock that forms, and the type of volcano that forms.


Lava flow after lava flow of low-viscosity, mafic lava builds up a basaltic shield volcano. Shield volcanoes are the largest type of volcano on earth, as exemplified by Mauna Loa on the Island of Hawaii. Shield volcanoes in the Pacific Northwest include: Medicine Lake Volcano in northern California; Newberry Volcano (Paulina Peak) in Oregon; and Simco Volcano in south Central Washington, on the Yakima Indian Reservation near the town of Goldendale.


Eruptions of intermediate lava as andesite flows, explosive eruptions of tephra, and intrusion within earlier flows to form dikes and sills, add up to form composite cones. Composite cones are the characteristic volcanoes of volcanic arcs along subduction zones. Composite cones of the Cascadia volcanic arc in the Pacific Northwest include Lassen Peak and Mt. Shasta in northern California, Mt. McGloughlin, Mt. Mazama (Crater Lake), Mt. Thielsen, the Three Sisters, Mt. Bachelor, Mt. Washington, Mt. Jefferson and Mt. Hood in Oregon, and Mt. Adams, Mt. St. Helens, Mt. Rainier, Glacier Peak, and Mt. Baker in Washington. Most of these volcanoes are over 10,000 feet above sea level in elevation. Along with abundant andesite and tephra, composite cones may also erupt lesser amounts of basalt, dacite and rhyolite. That is why they are called composite cones–they are a composite of many types of volcanic rock.


Fountain eruptions of volcanic ash, lapilli, blocks, and bombs, usually of mafic composition, pile up in a cone-shaped form to become a cinder cone. Cinder cones are usually less than 1,000 feet tall. Many composite cones and shield volcanoes have cinder cones on their flanks or close by. Cinder cones are small bumps compared to the much larger shield volcanoes or composite cones.


Lava domes are steep-sided domes formed by the pile-up of viscous, felsic magma. It is not uncommon for a composite cone to erupt explosively, forming a crater, and then to erupt high-viscosity lava into the crater, forming a lava dome. The lava that forms the dome may be from the same magma that erupted explosively but, after venting most of its gas in the explosive eruption, it has lost its explosiveness and erupts as flows.


Spatter cones are small volcanic landforms that from when lava spatters up in a small fountain and piles up in blobs of semi-molten material that accumulate into a solid, roughly cone-shaped form a few feet or a few tens of feet tall. A lava flow coming down the flank of a shield volcano or composite cone may spawn a subsidiary spatter cone by bursting out of a solidified crust.


Flood basalts usually originate from fissure eruptions, cracks in the surface of the earth that rapidly disgorge huge volumes of lava. Flood basalts are high-volume, widespread flows of mafic magma that solidify into basalt flows that in some cases cover thousands of square miles. The Columbia River Basalts comprise one of the world’s great flood basalt provinces. Some of the Columbia River basalt flows originated near the border of Idaho, covered much of eastern Washington and north central Oregon, and flowed down through the Columbia River gorge beyond the Cascade Range to cover parts of the Portland area and reach the Pacific Ocean, all in gigantic individual flows.


Ash flow tuffs are large-volume tuffs that originate from huge, explosive eruptions of felsic magma. They are sometimes referred to as ignimbrites. Besides volcanic ash, ash flow tuffs may contain pieces of pumice, pieces of broken rock that were caught up in the eruption, and crystals that had already solidified in the magma before it erupted. All of these materials become welded together in the solidified ash flow tuff. Some ash flow tuffs cover hundreds or, in rare cases, thousands of square miles. The yellow rock that gives Yellowstone National Park its name is an ash flow tuff. The explosive eruptions that form ash flow tuffs usually create a caldera, as the volcano collapses on itself after venting tephra across the landscape.


Pillow basalts form when mafic lava erupts underwater. The lava freezes on the outside against the cold water, but keeps flowing on the inside, bulging out into pillow-like shapes which break off from the main flow. These pillows can accumulate into a large volume of pillow basalts. Pillow basalts serve as an indicator that lava was erupted into a body of water. Much of the world’s oceanic crust consists of basalt, most of which erupted onto the seafloor to form pillow basalts. Pillow basalt can also form when mafic lava erupts on land and flows into a lake or into the ocean. In some places, there are layers of pillow basalt in the Columbia River Basalts, showing that the basalt flowed into lakes, forming pillows as it entered the water.


Tephra is also known as pyroclastic material—volcanic material that erupted explosively and flew through the air for some distance before landing on the ground. Tephra results from pyroclastic eruptions, which means explosive volcanic eruptions that hurl material into the air. There are several types of pyroclastic material, or tephra:

Volcanic Blocks

Blocks are large (>5 cm) pieces of tephra that were solid during the eruption. They tend to have blocky or jagged shapes.

Volcanic Bombs

Volcanic bombs are large (>5 cm) pieces of tephra that landed while still molten or semi-molten. They tend to have shapes that were streamlined while passing through the air, or else have a sort of splattered shape from hitting the ground while soft.


Lapilli are pieces of tephra between 0.5 and 5 cm across.

Volcanic Ash

Volcanic ash is fine-grained (<0.5 cm) tephra. It mainly consists of fine pieces of volcanic glass, which form by solidification and splintering of small pieces of the magma during the eruption. It may also contain pieces of crystals, from minerals that had already solidified in the magma before it erupted.


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