Volcanoes are dramatic evidence of Earth’s unsettled nature and are not randomly distributed over the Earth’s surface.  Most are concentrated on the edges of continents, along island chains or beneath the sea. Seventy-five percent of the world’s active and dormant volcanoes on land form a ring around the Pacific Ocean basin called the Ring of Fire. Frequent volcanic eruptions and earthquakes occur along its boundaries.  Earth’s crust is made up of large, irregularly-shaped slabs of solid rock that float on slow-moving viscous rock. Where these tectonic plates converge, they collide with each other or are forced under other plates. When one plate dives beneath another, the process is known as subduction. At plate boundaries along the coast of Northern California, the Juan de Fuca Plate is spreading away from the Pacific Plate and submerging beneath the North American Plate. The Cascade Range volcanoes were formed by subduction, including Lassen Peak, the southernmost active volcano in the chain.

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ERUPTION

The Peak Awakens
On May 30, 1914 Lassen Peak awoke from a 27,000 year long sleep when it was shaken by a steam explosion. Over the next year, more than 180 steam explosions blasted a 1000-foot (305 m) wide crater at the summit. 
 
Blast from the Past
On May 14, 1915 the character of the eruption changed. Incandescent blocks of lava could be seen cascading down the flanks of Lassen Peak from 20 miles (32 km) away, indicating that a lava dome had filled the volcano’s crater.

 On May 19, a large steam explosion shattered the crater’s central dome. Glowing blocks of lava fell on the snow-covered peak launching a half-mile wide avalanche of snow and rocks that raced down the volcano’s steep sides. Lava melted the snow and formed a lahar, a mudflow of volcanic rock, that leveled everything in its path. Lava spilled from the crater and poured 1000 feet (305 m) down the mountain.

 On May 22, after 2 quiet days, Lassen Peak exploded again in a powerful eruption that blasted rock and pumice high into the air. A huge column of ash and gas rose more than 30,000 feet (9,144 m) into the air and was visible from 150 miles (241 km) away.

Collapse of this column of ash and gas onto Lassen’s slopes generated a high-speed avalanche of hot ash, pumice, rock fragments and gas, called a pyroclastic flow that devastated an area of 3 square miles (8 km). Another mudflow followed, rushing down the slopes for nearly 12 miles (19 km).

 This eruption deposited pumice and volcanic ash 25 miles (40 km) away and rained fine ash over an area up to 200 miles (322 km) northeast of Lassen Peak. 

Coincidence or Correlation?
Both Lassen Peak and Mount St. Helens erupted in May during spring-time snowmelt.  Research suggests that melting snow may provide the added fuel for an eruption already primed to happen. Water from melting snow seeps into cracks at the surface and is heated by an already volcanically active system. 

The water provides a mechanism for transferring heat from one place to another and can trigger a phreatic, or steam eruption.

Flying Objects
Rock fragments erupted into the air by volcanoes are classified according to size, shape, composition and the way in which they form and travel. Lava may explode out of the volcano as blocks of rock, pellets and ash.

This material is called pyroclastic rock and may fall nearby, forming part of the volcano, or may be spread over great distances.

Bombs are larger lava fragments that ejected while partially molten and solidify during flight. Many bombs develop rounded shapes as they fly through the air, and others fly through the air as ribbons, folding into contorted shapes as they hit the ground. Bombs are named according to shape, such as ribbon, spindle, cow-dung and round bombs.

Scoria is a glassy volcanic rock usually composed of basalt or andesite. Its bubbly texture is caused by the escape of volcanic gases. Scoria is high in iron and may have an iridescent blue color. Oxidation may lead to a deep reddish-brown color.

Volcanic ash are tiny pieces of shattered rock formed during explosive eruptions and are no bigger than the head of a pin, less than 1/1,000th of an inch (.025 mm) in diameter. Ash consists of rock, minerals and volcanic glass fragments that are extremely abrasive.

FOUR TYPES of VOLCANOES

A Volcanic Laboratory
Lassen Volcanic National Park preserves within its boundaries all 4 types of volcanoes.
Volcanoes in subduction zones, which form the Cascade Range, are often characterized by explosive eruptions of ash, pumice, cinders and molten lava bombs.

Lassen Peak
Plug dome volcanoes are formed by lava that is too thick to flow any great distance. As a result, the lava piles up around the vent to form a plug. Domes sometimes form within craters or along flanks of larger composite volcanoes. Typically, lava domes are relatively small and are active for only a short time, usually less than 20 years.

Cinder Cone
These are the simplest type of volcano and only erupt once. Small particles of lava blown from the volcano fall around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and are rarely higher than 1000 feet (305 m) above the surrounding landscape.

Brokeoff Mountain
Composite volcanoes are steep-sided symmetrical cones built from alternating layers of lava flows, volcanic ash, cinders, lava blocks and bombs. They can rise over 10,000 feet (3048 m) above their base and have a central lava vent at the summit. Composite volcanoes can be active for half a million to a million years. Brokeoff Mountain is a remnant of Brokeoff Volcano, a larger, but now degraded, composite volcano.

Prospect Peak
Shield volcanoes are formed slowly from the build-up of many basalt lava flows that spread thinly over a great distance. Flow after flow pours from a vent building a broad, gently-sloping symmetrical cone, shaped like a shield. They are among the largest volcanoes in the world and include Mauna Loa in Hawaii. The planet Mars has a shield volcano 100 miles (161 km) across.

FOUR TYPES OF LAVA

Four types of lava are produced by the volcanoes at Lassen Volcanic National Park and are classified by their mineral content.

Magma is molten rock found deep below Earth’s surface. Igneous rocks are formed when molten rock cools and crystallizes. This can happen slowly, deep in the Earth, or quickly, at the surface, in a volcanic eruption.     

Plutonic, or intrusive igneous rocks, like granite, form when magma solidifies deep in the Earth’s crust. It cools slowly and crystals have time to grow to large size, from several millimeters to a centimeter (1/8 – 2/5”).

Lava is extrusive igneous rock, or volcanic rock that forms when magma is erupted onto the Earth’s surface.  Lava cools and solidifies rapidly, causing small mineral grains to form quickly. The composition, mode of eruption and cooling rate of the magma all determine what type of rock is formed.  

Basalt is a dense volcanic rock and contains more iron and magnesium than other lavas. Basaltic lava has a low melting point, stays fluid longer than other types and flows rapidly and long distances. It is the major rock type in shield volcanoes. Eruption temperatures are 2012 to 2282 °F (1100-1250 °C). 

Andesite commonly erupts from stratovolcanoes. It is thicker than basalt and contains more silica. Andesite can flow far down steep slopes, but slows as the ground levels. Since it tends to block vents, andesitic magma can generate enormous explosive eruptions. Eruption temperatures are 1652 to 2012 °F (900-1100 °C).

Dacite is between andesite and rhyolite in composition, and is usually light colored, but it can also be dark gray to black or red. The principle minerals are feldspars, quartz, pyroxenes and hornblende. Eruption temperatures are 1472 to 1742 °F (800-950 °C).

Rhyolite is high in silica and low in iron and magnesium, rhyolite is similar to granite in chemical composition. It’s texture is different from granite because rhyolite cools rapidly and the minerals, which form quickly, are small. Eruption temperatures are 1292 to 1562 °F (700-850 °C).

GEOTHERMAL
A Hint of What is Below
Lassen Volcanic National Park contains an extensive network of bubbling mudpots, boiling hot springs, steaming fumaroles and the smell of sulfur-rich gases. These hydrothermal activities are hints of active volcanism. They serve as a reminder of the potential for future eruptions in the Lassen area.

 All of these features are driven by steam, generated from an underground reservoir of boiling water. Cold surface water percolates deep into the ground, where it is heated by a body of magma or hot solid rock and is converted to steam.

As the steam rises, it causes ground water near the surface to boil. Hot springs form where steam-heated ground water accumulates at the surface. 

When steam comes directly to the surface without coming into contact with ground water it creates fumeroles. Mudpots form where water combines with ash or soil. The bubbles form when gas comes up through a vent.

Some Like it Hot
It’s hard to believe that anything could survive in hot springs or fumeroles. However, single-celled micro-organisms called Sulfolobus, that belong to a newly discovered life form called Archaea, live deep in hydrothermal systems. Studies show that microbes similar to Archaea may have spawned life on Earth 3.5 billion years ago in undersea hotsprings.

Sulfolobus survives in hot, acidic water and can easily withstand temperatures of 175° F (79.4 °C). They do not depend on sunlight or other organisms for food and metabolize sulfur compounds instead.

Fire and Brimstone
Sulfur often forms around the rims of volcanic vents where it is deposited by gases rising from below the surface.  When sulfur gases from the magma combine with hydrogen, they produce a toxic hydrogen sulfide gas that smells like rotten eggs.

Bumpass Hell, Lassen Volcanic National Park
This is the largest hydrothermal area in the park and marks the main area of upflow and steam discharge. While most geologic processes are slow, conditions in an area
like Bumpass Hell are dynamic and constantly changing.