Volcanic eruptions are linked to the slow grinding of the continent-sized plates drifting across the surface of our planet which float on top of a softer underlying mantle. From the Pacific coast of Asia to the Pacific coasts of North and South America, these oceanic plates have been colliding with the continental plates, causing the ocean floor to be subducted into the mantle beneath the continental plate. Subduction is the slow process in which the heavier oceanic plate collides with the lighter continental plate and is forced down into the superheated magma in the Earth’s mantle far below the surface. The high pressures and temperatures of the mantle along with the heat produced by friction cause this subducted sea floor to release water that has been chemically bound in the rocks. This heat and water lead to melting of the rocks in the mantle and continental plates, eventually causing huge, unstable pools of magma that lie miles underground and inland from the coast. This magma is the source of the volcanoes that lie along the entire rim of the Pacific Basin which are known as the "Pacific Ring of Fire." 

Lassen Peak first erupted about 20,000 to 30,000 years ago and began a pattern of volcanic activity for the Lassen area. J. S. Diller studied the Lassen area for the U.S. Geological Survey in 1895. He found a depression between the Sierra Nevada and the Klamath Mountains which he called the Lassen Strait where the Lassen volcanoes are now located. Diller believed that this strait had once been a seaway connecting the Pacific Ocean with a great inland sea which lay to the northeast. The Klamath Mountains and the Sierra Nevada were one continuous mountain range before the appearance of this depression or strait. A combination of rifting, faulting, and rotation caused the Klamath Mountains to move northwest, away from the Sierra. This also caused the rock layers to deteriorate while creating a broad depression. About 3.5 million years ago great lava flows began to fill in the depression and began the volcanic activity of the Lassen region which continues to this day.

There are three major volcanic centers in and near Lassen Volcanic National Park, each of which began with violent explosions and thin lavas that created  a large composite volcano. Thicker lavas pushed up forming domes along the sides of the main volcanic peak. The Dittmar Volcanic Center, the oldest of the three centers, is located in Warner Valley in the southeast corner of the park. It was active about two million years ago and was a large composite volcano similar to Mt. Shasta. Warner Valley, which supports a mature forest now, is the eroded center of the ancient volcano named Mount Dittmar. As its eruptions stopped, erosion began wearing down the cone until only the peaks and cliffs surrounding the Warner Valley remain.

 Mount Maidu occupied the area where the town of Mineral and Battle Creek Meadows now lie just south of Lassen Park. Following a similar pattern of development and erosion as did Mount Dittmar, only the ridges now surrounding Mineral remain as remnants of the once great volcano. Mount Maidu was active from about two million to 800,000 years ago and is now only a hollowed-out core with meadows and open areas due to erosion of the immense volcanic core.

A new center arose about 600,000 years ago creating a large composite volcano called Mount Tehama, now known as the Lassen Volcanic Center concentrated around Brokeoff Mountain in the southwest corner of Lassen Park. Mount Tehama towered 11,500 feet in elevation and measured 11 miles across at the base. About 400,000 years ago, volcanic activity of Mount Tehama declined causing domes of thick, pasty lava to erupt from new vents at its sides. Lassen Peak, which geologists think is between 15,000 and 25,000 years old, was the largest of one of these later domes that pushed up around the base of Mount Tehama. Lassen Peak has had repeated eruptions over thousands of years and is possibly the largest dome volcano in the world.

As these other domes grew around Mount Tehama, the great volcano began to deteriorate. The active hydrothermal systems weakened the rocks in the central core and along with erosion from wind, running water, and the grinding effect of glaciers during the ice ages, caused the central cone to be destroyed. Diamond Peak occupies the central core area with Brokeoff Mountain, Mount Diller, Pilot Pinnacle, and Mount Conrad the last remnants of the once mighty Mount Tehama volcanic peak. The Sulphur Works still continue to release hot steam, boiling water and mud and are probably where the central vent of Mount Tehama once existed.

Lassen Peak is a lava dome or plug dome volcano that formed on the flank of its ancestral Mt. Tehama. This means that its lava flow is quite thick and pasty and therefore, does not flow very far but bulges up from the vent. This thick and sticky dacite lava solidifies as it cools and becomes a "plug" in the volcanic vent causing expansion mostly from within and below its surface. Lava domes are usually small in size, normally less than 2,000 ft. high, making Lassen Peak quite possibly the world’s largest plug dome at 10,457 feet in elevation. Its magma is high in silica which creates a more acidic and cooler lava flow combined with explosions of pent-up gases. The past eruptions of Lassen Peak which included slowly oozing lava as well as pyroclastic explosions of gas, steam, ashes, and huge boulders, are a good example of the characteristic eruptions of a lava dome type of volcano.