USGS: 

In the late winter and early spring of 1980, magma started penetrating the Mount St. Helens structure. On May 18, the north flank's cryptodome (bulge) had probably reached the point of instability and was moving more quickly in the direction of failure.

A debris avalanche and a magnitude-5+ earthquake that occurred on May 18, 1980, released the cryptodome and released the pressure that had been building up at the volcano's summit. This sudden release of pressure caused the system's hot water to flash to steam, which then exploded and started a hydrothermal blast that was directed laterally through the landslide scar. The pressure on the system of magma beneath the volcano diminished because the upper part of the volcano was gone. A 9-hour Plinian eruption was sparked by a wave of decreasing pressure that traveled down the volcanic conduit to the underground magma reservoir. The magma reservoir then started to rise, create bubbles (degas), and explode violently.

Earlier Activity

The first indication of activity at Mount St. Helens was as a series of minor earthquakes on March 16, 1980. Following hundreds of further tremors, the volcano finally erupted on March 27 for the first time in more than a century. Steam explosions caused a 60 to 75 m (200 to 250 ft) broad crater to form in the volcano's top ice cap and dumped dark ash across its snow-covered southeast region.

Within a week, the crater had expanded to a diameter of roughly 400 m (1,300 ft), and two enormous crack networks had traversed the whole summit region. By the 22nd of April, when the initial phase of activity came to an end, eruptions were happening on average from approximately one every hour in March to around one every day. On May 7, minor eruptions resumed, and they continued until May 17. By that point, the volcano had been jolted by more than 10,000 earthquakes, and the north flank had expanded outward by around 140 meters (450 feet), forming a noticeable bulge. The bulge expanded outward—almost horizontally—from the beginning of the eruption at regular rates of around 2 m (6.5 ft) each day.

Strong evidence that molten rock (magma) had climbed high into the volcano was provided by the extreme deformation of the volcano. A cryptodome that had gotten within the volcano's structure but hadn't yet erupted on the surface was actually there beneath the surface bulge.

Avalanche

At 8:32 a.m., a magnitude 5.1 earthquake struck without any prior warning. on May 18, 1980, and was followed by a flurry of activities. The northern bulge and summit of the volcano were destroyed by a massive landslide that occurred simultaneously with the earthquake, making it the greatest debris avalanche in human history. Two eruption plumes, one from the summit crater rising to a height of around 200 meters (650 feet), were both modest, dark, and ash-rich.

However, the majority of the debris avalanche diverted westward and traveled up to 23 km (14 mi) along the valley of the North Fork Toutle River before forming a hummocky deposit. The debris avalanche also traveled around and up mountains to the north. The volume of the entire avalanche is approximately 2.5 km3 (3.3 billion cubic yards), or 1 million Olympic swimming pools.

Lateral Blast

Nearly 30 km (19 mi) from west to east and more than 20 km (12.5 mi) north of the previous top were completely destroyed by the lateral blast. There are almost no trees left in the inner zone, which extends about 10 km (6 mi) from the peak. This area was originally covered in lush forest. The blast's outer boundary completely charred the remaining trees, and just beyond that, all remaining standing trees were blown to the ground. A pile of hot debris delivered by the blast covered the 600 km2 (230 mi2) destroyed area.

Plinian Eruption

Pressure on the summit of Mount St. Helens' plumbing system was released as a result of the cryptodome and flank being removed from the volcano. This allowed the trapped magma to spread higher toward the vent opening by causing a depressurization wave to go down the conduit to the volcano's magma storage zone. A Plinian eruption that produced a large amount of tephra plume high into the atmosphere began less than an hour after the eruption first began due to this decrease of conduit pressure. Swift pyroclastic flows that began just after noon stretched as far north as 8 km (5 mi), at a speed of 50 to 80 miles per hour, forming the Pumice Plain.

A large eruption column, many pyroclastic flows, and ash fall downwind of the eruption were all produced during the 9-hour Plinian phase. According to scientists, the eruption peaked between three and five in the afternoon. After the Plinian phase, a new top amphitheater measuring 1.9 x 2.9 km (1.2 x 1.8 mi) in size was discovered.

Lahars

Parts of the blast cloud surged over the freshly formed crater rim and down the west, south, and east sides of the volcano during the first few minutes of this eruption. Quickly eroding and melting some of the snow and ice covering the volcano, the turbulently flowing hot rocks and gas produced surges of water that combined with loose rock debris to form lahars. A number of lahars flowed from the volcano into river valleys, uprooting trees, demolishing bridges and roads, and pulling them from their roots.

The North Fork Toutle saw the formation of the largest and most damaging lahar, which was caused by water (initially groundwater and melting glacier ice blocks) emerging from underneath the massive landslide deposit during much of the day. Material from the North Fork Toutle River's channel and landslide deposit were both eroded by this potent slurry. The lahar grew in magnitude as it moved downstream, destroying homes and bridges along the way before emptying into the Cowlitz River. Around midnight, on the Cowlitz River, 80 kilometers (50 miles) downstream from the volcano, it grew to its largest size.