Haleakala, also known as the East Maui Volcano, is a massive, dormant volcano that forms over 75% of the island of Maui. Peaking at 10,023 feet above sea level, its formation involved a complex, multi-stage geological history stretching back millions of years. This process included sustained volcanic growth, a prolonged period of deep erosion, and a final phase of smaller, localized eruptions.
The Foundation: Shield Volcano Growth
The initial and most voluminous phase of Haleakala’s life began approximately two million years ago. This occurred as the Pacific tectonic plate moved over the stationary Hawaiian Hotspot, which provided the magma necessary to build the volcano from the seafloor. Haleakala is a classic shield volcano, characterized by broad, gently sloping flanks built by thousands of thin, highly fluid lava flows.
These flows consisted of low-viscosity basaltic lava that spread out widely before solidifying. This gave the volcano its immense size rather than a steep profile. The shield-building stage was the most productive period, during which the volcano grew to an estimated height of 12,000 to 15,000 feet above sea level. This structure eventually transitioned into a less active post-shield phase as its connection to the hotspot lessened.
The Great Depression: Erosion and Valley Formation
After the primary shield-building phase ended, the volcano entered a long period of quiescence, allowing erosion to take hold. Rainfall, driven by trade winds, began carving deep valleys into the mountain’s flanks through headward erosion. The northern and southern slopes were deeply incised by the Koʻolau and Kaupō stream systems, respectively.
These streams relentlessly eroded the rock, extending their valleys toward the summit. Over tens of thousands of years, the ravines of the Koʻolau and Kaupō valleys eventually met and merged at the summit. This coalescence of massive, water-carved valleys created the spectacular, bowl-shaped feature seen today. This depression, which is about 11.25 miles long and 3.2 miles wide, is often mistakenly called a crater. It is not a true volcanic caldera formed by collapse, but rather a colossal erosional feature that exposes the internal structure of the shield volcano.
The Final Stages: Post-Erosional Activity
Long after the main shield was built and the great valleys were carved, the volcano experienced a final, localized phase of volcanism known as the post-erosional or rejuvenated stage. This activity occurred hundreds of thousands of years after the main shield-building eruptions had stopped. These later eruptions were smaller and focused along the volcano’s two main rift zones, which pass through the erosional depression.
The lavas from this stage were chemically different, often more viscous and alkalic than the original shield-building basalt. This change in magma composition, coupled with a higher gas content, led to explosive eruptions. These eruptions built numerous cinder cones, such as Puʻu ʻUlaʻula, visible on the floor of the summit depression. Their lava flows partially filled the massive erosional valley. Haleakala is classified as dormant, not extinct, because these post-erosional eruptions occurred relatively recently. The most recent eruption is estimated to have taken place between 1480 and 1600 AD, confirming the volcano’s potential for future activity.