Half Dome, the iconic massive monolith towering over Yosemite Valley, is a striking example of geological history. This massive rock formation is primarily composed of granodiorite, an intrusive igneous rock. Identifying this rock type is the first step in understanding the monument’s deep origins and its distinctive, half-domed shape. Its presence is a testament to the immense forces of plate tectonics and the slow, powerful work of erosion that sculpted the Sierra Nevada landscape over millions of years.
The Core Composition
The rock making up Half Dome is specifically known as the Half Dome Granodiorite. This intrusive igneous rock is defined by its mineral composition, placing it on a spectrum between true granite and diorite. It is characterized by a high content of quartz, which is the glassy, clear-to-gray mineral visible within the rock structure.
The distinguishing feature of granodiorite is the ratio of its feldspar minerals. Granodiorite is richer in plagioclase feldspar than true granite, giving it a slightly darker appearance. Other prominent minerals include the dark mica biotite and the dark amphibole hornblende, which form visible, interlocking crystals. This relatively strong composition allowed the rock to resist erosion, leaving it exposed as a massive dome when surrounding, weaker rocks wore away.
Deep Origins of the Rock
The material that forms Half Dome originated as molten rock, or magma, deep beneath the Earth’s surface approximately 87 million years ago during the Mesozoic Era. This process, known as plutonism, involved magma rising from a subduction zone where the ancient Farallon Plate slid beneath the North American Plate. The magma stalled and pooled several miles below the surface, never erupting to form volcanoes.
The immense pressure of the overlying crust caused the molten rock to cool extremely slowly over millions of years. This slow cooling rate is responsible for the coarse-grained, crystalline texture characteristic of granodiorite. The interlocking crystals of quartz, feldspar, and mica had ample time to grow large, forming the durable rock of the Sierra Nevada Batholith. The Half Dome Granodiorite intrusion is considered one of the youngest in the Yosemite Valley area.
Sculpting the Monolith
Once solidified deep underground, the granodiorite began its journey to the surface through regional uplift. Over tens of millions of years, the entire Sierra Nevada block was slowly pushed upward, starting around 10 million years ago, while erosion removed the overlying rock layers. As the burden of rock was removed, the pressure on the deep granodiorite was released, allowing the rock to expand slightly.
This pressure release caused the outer layers of the rock to fracture in curved, parallel sheets, a weathering process called exfoliation. Exfoliation created the smooth, rounded, dome-like profile on three sides of Half Dome, peeling away rock like the layers of an onion. This process continues today, occasionally resulting in large rockfalls from the steep faces of the dome.
The final shaping of the monolith occurred during the Ice Ages, when massive glaciers flowed through the region. Glaciers repeatedly carved out the U-shaped valleys of Yosemite, deepening and widening Tenaya Canyon, which runs alongside Half Dome. The ice exploited a major, pre-existing vertical fracture, or joint, in the granodiorite on the northwest face. Glacial shearing off the side along this weakness created the iconic, sheer vertical face that distinguishes Half Dome from a fully rounded dome.