Rangitoto Island's shield shape is not directly caused by basalt magma alone. While basalt is the primary magma type responsible for the island's formation, it's the eruption style and magma viscosity that play the key roles in creating the characteristic shield shape.

Here's a breakdown:

* Basalt Magma: Basalt magma is relatively fluid (low viscosity) due to its low silica content. This allows it to flow easily and spread over large areas.

* Eruption Style: Rangitoto's eruptions were predominantly effusive, meaning the magma flowed out of the vent as lava rather than exploding violently. This continuous flow built up the island's broad, gently sloping flanks.

* Shield Shape: The combination of fluid basalt magma and effusive eruptions results in the formation of a shield volcano. These volcanoes are known for their broad, low-angle slopes, resembling a warrior's shield lying on the ground. The low viscosity of the magma allows it to spread far from the vent, creating a wide base and gentle slopes.

In summary, it's not just the basalt magma but also the effusive eruption style and the magma's low viscosity that together contribute to the shield shape of Rangitoto Island.