The term geological hazard (geohazard) principally covers earthquakes and volcanic activity, as well as the tsunami and landslides they can generate. Pacific island countries and territories are all highly vulnerable to geological hazards because they are located within the Pacific Ring of Fire, home to about two-thirds of all the active or dormant volcanoes on Earth.

In January 2022, a volcanic eruption at Hunga Tonga–Hunga Ha'apai generated a tsunami that affected many Pacific islands, especially Tonga. In 2016, a magnitude 7.9 earthquake hit Papua New Guinea's Highlands region, causing landslides.

How and why do these geohazards occur? The seven large and numerous small plates which make up the Earth’s surface fit together like the pieces of a jigsaw puzzle – a puzzle with moving parts. The plates move because each of them rests on hot liquid rock (magma). The heat and pressure within the Earth cause this liquid magma to flow which, in turn, causes the plates to move a few centimetres each year at about the speed at which your fingernails grow.

Inevitably, these plates end up colliding, sliding underneath one another or pulling apart. This friction causes pressure to build up. When the pressure becomes too strong, it is released – a bit like a pressure cooker letting off steam – and can generate an earthquake.

When the epicentre of the earthquake is on the seabed, the release of this pressure can displace large quantities of seawater, generating a tsunami wave on the ocean surface which can rise more than 30 m in the air as it reaches shallow waters. The 9.2 magnitude earthquake which struck Indonesia in the Indian Ocean in December 2004 created the most lethal tsunami in recorded history; it lifted the sea floor by 6 m over 1,000 km and a width of 130 km in just eight minutes, displacing trillions of litres of water and inundating coastlines thousands of kilometres from the epicentre. More than 160,000 people died from the earthquake and tsunami in Indonesia but the tsunami also killed thousands in countries as far away as India, the Maldives, Sri Lanka and Thailand.

About 70% of the major tsunami that occur in the Pacific Ocean are triggered by earthquakes and 15% of these earthquakes originate from the Southwest Pacific region.

A volcanic eruption occurs when magma escapes to the Earth’s surface to emerge as lava. This typically happens at or near plate boundaries. Pacific island countries such as New Zealand, Papua New Guinea, Samoa, Solomon Islands, Tonga and Vanuatu were all created long ago by volcanic eruptions. These volcanic islands are made of rock (minerals), whereas atolls are made up of biological material that has accumulated from the surrounding coral reefs, including shelled animals and corals –,  which are themselves animals.

There are two types of volcanic eruption: effusive – meaning that it generates lava flows – or explosive. In an explosive eruption, a lot of gas is trapped within the magma. Pressure builds until the magma eventually erupts explosively out of the volcano crater, projecting ash and rock into the air. Imagine shaking a fizzy drink then removing the bottle cap to picture an explosive eruption.

Mount Manaro in Vanuatu has been described as one of the world’s ten most dangerous volcanoes. One theory posits that, in a large eruption, the water in Mount Manaro’s crater lakes could become superheated steam and cause a massive explosion, ejecting water, ash, rock and magma. The resulting landslides down the island’s steep slopes could generate a tsunami that would affect Vanuatu’s northern archipelago and beyond.

Explosive eruptions can be accompanied by emissions of carbon dioxide or hydrogen sulfide. These gases can suffocate people at sufficient concentrations, as in Java in 1979 when an eruption killed 149 people, most of whom were overcome by poisonous gases.

The eruption of the Manam Volcano in Papua New Guinea in 2014–2015 caused fast-moving flows of hot gas, rocks and boulders that disrupted air travel. These debris flows, known as ‘pyroclastic flows,’ can travel at speeds of more than 700 km per hour, burning everything in their path and suffocating people.

Earthquakes, volcanic eruptions and tsunami are all tied to the movement of the Earth’s plates, so these phenomena are distinct from climate-related hazards which are caused by either by too much water (such as cyclones) or too little (drought).

However, climate change can affect the soil and bedrock, which are mineral-based and, thus, geological. Sea-level rise is causing coastal erosion, for instance, and heavy rains can cause landslides and, over time, sinkholes by dissolving rocks underground to form unstable caves that end up collapsing.