You're probably thinking of sulfur (or sulphur)
SULFUR
Water vapor and carbon dioxide. Water vapor is the most abundant.
When magma is formed, it is composed of water vapor, carbon dioxide, and sulfur dioxide. The most abundant gas in magma is water vapor.
The materials associated with volcanic eruption are Lava flows such pahoehoe lava and Aa lava. Gases that contain mainly water vapor and carbon dioxide. Pyroclastic materials such fire fragments, ash, dust, pumice, lapilli, cinders, harder lava and hot lava as bombs.
The most likely combination to result in a violent volcanic eruption is the presence of highly viscous magma (such as rhyolitic or andesitic magma) and a large amount of dissolved gas (particularly water vapor and carbon dioxide). These conditions can lead to a buildup of pressure within the magma chamber, causing a violent and explosive eruption. Other factors such as the presence of a vent or conduit for the magma to reach the surface and the structural weakness or faults in the volcano can also contribute to the severity of the eruption.
SULFUR
The more gases, water vapor and silica there is in magma the bigger the explosiveness of a volcanic eruption.
The most common volcanic gas released in an eruption is water vapor. Water vapor is followed by carbon dioxide and sulfur gas.
Water vapor and carbon dioxide. Water vapor is the most abundant.
Emission of water vapor and other greenhouse gasses!
When magma is formed, it is composed of water vapor, carbon dioxide, and sulfur dioxide. The most abundant gas in magma is water vapor.
The materials associated with volcanic eruption are Lava flows such pahoehoe lava and Aa lava. Gases that contain mainly water vapor and carbon dioxide. Pyroclastic materials such fire fragments, ash, dust, pumice, lapilli, cinders, harder lava and hot lava as bombs.
The most likely combination to result in a violent volcanic eruption is the presence of highly viscous magma (such as rhyolitic or andesitic magma) and a large amount of dissolved gas (particularly water vapor and carbon dioxide). These conditions can lead to a buildup of pressure within the magma chamber, causing a violent and explosive eruption. Other factors such as the presence of a vent or conduit for the magma to reach the surface and the structural weakness or faults in the volcano can also contribute to the severity of the eruption.
What produced by volcanic outgasing is carbon dioxide and water vapor. The water vapor condensed to form part of Earth's oceans as the surface cooled.
water vapor is a compound.
Hydrogen is a minor constituent of volcanic gas. This will burn in air to form water vapor. However, volcanic gas also includes water vapor anyway.
Water vapor and silica