Which type of underground magma formation is usually dome shaped?

Lave-dome in underground magma formation is usually in dome shape.

A lava dome is a type of crater that is circular and forms when eruption of magma is slow later forming a pile around the vent. The volcanic eruption of this type of lava doesn't flow a great distance due to its thickness and stickiness but it forms a pile.  

The thickness and slow movement are caused by the high silica content or by degassing of the fluid magma. Lava domes volcanoes are typically found on the convergent boundaries. Rocks formed from the eruptions include basalt, dacite, rhyolite, etc.

i'm sure the answer you are looking for would be

a. an ice age

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