What feature forms over the vent of a volcano when very thick lava builds up?

lava dome

Explanation:

The domes are accumulations of very viscous lava that is located on the emission ducts of the volcanoes. The domes, depending on the characteristics of magma, have dimensions and characteristic evolution. They are arranged in the form of a dome that increases in size as the internal pressure of the gases increases. This pressure increase can end in an explosive event of greater or lesser intensity that results in the formation of explosion gaps that are dispersed at different distances from the crater. The collapse and explosion of the domes can also generate pyroclastic flows. The location of the domes entails the development of seismic activity.

b - the respond is innate and varies over time.

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the answer for this question would be d. glaciers