北师大高中英语模块2 Unit 4 Culture Corner: North Island has a warm climate and there is quite a lot of volcanic activity.

解释：

activity既可作可数名词，也可作不可数名词。上句中activity作不可数名词，意为“[U]the condition in which things are happening or being done（事情发生或发展的）状况”。例如：

These environmental changes are the result of human activity.

这些环境的变化是人类活动的结果。《麦克米伦高阶英汉双解词典》

Vocalnic activity characterizes extrusive facies - spurt - deposit facies, which constitutes multi-rhythmic volcanic spurting.

火山活动具有由喷发相(喷溢亚相)-喷发-沉积相的特点，构成了具多韵律性的火山喷发特征。《英汉通用网络句库》

另：activity作可数名词，意为"(一般作activities)something that a person or group does or has done to achieve an aim 活动"。例如：

Will you join with us in this activity?

你要不要参加我们的活动？

We suspect he may be involved in illegal activities.

我们怀疑他可能从事非法活动。