核心问题
Why do earthquakes happen? Scientists explain that the outside of the earth is made of a number of different plates. [《高中英语教科书》第一册(上), 人教社, p.2]
有一本教学辅导材料将上面的 explain 改为 explained。我认为这种修改是正确的，因为科学家是在过去作的解释呀。
词典解答
这种修改是错误的,见 simple present tense 。
[附注]上句中的 made of 应改为 made up of, 见 make of, make up of。