I have tutored multiple lower-division Earth Science classes in the past two years, including Oceanography, Geology of National Parks, and Earth Catastrophes. Though these classes have different topics, they have very similar basic concepts and problems that students run into in the course of the class. Because of this, I'll talk about these classes for the most part as a group.
I had the advantage of a broad background in earth-science-related topics from being involved in 4-H and school projects before college, so tutoring these classes was easy for me. I had a good enough grasp of the concepts that I could bring up a real-world example for almost anything we talked about – which sometimes led to tangents about glaciers or the 1906 San Francisco earthquake or ocean currents in discussions of unrelated topics. However, I don't think this is a bad thing – I've had students tell me my absurd enthusiasm for these subjects makes the concepts more interesting for them.
Something that I keep running into is the fact that many students have no background in science, and therefore have no basis of understanding for concepts like plate tectonics, volcanism, and geologic time – which are inherent to Earth Science classes. We often revisit these concepts throughout the quarter as the students gain a better grasp of the ideas, and can look at them in a more complex sense. Earth Science classes are fantastic in that the concepts nearly always have visual elements to them – usually impressive ones, too. This quarter, I spent twenty minutes in one session watching videos of Japanese tsunamis with two students so they could see how the water acted. Hollywood movies had given them the idea that tsunamis are giant cresting waves, while I wanted to show them that tsunamis are more like massive walls of water that act like floods on steroids. In this case, YouTube helped. Don't hesitate to seize opportunities like this – if they want to learn about something that is related to what the professor is discussing, which will ultimately help their understanding of the class, then who are we to stand in their way? Be flexible, and your students will surprise you.
I found that having my notes from when I took these classes was invaluable – if the class is being taught by the same professor, 95% of it will be the same – and even if it's being taught by a different professor, it will still be very similar. Also, if you still have the class textbook, bring it along. You being prepared usually gives the students more incentive to be prepared themselves – plus the book can be helpful if you need visuals or the exact definition of something from the glossary.
I have found that allowing the students to drive the course of the sessions gets better results and gives the students more power over their education. I'll come to the session with a few concepts I want to cover, but if the student feels their time would be better spent discussing different topics covered by the professor in the past week, I'll drop my schedule and go with theirs – as long as they understand the main concepts I would have talked about. For the most part, my sessions are question-and-discussion. The students ask questions, then discuss amongst themselves and/or with me (if it's a single session) what the answers might be, and why. The “why” is very important in Earth Science classes, because many “whys” are related. Earthquakes and volcanoes are both tied to plate tectonics, tsunamis and landslides are both tied to earthquakes, and so on – and if the students understand the “whys” of one concept, that gives them a sounding board for the “whys” of another concept.
When engaging in such discussions, I find it helps to ask “does that make sense?” instead of the more accusatory “do you get it?” or “do you understand this?” Not saying “you” makes the question less aggressive, and makes it lower-risk for the student to answer “no” to, because it doesn't put the blame on them. It just means they need a different explanation.
Last but not least, professors are your resources as much as they are the students'. Contact the professors (though Earth Science professors I've contacted are often slow with emails) and see if they have review sheets or practice tests you can use in your sessions. If not, don't hesitate to make your own, if the students want. Remember, we are here for our tutees, so don't hesitate to ask how their weekend was, or how their day is going. It will remind them we're students, too, and we know what the class was like.