The material included below is an example of an actual journal entry submitted by a student in my Spring, 1999 Honors Earth Science class. I hope it gives you some idea of what I have in mind for your journals.

Note: the comments [in brackets] have been added by me, your tireless instructor, to make sure that you don't acquire this student's misconceptions.

Entry day/date: Journal entry for Wednesday, January 27, 1999 lecture

Textbook readings: Chpt. 7, p. 229-235; Chpt. 9, p. 268-171 (short assignment!)

Key concepts: We discussed the concepts of subduction, accretion and obduction and how they help to construct continents as well as volcanic island arcs. These processes are responsible, acting over huge amounts of time, for building up the first continents. Partial melting, pressure relief melting, rock weathering and plate subduction together produced the felsic rocks of continents from the ultramafic rocks of the mantle!

"Feelings": The most interesting part of lecture for me was the talk of the trench on the eastern side of the Lesser Antilles where subduction is taking place.

Insights: For one thing, I did not realize that subduction could be influenced by deposition of sediment from rivers on the continents. I knew that sediments went into trenches, but I hadn't realized that accretionary wedges form because sediment is scraped off of the subducting plate. However, I guess I just hadn't thought about where the sediment came from and how much of it there might be. I guess I imagined it coming from dead sea animals like shells at the beach. I remember that sediment is carried to these plate boundaries, but I had not put two and two together to figure that the sediment was carried by rivers. Maybe it was because when you started talking about the trench, I couldn't see how a river could affect a trench hundreds of miles beyond the continental margin.

Secondly, I had not realized that plates, while being subducted, as well as sediment on the plates, could be pushed onto the overriding plate [obduction] and not just drawn under it. Why does this only occur at volcanic mountain arcs and volcanic island arcs? Why can't oceanic-oceanic subduction include obduction or accretion? [it does -- volcanic island arcs form at O-O margins and accretion & obduction do occur there]

Or can oceanic-oceanic subduction zones have accretion and just not obduction? [all three can occur] Which one is occuring at the trench by the Lesser Antilles? [primarily subduction and accretion; in the geologic future, obduction may occur as well]

Other stuff: Thanks for going over the map of the Caribbean in more detail in your office. It helped me to see and understand what you see and understand.

Geo-topic: [as you can see, this student chose a challenging article and wrote quite a bit about it]

Hirose, K., Yingwei F., Yanzhang M. and Ho-Kwang M., 1999, The fate of subducted basaltic crust in the Earth's lower mantle: Nature, vol. 397, p. 53-56.

Comment #1 [this student included this on her own, it was unsolicited by me]: How excited I was to find this article, considering it is exactly what we have been talking about in class! And the article makes sense to me (mostly). I have read through it a number of times and also using my notes from class and tried to write notes about it and define words that I didn't know. Let's just hope I can summarize it as well as I think I understand it ("think" being the key word here-- I may be completely off-base) . . . .

Basaltic lithospheric crust from the mid-ocean ridge is supposed to have been subducted and descended to the lower mantle where it would then remelt and ascend to the crust again as ultramafic rock. While not all basaltic crust that is subducted descends to the lower mantle after being transformed to perovskite, the basalt crust that does descend experiences higher pressure and extreme temperatures that melt the contents of the mid-oceanic ridge basalt (MORB).

The chemical composition of the basalt determines which compounds will be trapped in the upper mantle between 660km and 720 km under the earth's surface. Since each chemical compound (mineral) has separate melting points, those with higher melting points descend farther because they remain solid and therefore more dense than the surrounding liquids and are more likely to be transformed to the mineral perovskite. Perovskite consists of less than half silica and includes iron and magnesium and other such elements that are more dense.

Higher densities draw the rocks deeper into the mantle by gravity where they descend to the mantle-core boundary in slabs. Under the influence of the pressures in the mantle, basalt crust has a melting point of about 3,750 degrees Kelvin (I think), mantle peridotite has a melting point of 4,000 K. Thus peridotite can descend to the lower mantle without melting until it arrives at the core boundary. At this boundary, if the temperatures do indeed exceed this melting point, the transformed basalt lithosphere partially melt and can then be convected through the mantle and lithosphere again as ultramafic rock (which contains less silica than felsic rock) because the liquid form would be less dense.

Comment #2: Okay, I guess that's the best I can do for now at explaining what I think I know. After going over this stuff for a couple hours, I just needed to try writing down my thoughts without delving any further for the time being since it is due by tommorow. So I hope you enjoyed reading this as much as I enjoyed learning it. Now I need to spend time reading the book. I've been reading the article and my notes, so now I'll move on to another source . . . .