Well since some 3/4 of the Earth's surface is covered by oceans, its pretty easy to see that the manufacture of seafloor could easily be a full time occupation. And since we know that average age of the ocean floor (about 200 million years) is about 1/20 of the age of the Earth (4.5 billion years), we can guess that all of the ocean crust could have been remanufactured about 20 times during this time period. Well that's enough work to keep Vulcan at his forge or Santa and his little elves busy for a long, long time. The older the ocean crust gets, the more dirt, grime, and ooze builds up on it (kind of like a neglected bathroom).
Though this is pretty interesting, there's some more stuff that we can figure out from looking at the kinds of basaltic lava that we see forming in the oceans today. The part of the mantle that melts to form the ocean crust must have been used a bunch of times. The source area turns into kind of a residue. You can kind of imagine a mixture of half lead solder and half pennies. If you apply a little heat to the mixture, the lead solder will melt way before the copper. If this is all of the heat and melting then you would end up with volcanoes that erupt lead and copper pennies in the residue. Eventually if you melt the residue enough times you will get lava that is richer in copper and poor in lead. In a similar way we can look at the chemistry of the basalts from mid-ocean ridges and ocean islands and see that they are different due to repeated melting of the source for MORB basalts. The second is that since there is no old ocean crust, it must have been recycled back into the source area. We know that this recycling takes place at the convenient recycling subduction zones located handily around the globe for just this purpose.
What we don't know is just how much of the recycled material gets mixed back into the source area. What we do know is that MORB comes from the part of the mantle that has been used again and again for making ocean crust. We're pretty sure that this is the shallow part of the mantle. In contrast, OIB like that in Hawaii comes at least partially from a different source that hasn't been used as much and may have some recycled stuff in it. So we can tell these two types of basalt apart and this information is telling us something about the hotspot we call home.
The types of basalt we get in Hawaii have not been as depleted as MORB. This gives us an important constraint on the type of model that we favor for hotspots. If simple fracturing caused the hotspot, melting would occur in the upper part of the asthenosphere, the same source as MORB. So if the hotspot was the result simply of a propagating fracture system, we wouldn't get a different type of basalt in Hawaii or on other ocean island hotspots. So we know at least some component of the magma feeding Hawaii is coming up from depth and melting due to decompression (release of pressure). This rising plume would not only cause a temperature increase, but also would bring up deeper material releasing pressure without releasing heat. Both of these could cause melting at the base of the lithosphere (rigid mantle and crust). We infer that there is a shallow residual source for MORB and a deeper possible partial source for Hawaiian basalts. There may also be a component of recycled ocean floor in the deeper source area.
When the island is directly over the hotspot, there is fairly extensive melting of the mantle. The molten rock is lighter than the surrounding rock, so it rises by working its way up through a series of fractures in the mantle and crust. The heat necessary for melting must come from deep in the earth as does part of the magma material. Some of the magma could be melting in place near the top of the plume.
Going back to our lead and copper model of melting, if you melted all of a 50-50 lead and copper mix, you would get lava with the same mixture. If you melt just a little of brand new stuff, the melt would be all lead. If you melt some of the residual used up stuff, there would be a lot of copper in the melt.
Thinking this way, its not surprising that over the hotspot where a lot of material melts, we get a composition more similar to the source. These basalts are called tholeiites and tend to have a lower overall percentage of easy melting components in them.
As the volcano moves off of the hotspot, the heat becomes more erratic and the degree of melting becomes less. So we should either get small amounts of melt from new type sources (like lead) or from residual type sources (like the pennies). What we see is that the stuff away from the hotspot looks like it came from a residual source, while some of the stuff on the hotspot came from a new source.
Coupled with this, the smaller amounts of melting also favor the concentration of the easy melting fraction in the magma produced. So sometimes even if you have only 1% of this fraction of easy melting stuff, if you only melt 1% of the rock you can get a magma that looks like it might have a new component in it.
In Hawaiian basalts, the massive melting over the hotspot produces lavas called tholeiitic basalt that are poor in alkali elements (potassium and sodium), whereas the later melting away from the hotspot produces melts called alkali basalt that are very high in potassium and sodium (see the figure). This is due to the much smaller amount of melting that produces the alkali basalt. The variation in types of akali rocks and tholeiites is largely due to cooling and crystallization of the original melted rock (the removal of crystals tends to make the rock richer in SiO2).
| Kilauea | MORB | Hawaiian Alkali Basalt | ||
| SiO2 | 50.5 | 48.8 | 46.4 | |
| Al2O3 | 13.5 | 15.9 | 14.2 | |
| FeO | 11.2 | 9.8 | 12.6 | |
| MgO | 7.4 | 9.7 | 9.5 | |
| CaO | 11.2 | 11.2 | 10.3 | |
| Na2O | 2.3 | 2.4 | 2.9 | |
| K2O | 0.5 | 0.1 | 0.9 | |
| MnO | 0.2 | 0.2 | 0.2 | |
| TiO2 | 2.6 | 1.2 | 2.4 | |
| P2O5 | 0.3 | 0.3 | 0.3 | |
| La | 13.4 | 2.1 | 18.8 | |
| Ce | 35.5 | 43.0 | ||
| Sm | 6.1 | 2.7 | 5.4 | |
| Eu | 1.9 | 1.1 | 1.8 | |
| Yb | 2.0 | 3.2 | 1.9 | |
| Rb | 9.2 | 0.6 | 22.0 | |
| Sr | 371.0 | 89.0 | 500.0 | |
| Ba | 150.0 | 4.2 | 300.0 | |
In order to tell if this small amounts of lava rich in alkalis is due to a source area enriched in alkalis or due to small amounts of melting, we use isotopic tracers found naturally in the rock. This isotopic tracers are products of natural radioactive decay and are the same types of elements that can be used to date the rocks.
What we find is that the basanites or alkalic rocks shown the isotope diagram have isotopic compositions much closer to MORB than do the tholeiites, though the general composition of the tholeiites is much closer to MORB. This shows us that the alkalic rocks are probably derived from very small amounts of melting of the upper mantle used to make MORB, whereas the Hawaiian tholeiites are derived from large amounts of melting that includes a deep mantle component.
As you move farther away from the hotspot these late erupted lavas become increasingly richer in alkalis due to smaller and smaller amounts of melting. In contrast, the tholeiite lavas change composition only because of cooling and sinking of crystals from the melt, which causes the silica content to increase.
The change in composition of lavas as volcanoes move from the hotspot is shown in the form of the volcanoes themselves. Mauna Loa and Kilauea are made of very fluid tholeiite lavas and have smooth round shapes. In contrast, Mauna Kea is capped by stickier alkalic lavas that form cones and give Mauna Kea its characteristic peak shape. More on this in the next lecture....
If you have comments or suggestions, email me at kenhon@hawaii.edu