Monthly Meeting – Speaker Graham Cunningham21st Jan 2013
It’s important to develop ideas about a planet by examining images of its surface, and the talk stressed the importance of looking at such evidence, and comparing it with the surfaces of the other rocky planets – still most of what we know. James Hutton proposed in 1785 that by looking at the relationships between rocks exposed at the Earth’s surface, we could identify repeated cycles of deposition, uplift and erosion, showing a continuity of geological processes over an immense time-scale, contradicting many contemporaries who wanted to argue that catastrophic events governed the Earth’s history. Hutton’s ideas implied that large-scale dynamic processes produce these ‘earth-movements’, but it wasn’t until the last few decades that we have been able to look at the topography of the oceans, and this led to the hypothesis of plate-tectonics, where basalts are erupted to form new ocean crust at volcanic ridges and this crust first spreads to form the oceans then dives below adjacent plates at great linear features marked by deep trenches. Importantly, the lines of volcanoes which mark this ‘subduction’ process typically produce andesite rather than basalt – while basalt is the product of melting essentially-dry mantle rock, water is needed to allow melting at lower temperatures to give the more silica-rich andesite. The Earth has a unique supply of liquid water in its oceans, and this may be essential for plate tectonics as well as life. There remain many things we don’t know about plate tectonics – just how it’s driven and even whether in fact its processes only involve relatively shallow depths of a few hundred metres into the mantle. Nor do we know if the process can be tracked close to the formation of the Earth’s crust over 4 billion years ago, although we do find very ancient rocks of types we now associate with plate tectonics. So ideas about other planets need to be guarded. On Mars and the Moon we seem to see large old (because heavily impact-cratered) upland areas which we might infer represent the earliest crust, not subject to the deformation and erosion processes seen by Hutton, and smoother areas – the North polar region on Mars, the ‘maria’ on the Moon. Both regions are relatively low and filled with basalt (we’re sure of this on the Moon). This phase of impact and volcanism is 3.9 billion years old, and on Mars it seems to have partly erased weak (?) magnetism in the crust of Mars. This apparently-similar geological ‘history’ could only be plausible if the impact event was widespread at least in the inner solar system, and its timing would leave little scope for Mars to have developed an atmosphere and liquid water, essential for life. No doubt Curiosity and its successors will add greatly to what we know, and it will be interesting to find out more about the huge martian volcanoes and whether the other large rocky planet, Venus, shares some of the Earth’s features; who knows? – plate tectonics of a sort, perhaps.