Rock And Turn: They decided to live with the rock and turn it into a moss, lichen, and whatever-else-happened-to-show-up garden. Last weekend on an August afternoon under a misty sky with a light, drizzling rain— the first rain we've had in weeks—Janet and I took inventory of the rock. Except for a few choice clumps of lichens and mosses brought back from nature walks to other parts of the property, everything growing on the rock has started on its own.
The movement of the earth's crust may carry the rock as much as 700km (454 miles) below the surface. Here the temperature and pressure will be even higher and the rock will begin to melt. Molten rock is lighter than solid rock and it will begin to rise up through the overlying rock towards the surface. If it reaches the surface as a lava flow it will immediately be ready for weathering and erosion and the start of a new cycle. More often the molten rock solidifies underground and then all the rock above it must be eroded away before it can begin the cycle again.See Also Bare Rock Patchily:Stream beds in their upper section are often bare rock patchily covered by pebbles. Here the stream has greatest capacity to erode and transport farther downstream all but the largest stones. The valley in the upper course has steep sides and a V-shaped cross-section and most pools, rapids, waterfalls [6] and pot-holes [3] occur here, caused by the stream wearing away softer rock more quickly than hard rock. This results in rapids such as the cataracts of the Nile, and where a river flows from a hard bed of rock to a soft one the latter will be eroded away and a waterfall will be formed as a result.
Both lichens and mosses can exist on bare rock. By chemical means they produce food. By mechanical means they can remove tiny pieces of hard rock, threading their rhizoids, or tiny roots into microscopic pores in the rock's surface, gaining trace elements for nutrition and slowly, over eons, making soil. Even airborne dust from mountain roads and shoulders is trapped by the tiny projections of the lichens and the leaves of the mosses, eventually to combine with old crumpled and dehydrated plants to form new accumulations of dirt. Given enough time, the little white pine will someday find soil on the top of Janet's rock.
On The Other Hand See Solid Rock Over:Melting a rock like peridotite, however, results in about a 10% in¬crease in volume, so that the melt is less dense than the solid rock over rock. Therefore, if peridotite be¬gins to melt, the molten rock—being lighter than the solid rock over—should tend to rise through the man¬tle. Numerous experiments showed that the first-iformed melt of a rock of peridotitic composition {would be a basalt.
The process of producing molten basalt in the upper mantle should be that of partial melt¬ing, because compositions of basalts are so different from those of peridotites. The experi¬ments showed that peridotite melts over a range of temperatures rather than having a fixed melt-ting point. Because certain minerals in the rock [melt before others, the composition of the first melt that forms differs from that of the whole •ock. As the temperature rises, more and more solid rock over matter reacts and melts, causing the com¬position of the melt to become more like that of the original solid rock over rock.
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