Do you know your rocks#8 EarthCache
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Do you know your rocks#8
As with the other earthcaches in this series we will be taking a look at different rock types and formations and learn how they are formed and hopefully be able to recognize them in nature. In this Earthcache we will be looking at the rock type Dolerite.
DOLERITE FORMATION:
Dolerite is a igneous rock that forms when magma from basaltic volcanoes (volcanoes thats lava is rich in iron and magnesium and low in silica) moves up into fractures and weak zones below the volcano. When the dolerite cools down in these fractures it forms one of two distinct formations - dikes (tabular igneous rock bodies that cut across pre-existing rock layers or bodies) or sills (tabular igneous rock bodies that form parallel to pre-existing rock layers).
Dolerite cools moderatly quickly and this moderate cooling rate allows visible crystals to form in the rock, all the crystals are not the same size though. As the magma rises to the surface it does cool down at different rates due to the fact that the closer magma gets to the surface the colder the surrounding rock and various elements are causing the magma to cool down faster (the faster magma cools the less time minerals have to form crystals) so dolerite will have both large and small crystals.
MINERALS IN DOLERITE:
Minerals in dolerite include olivine, plagioclase feldspar, pyroxene, quartz and sometimes ilmenite, biotite, hornblende and magnetite. When fresh it is black, dark-grey or green; may be mottled black and white. Olivine has a greenish color so when dolerite has a lot of olivine in it - it causes the dolerite to have a green tinge or even have green speckles. Where olivine occurs as an additional mineral, the rock is termed an ‘olivine dolerite’. Where quartz occurs as an additional mineral in the ground mass, the rock is termed a ‘quartz dolerite’ and so forth.
Generally Dollerite is dark green to black, sometimes with some white crystals scattered through it. When exposed to the weather its surface often turns reddish browndue to the iron.The two dominant minerals in dollerite is plagioclase and pyroxene.
Plagioclase is white, gray, bluish white, reddish white and greenish white in colour and form large crystals. Pyroxene is Usually dark green, dark brown or black, but some varieties are white to light green and the crystal size is usually smaller than that of plagioclase.
USES FOR DOLERITE:
Dolerite is used as building material, concrete aggregate, paving stones and as an ornamental stone.
At way point #1 you will see a cement man-made structure. Just behind it there is a small rock formation that will show evidence of the upward force of dolerite. There is a band of pmb shale that runs vertical to the dollerite. This shows that the horizontal plane of the shale was moved vertically and the dollerite cooled down on both sides of the shale.
When you reach gz you will be standing on a rock formation around 135 million years old. Take a break and enjoy the view.
The above information from wikipedia and geology.com
On site questions.
#1 Is this area a dyke or a sill dollerite formation?
#2 What color is the dollerite.
#3 In your own words describe the surface of the. dollerite that is closer to the waves than the surface further away from the wave action.
Please forward answers ASAP
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