The Lebombo Mountains run North South along the East border of the Kruger National Park and later form part of the Drakensberg Range.
Your tasks – the C.S.I. work!
At GZ you will at a hide where you are allowed to alight from your vehicle. PLEASE BE AWARE THAT THERE ARE NO BARRIERS BETWEEN YOU AND THE WILD ANIMALS – YOU DO SO AT YOUR OWN RISK!
In order to substantiate your visit and be able to claim this EarthCache a number of questions need to be answered and submitted to the cache developer, via e-mail, to “cincolcc [at] gmail.com”
1 – As you enter the parking area and when you look at the hide, to the right, you will see 2 rocky outcrops. These outcrops are conspicuous as the surrounding area is relatively flat with vegetation cover. Estimate the width and length of the outcrop nearest the hide structure.
2 – Look at the colour of the outcrops and explain why this is rhyolite and not basalt.
3 – Explain in your own words what the difference is between basalt and rhyolite.
4 – Describe any other interesting things you saw or experiences you had on your journey to the site from the main road – especially any geological features that you might have seen. Photos are always welcomed in the logs but please don’t post photos of the outcrops themselves.
5 – OPTIONAL – post a photo of your team at GZ.
NOTE: You may log your visit prior to approval, but e-mail submissions that do not meet the above criteria will be deleted.
The Karoo Subgroup
In Southern Africa, rocks of the Karoo Supergroup cover almost two thirds of the present land surface, including all of Lesotho, almost the whole of Free State, and large parts of the Eastern Cape, Northern Cape, Mpumalanga and KwaZulu-Natal Provinces of South Africa. Karoo supergroup outcrops are also found in Namibia, Swaziland, Zambia, Zimbabwe and Malawi, as well as on other continents that were part of Gondwana. The basins in which it was deposited formed during the formation and breakup of Pangea. The type area of the Karoo Supergroup is the Great Karoo in South Africa, where the most extensive outcrops of the sequence are exposed. Its strata, mostly shales and sandstones, record an almost continuous sequence of marine glacial to terrestrial deposition from the Late Carboniferous to the Early Jurassic. These accumulated in a retroarc foreland basin called the "main Karoo" Basin. This basin was formed by the subduction and orogenesis along the southern border of what eventually became Southern Africa, in southern Gondwana. Its sediments attain a maximum cumulative thickness of 12 km, with the overlying basaltic lavas (the Drakensberg Group) at least 1.4 km thick.
Drakensberg and Lebombo Groups
About 182 million years ago the crust under the Karoo Supergroup ruptured releasing huge volumes of basaltic lava over the Clarens desert, covering nearly the whole of Southern Africa and other portions of Gondwana.The pile of lava that accumulated over the course of several eruptions was more than 1600 m thick, especially in the east (in present-day Lesotho). This massive lava outpouring brought the Karoo sedimention to an abrupt end.
The name Drakensberg group is derived from the fact that this layer forms the uppermost 1400 m of the Great Escarpment on the international border between Lesotho and KwaZulu-Natal, often referred to as the Drakensberg (although technically the "Drakensberg" refers to the entire 1000 km long eastern portion of the Great Escarpment, only about a third of which is capped by the Drakensberg lavas).
The outpouring of lava coincided with uplifting of the Southern African portion of Gondwana, and the formation of rift valleys along what were to become the sea borders of the subcontinent. As these rift valleys widened they became flooded to form the proto-Indian and Southern Atlantic Oceans, as Gondwana fragmented into today's separate continents of South America, Africa, Antarctica, Australia, India, Madagascar and Arabia.
In close association with this rifting, a second episode of basalt eruption occurred along the border with Mozambique to form the Lebombo Mountains. A layer of lava more than 4800 m thick was violently extruded at this time. While the Drakensberg lavas form nearly horizontal layers, the Lebombo lavas dip to the east, so it is difficult to gauge how far the lava spread laterally.
What is Basalt?
Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro. The difference between basalt and gabbro is that basalt is a fine-grained rock while gabbro is a coarse-grained rock.
Earth's Most Abundant Bedrock
Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is a very important rock.
Basalt on Moon and Mars
Basalt is also an abundant rock on the Moon. Much of the Moon's surface is underlain by basaltic lava flows and flood basalts. These areas of the Moon are known as "lunar maria." Large areas of the Moon have been resurfaced by extensive basalt flows which may have been triggered by major impact events. The ages of lunar maria can be estimated by observing the density of impact craters on their surface. Younger basalt flows will have fewer craters.
What is Rhyolite?
Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or gray in colour with grains so small that they are difficult to observe without a hand lens. Rhyolite is made up of quartz, plagioclase and sanidine, with minor amounts of hornblende and biotite. Trapped gases often produce vugs in the rock. These often contain crystals, opal or glassy material.
Many rhyolites form from granitic magma that has partially cooled in the subsurface. When these magmas erupt, a rock with two grain sizes can form. The large crystals that formed beneath the surface are called phenocrysts and the small crystals formed at the surface are called groundmass.Rhyolite forms in continental or continent-margin volcanic eruptions where granitic magma reaches the surface. Rhyolite is rarely produced at oceanic eruptions.
Lebombo Rhyolite
REFERENCES:
https://en.wikipedia.org/wiki/Karoo_Supergroup
http://christieatthecape.blogspot.co.za/2007/03/riftclastic-and-rhyolite-and-basalt.html
http://geology.com/rocks/basalt.shtml
http://geology.com/rocks/rhyolite.shtml
