|Avalonian||Avalonia was an ancient microcontinent or terrane whose history formed much of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States. The name is derived from the Avalon Peninsula in Newfoundland. In the early Cambrian, the supercontinent Pannotia broke up and Avalonia drifted off northwards from Gondwana. This independent movement of Avalonia started from a latitude of about 60° South. The eastern end of Avalonia collided with Baltica, a continental plate occupying the latitudes from about 30°S to 55°S, as the latter slowly rotated anticlockwise towards it. This happened at the end of the Ordovician and during the early Silurian.|
|terrane||A geological term for a crustal block or fragment that preserves a distinctive geologic history. It is different from the surrounding areas and is usually bounded by faults (not to be confused with terrain, which is a much more commonly used term meant to describe the physical features of a tract of land).|
|Proterozoic Z age||a geological
eon representing a period before the first abundant complex life on Earth.
The Proterozoic Eon extended from 2500 Ma
to 542.0 ± 1.0 Ma (million years ago). The Proterozoic is the most
recent part of the old informal Precambrian
The Proterozoic consists of 3 geologic eras, from oldest to youngest:
The well-identified events were:
|metamorphosed||Changed in form or nature, a metamorphic rock is created by heat and pressure such that the minerals, fabric, color are changed, but not the composition; Material (usually sedimentary) that has been altered by hear and/or pressure (eg. siltstone to schist, limestone to marble.)|
|sedimentary||Rocks formed from material, including debris of organic origin, deposited as sediment by water, wind, or ice and then compressed and cemented together by pressure.|
|igneous||Rocks solidified from molten magma at or below the surface of the Earth (as distinct from the melting that occurs in meteorites).|
|Middle Paleozoic||The Paleozoic (also spelt "Palaeozoic")
era lasted from about 540 to 250 million years ago, and is divided into
six periods The 320-odd million years of the Paleozoic era saw many
important events, including the development of most invertebrate
groups, life's conquest of land, the evolution of fish, reptiles,
and vascular plants, the formation of the supercontinent of Pangea,
and no less than two distinct ice ages. The earth rotated faster
than it does today so days were shorter, and the nearer moon meant
The Middle Paleozoic was a time of considerable stability. Sea levels had dropped coincident with the Ice Age, but slowly recovered over the course of the Silurian and Devonian. As plants took hold on the continental margins, oxygen levels increased and carbon dioxide dropped, although much less dramatically. The north-south temperature gradient also seems to have moderated, or metazoan life simply became hardier, or both. At any event, the far southern continental margins of Antarctica and West Gondwana became increasingly less barren. The Devonian ended with a series of turnover pulses which killed off much of Middle Paleozoic vertebrate life, without noticeably reducing species diversity overall.
|gneiss||A form of granite, but having the component materials,
especially the mica, arranged in planes so that it breaks rather easily
into coarse slabs or flags. It is a metamorphic rock, commonly
rich in quartz and feldspar, with a banded and foliated texture, formed
at temperatures above about 550° Centigrade
|schist||A fine-grained rock, altered after formation by heat or
pressure or both, so that mineral content is in roughly parallel layers.
It can therefore be split into thin plates, as shale or slate can be.
|quartzite||A stone which was formed in water deposited sediments and
consists of sand grains (quartz) which have been cemented together. It
can be chipped, but is difficult to work. Quartz is the mineral
SiO 2 (silica dioxide) that occurs in transparent and colored crystals
and crystalline masses.