Geology of the Appalachian Mountains
A brief overview of Appalachian Geology
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A cordilheira dos Apalaches é um dos sistemas montanhosos continentais mais antigos do mundo. A montanha mais alta da cordilheira é o Mount Mitchell, de 6.684 pés, localizado na Carolina do Norte. Comparado com as Montanhas Rochosas do oeste da América do Norte, que têm 50 picos mais de 14.000 pés de altitude, os Apalaches são bastante modestos em altura. No mais alto deles, no entanto, eles subiram para as alturas da escala do Himalaia antes de serem desgastados e erodidos nos últimos 200 milhões de anos.
A Physiographic Overview
As montanhas dos Apalaches tendem a sudoeste a nordeste desde o centro de Alabama até Newfoundland e Labrador, no Canadá. Ao longo desse caminho de 1.500 milhas, o sistema é dividido em sete províncias fisiográficas diferentes que contêm origens geológicas distintas.
In the southern section, the Appalachian Plateau and Valley and Ridge
provinces make up the western border of the system and are composed of
sedimentary rocks like sandstone, limestone and shale. To the east lie
the Blue Ridge Mountains and the Piedmont, composed primarily of metamorphic and igneous
rocks. In some areas, like Red Top Mountain in northern Georgia or
Blowing Rock in northern North Carolina, the rock has eroded down to
where one can see basement rocks that formed over a billion years ago
during the Grenville Orogeny.
The northern Appalachians are made up of two parts: the St. Lawrence
Valley, a small region defined by the St. Lawrence River and St.
Lawrence rift system, and the New England province, which formed
hundreds of millions of years ago and owes much of its present
topography to recent glacial episodes.
Geologically speaking, the Adirondack Mountains are quite different
than the Appalachian Mountains; however, they are included by the USGS
in the Appalachian Highland region.
Geologic History
Para um geólogo, as rochas das Montanhas Apalaches revelam uma história de bilhões de anos de violentas colisões continentais e a subseqüente formação de montanhas, erosão, deposição e / ou vulcanismo que vieram com. A história geológica da área é complexa, mas pode ser dividida em quatro grandes orogenias, ou eventos de construção de montanhas. É importante lembrar que entre cada uma dessas orogenias, milhões de anos de intemperismo e erosão desgastaram as montanhas e depositaram sedimentos nas áreas circundantes. Esse sedimento era frequentemente submetido a calor e pressão intensos, à medida que as montanhas se elevavam novamente durante a orogenia seguinte.
- Grenville Orogeny: This mountain building event occurred around 1 billion years ago, creating the supercontinent Rodinia. The collision formed tall mountains along with the igneous and metamorphic rocks that make up the very core of the Appalachians. The supercontinent began to break apart around 750 million years ago and by 540 million years ago, an ocean (the Iapeus Ocean) existed between the paleocontinents.
- Taconic Orogeny: Approximately 460 million years ago, as the Iapetus Ocean was closing, a volcanic island arc chain collided with the North American craton. The remnants of these mountains can still be seen in the Taconic Range of New York.
- Acadian Orogeny: Starting 375 million years ago, this mountain building episode occurred as the Avelonian terrane collided with the North American craton. The collision did not happen head on, as it struck the northern section of the protocontinet and then slowly moved southward. Index minerals show us that the Avelonian terrane struck the North American craton at different times and with different collisional forces.
- Alleghanian Orogeny: This event (sometimes referred to as the Appalachian orogeny) formed the supercontinent Pangea ~325 million years ago. The ancestral North American and African continents collided, forming Himalayan-scaled mountain chains known as the Central Pangean Mountains. The modern day Anti-Atlas Mountains of northwestern Africa were part of this chain. The mountain building ended some 265 million years ago, and the ancestral North American and African continents began rifting apart ~200 million years ago (and continue to do so to this day).
Os Apalaches foram desgastados e erodidos ao longo das últimas
centenas de milhões de anos, deixando apenas remanescentes do sistema
montanhoso que alcançaram recordes de altura. Os estratos da Planície
Costeira Atlântica são formados por sedimentos de intemperismo,
transporte e deposição.
