Earth’s First Mountains

The area of the Minnesota/Canada border has an interesting early geological history.

Granite layers in Voyageurs National Park                              photo from NPS

In its early life, the planet Earth was a hot ball of rock that was constantly pounded by impacts. The tectonic plates which are such a familiar part of our modern geology did not yet exist, and the planet’s surface, softened by the immense heat, was instead roiled by vertical convection cells which brought up molten magma from the Earth’s interior and spilled it out onto the surface.

Eventually, over about 2 billion years, the Earth cooled enough for granite-like rock masses to form, and these began to slowly move across the surface as the planet’s tectonic plates began to form. These early rock masses are known as “cratons”. In between cratons, small volcanic islands would have formed in the early oceans.

Over a period of around 300 million years, eight of these early craton masses were crushed into each other by the motion of their tectonic plates and were welded together at the edges to form most of the landmass of what is now North America. During this same time, other cratons were assembling to form other landmasses: four ancient cratons make up most of what is now Australia, and three more form much of Europe.

Until this time, most of the planet’s surface was relatively flat. The rock crust was so softened by the heat that it could not support the weight of mountains, and whenever they tried to form (such as around the volcanic pipes which carried magma to the surface), the would-be mountains slowly sank into the ground and disappeared.

But by the time the cratons began crushing into each other to form “continents”, the Earth’s crust had cooled and strengthened, and this led to the formation of the first mountain ranges—a process known as “orogeny”. It is the same process which forms most mountains today, and it is based on the movement of tectonic plates. When two plates collide, the immense force will cause the edges of the landmass to crumple and fold, such as what happens when you push two rugs together. This process of “uplift” forms mountain chains that run along the plate borders. The modern Himalayas, for instance, were formed when the tectonic plate carrying India crashed into southern Asia about 45 million years ago. That slow-motion collision is still happening today, and the Himalayans are still slowly being pushed higher and higher, about a half-inch per year, from this immense force.

The earliest instance of mountain orogeny that we know about happened about 2.7 billion years ago, in what is now North America. A relatively small plate of continental rock known as the Minnesota River Valley Gneiss Terrane was carried into the much larger Superior Craton, which stretched from what is now Quebec and Ontario all the way across Michigan and Minnesota to North and South Dakota. This impact resulted in the Algoman Orogeny, pushing up the edges of the Superior Craton and forming a long mountain chain, the Algoman Mountains, that ran from modern-day eastern Canada over to the Great Plains of the United States. (The name comes from the town of Algoma WI, where some of the first evidence for their existence was found.) The heat and pressure were enough to alter some of the granite rocks, metamorphing them into schist and gneiss.

The Algoman Mountains were immense, soaring at least as tall as today’s Andes and perhaps as tall as the Himalayas. Today, 2.7 billion years later, they have been worn away completely, eroded by water and wind. We only know of their existence because we can see the vast sedimentary deposits resulting from their erosion and the ancient folded schist, gneiss, and granite layers that once formed their base.

Over time, more cratons were joined to this North American core to eventually form the northern super-continent that became known as Laurasia, containing most of what is now North America, Europe, and Asia. This was accompanied by another super-continent in the south, called Gondwana, which contained much of South America, Africa, Australia, India and Antarctica. Since then, geological history has consisted of these super-continents periodically breaking into pieces and then rejoining.

The ancient metamorphic schist and gneiss bands from the Algoman Orogeny are now exposed at the surface inside Minnesota’s Voyageurs National Park, having survived some 2.5 billion years of erosion, plate subduction, volcanism, and glaciation. They make up some of the oldest still-existing rocks in North America. Layers of gneiss and schist can be seen at the Kabetogama Lake and Rainy Lake areas of the park, while there are bands of exposed basal granite at Sand Point Lake and Namakan Lake.

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