Breaking Up is Hard to Do: Continental Rift

Wisconsin side of St. Croix Dalles as seen from Minnesota

One of my favorite questions to ask students in Biogeography or Evolution is "What evidence do you see around you of a changing Earth?"  It is an important question; one of the many that were just starting to be addressed as Charles Darwin set foot on the HMS Beagle.  Our time on Earth is so fleeting and the time required for massive change is so long, that it is no wonder that many people assume that the Earth is essentially as it always has been.  If only time machines were a thing.  Where are Doc Brown and Marty McFly when you need them?

Nadienne blending in

The evidence is there if you open your eyes and ask the right questions.  Many places exist that act as our time machine, allowing us to see structures on Earth that have been hidden for hundreds, millions, or even billions of years.  One of my favorite evidences for a changing Earth has to be Interstate State Park, which flanks the St. Croix River in Minnesota and Wisconsin, just north of the Twin Cities.  Here, Ice Age floods have scoured out a record of a time when the North American Craton (core continental crust) began to tear apart into new continents about 1.1 billion years ago.

Volcanic Rocks...In Minnesota and Wisconsin?

Pillow lava with flood basalt deposited on top

A lot of people are surprised to hear that there are deposits of basalt or other volcanic rock in places where there are no obvious volcanoes, but they are here.  When I walked down into one of the park valleys, even knowing that I was looking at basalt I was still a bit surprised to see signs of classic pillow lavas.  This lava breaks out to the surface underwater, and the water cools the surface of the lava, but the inside keeps flowing which causes big blobs of basalt to form as in the bottom half of the picture at right.

Bubble pattern shows successive volcanic episodes 

Reading the record in the rocks (yes, I do see the world in equation form and process diagrams), it is possible to see evidence for at least seven basalt flows at Interstate State Park.  When a basalt erupts to the surface, gases escape from the molten rock, and bubbles of gas are trapped in the thicker cooling rock at the surface of the deposit.  Looking closely at the rocks, you can see a layer of bubbles from one eruption, then see a smooth deposit with few bubbles from the next eruption, then the incidence of bubbles increases as you move upwards, until the cycle repeats itself.

Multiple volcanic episodes seen...bubbles...no bubbles...bubbles...no bubbles...bubbles

Some of those bubbles can be quite

Amygdules in basalt

large.  Over the course of a billion years, and under a pool of water like Lake Superior and its ancestral precursors, water percolates through the rock and drags minerals into these cavities.  When the concentration of mineral gets high enough, they crystallize.  Several of the rocks here contain amygdules of quartz and feldspar.  In the Lake Superior basin, many of these deposits are banded in beautiful reds and browns from iron, forming the famous Lake Superior agates.  These rocks were released from their lava prisons by cycles of freezing and thawing, then tumbled and transported south by glaciation.  We find them (rarely) in northern Missouri, and they are relatively common in the Mississippi River valley north of Missouri.

A large Lake Superior agate

The Midcontinent/Keweenawan Rift

Arms of rift in red with direction of rifting indicated

Most of the rocks in this volcanic system lie buried, but the metals that they contain create a gravity high that can be measured.  The system runs from Lake Superior east into Michigan, southwest into Kansas/Oklahoma and perhaps north to Lake Nipigon in Ontario, Canada.  The basalt ranges from two to twelve miles in thickness, and is buried by as much as six miles of sediment in some places.  Surface exposures are found only around Lake Superior and south through the St. Croix River valley.

The crust of the Earth is relatively thin and broken into a variety of continental and oceanic rocks.  These plates move with respect to each other, driven by the tremendous heat engine that is the mantle.  Seafloor spreads from an upwelling of magma that seeps through to the surface depositing igneous (volcanic) rock that is relatively dense.  Deposition of new material here will push two plates apart.  Heated magma will also rise and strike the bottom of the plate, dragging it along by conveyor action.  Eventually, two plates are going to collide with each other.  Oceanic crust is denser than continental crust, and tends to slip under the continental crust, pushing on the edge of the continent, elevating the continental edge.  A deep trench is evident where this subduction is taking place.  Gravity and density work together and pull the subducting plate downwards.

Stages of rifting - Midcontinent Rift stopped between steps  5 and 6

As the oceanic crust melts, the water in it superheats and may start melting the continental crust, far away from the subduction zone.  The rising plume of magma will cause a dome to form in the crust, eventually the heating by the magma will cause the lithosphere (crust and upper layers of mantle) to become more plastic and stretch.   The lithosphere will sag under the weight of the overlying crust, causing faults (breaks) to form in the crust, forming a series of valleys and ridges.  Magma may find its way to the surface in the region of the mantle plume or intrude into pre-existing rocks further away.  The valleys may fill with water, as with the Rift Valley Lakes in East Africa.

As igneous rocks are deposited in the rift, the rift widens.  If rifting continues long enough, a new ocean will open up, separating the continental land mass.  This process is easiest to see in East Africa, where a triple-junction has opened from a mantle plume under Ethiopia.  Extension of the rift to the northwest is tearing the Arabian peninsula free from Africa, and the Mediterranean Sea will be connected to the Indian Ocean, as well as the Atlantic. Rift extension to the southwest will separate the African Plate into the Nubian Plate and Somalian Plate, with an oceanic corridor between them.

East African Rift triple junction

Round and round

Pillow lava overlain by flood basalt

This predicts a cycle of events in which oceanic widening pushes continental landmasses together into a supercontinent.  Rifts that appear in the supercontinent results in oceanic intrusion and the landmasses float apart into separate continents.  Continued seafloor spreading pushes the landmasses together again and another supercontinent  forms.  Currently, spreading of the Atlantic Ocean basin is pushing the Americas away from Europe and Africa on the east, while the Pacific Ocean is closing on the west and the Americas and Asia are getting closer.  The 2011 earthquake in Japan caused Japan to shift closer to North America by 13 feet.  The usual rate of plate movements is about the same as the growth of your fingernails.

Rodinia reconstructed showing mountain building areas (green)

The Midcontinent rifting took place when the continents were assembling into the supercontinent of Rodinia.  Minnesota was located very close to the equator and directions were rotated about 90 degrees from present (west now was north then).  The mantle plume for the Midcontinent Rift was under present-day Lake Superior.  The rifting resulted in quite a bit of volcanic action which is evident on the surface in the north.  Pillow lavas which were deposited at the bottom of rift lakes were overlain by later flood basalts that erupted from fissures in the Earth.  From Iowa south, the volcanics rose up into pre-existing rocks, but probably did not break through to the surface.

Continental movement prediction for the next 250 million years

Rifting lasted for 15-22 (ish) million years, then stopped.  This break nearly opened up a way to the ocean; it is the deepest rift known to have healed.  Geology of the region suggests that the crust in this region thinned to 25% of its pre-rifting thickness.  The region of Lake Erie was about continents edge at the time of this rift.  The East Coast of North America was added through later collisional events.  During the formation of Rodinia, it is thought that the North American craton collided with the Rio Plato, Amazonia and Baltica cratons, resulting in the formation of mountains.  This mountain building episode is called the Grenville orogeny, and may have put enough pressure on North America to halt the spreading of the rift and reverse it.  Many blocks of reverse faulted crust can be found, being pushed upwards by compression.

Rio Grande Rift

Continents will continue to move, stretch and break as long as there is molten rock on the interior of Earth.  There is some thought that the New Madrid fault zone between Missouri and Tennessee is the product of a failed rift.  Active rifts abound, famously including the East African Rift.  The magnificent lava fountains of Iceland are formed by the Mid-Atlantic Rift reaching the surface of the water.  In North America, there are several active rift zones, including Death Valley and the Rio Grande River Valley.  Hotspot plumes of mantle trace a path in the Hawaiian Island chain and across Idaho into present day Yellowstone National Park.  Present models suggest that 250 million years from now, the landmasses of Earth will again coalesce into a supercontinent, this time called Pangaea Proxima.

Getting There

Waypoint: Latitude 43.3936727 N; Longitude 92.6709847 W
Street Address: US-8/St. Croix Trail and Milltown Rd., Taylors Falls, MN 55084


On I-35 north of the Twin Cities, take Exit 132 to Taylors Falls (US-8 East).  The entrance to the Park (Milltown Rd) is in Taylor Falls just west of the bridge to Wisconsin.  There is another part of the park in Wisconsin (hence the name Interstate State Park) just across the bridge at Taylors Falls.

Further Reading

Interstate Park - MN DNR

Midcontinent Rift System in Iowa - Aerial Surveys