Minnesota's Rocks and Waters

Minnesota Geology - The Big Picture

Geologic setting of Minnesota

• Boundary between Laurentian Upland and Interior Plains and Lowlands
• Laurentian Upland coincides approximately with Canadian Shield, an area within the stable craton where igneous and metamorphic rocks (crystalline rocks) of Precambrian age are exposed
• Minnesota lies entirely within the craton of North America. The craton is a portion of a continent which has been stable (that is, has not experienced major effects of Earth's internal processes, such as earthquakes, volcanism and mountain building) since Precambrian time
• The craton of North America is surrounded by belts of folded and faulted rocks uplifted in so-called fold-belt mountain chains. The Paleozoic Appalachian Mountains to the east are lower and less rugged than the Mesozoic/Cenozoic Rocky Mountains to the west because the older Appalachians have been more deeply eroded.
• The existence of fold-belt mountain chains at the edges of the North American continent can be understood in terms of plate tectonic theory. The folded Appalachians were formed when the eastern margin of the plate was colliding with plates to the east, and the Rockies were formed when the western margin of the plate was colliding with plates to the west.
• Note that Minnesota, at present, is nowhere near a plate boundary, which accounts for the fact that Earth's internal processes are not manifest in the state.

Bedrock of Minnesota

• The Laurentian Upland has igneous and metamorphic rocks of Precambrian age exposed at the surface of the shield.
• The Interior Plains and Lowlands are covered by a blanket of nearly horizontal strata of sedimentary rocks. In southeastern Minnesota these rocks are Paleozoic (mostly Cambrian and Ordovician) and in southwestern Minnesota these rocks are Mesozoic (mostly Cretaceous).
• The entire state is overlain by a blanket of Quaternary glacial sediments (drift) of variable thickness, except for southeastern Minnesota which is located in the so-called "driftless area". The thickness of drift is variable across the state and is thickest in areas where pre-glacial valleys were present. See the map of surficial deposits and the map of bedrock exposure for further information.
• The topography of Minnesota is governed in part by the different geological materials exposed at the surface. These different types of materials have different resistances to weathering and erosion, and also reflect different types of processes responsible for evolution of the geology.
• Based on topography, the state can be divided into several different physiographic provinces whose differences are attributed to the nature of the bedrock and the different processes that shaped the landscape.

Minnesota's geology and topography reflects a dynamic Earth

• The geologic evolution of Minnesota can be understood in terms of Earth systems. A system is a group of interdependent materials that interact to form a unified whole and are influenced by related forces. Most geologic systems are open; that is, they can exchange matter and energy across their boundaries.
• Important Earth systems include rock systems, water (hydrologic) systems and plate tectonic systems.
• Systems generally involve cycles which operate as the system evolves. The rock cycle is perhaps the most important of these cycles. You should study it now to understand the way in which geologic processes and earth materials interact to produce a dynamic Earth.
• Earth materials include minerals (the letters of the geological alphabet) and rocks (the words of the geological record).
• Earth processes can be subdivided into internal (operating in Earth's interior) and external (operating on Earth's surface).
• Internal processes are driven by heat energy from remnant and radioactive sources. We sense these processes at the surface in the form of earthquakes, volcanic activity and mountain building.
• External processes are driven by heat energy from the sun and include weathering, (another weathering site) mass wasting, erosion and deposition. Agents of erosion and deposition include running water, ground water, glaciers, waves and currents, and wind.
• Internal processes build up the Earth's surface, while external processes level the surface.
• The Earth's surface is not level, because internal and external processes struggle with one another to produce an ever-evolving geologic framework and landscape.
• The interaction between geological processes and materials has been ongoing for 4.6 billion years (the age of the Earth).
• Plate tectonics is the overriding system which controls the evolution of Earth's geologic framework. We will spend considerable time studying this important theory and you should begin your reading on this topic now.