Now we shall look more closely at the geologic materials that make up the bedrock of Minnesota. We have already noted that surface bedrock in the portion of Minnesota that lies in the Laurentian Upland (Canadian Shield of northern Minnesota) is composed of igneous and metamorphic rocks made up of silicate minerals. And surface bedrock in the portion of southern Minnesota located in the Interior Plains and Lowlands is largely composed of sedimentary rocks of Paleozoic (southeast) and Mesozoic age (southwest) made up of a variety of minerals, including silicates and carbonates.

Minerals

• The letters of the geological alphabet. Minerals put together in certain ways make up rocks
• Chemical compounds, each with a fixed composition and an orderly internal arrangement of atoms (structure)
• Composition and structure are reflected in the properties of each mineral
• Silicate minerals are most abundant minerals in the crust. Made up of various chemical elements in combination with silicon and oxygen (see silica tetrahedron, silicate structure 1 and silicate structure 2 for diagrams that show how silicate structures are built)
  • If iron and magnesium are abundant together with silicon and oxygen, we call the minerals ferromagnesian silicates. These minerals are generally dark-colored and relatively dense
  • If iron and magnesium are absent, and the minerals are more rich in silicon and aluminum, we call the minerals non-ferromagnesian silicates. These minerals are generally light-colored and less dense. Feldspar and quartz are the most common non-ferromagnesian silicates.
• Non-silicate mineral groups
  • Carbonate minerals are the most important non-silicate mineral group for our study. These minerals contain various chemical elements in combination with carbon and oxygen.
    • Calcite (calcium carbonate) and dolomite (calcium-magnesium carbonate) are the most common carbonate minerals.
    • These minerals are soluble in chemically active water solutions and will dissolve in a combination of fresh water and carbon dioxide (carbonic acid).
  • Oxide minerals are important in weathering environments, where oxygen has combined with metallic elements such as iron to produce reddish or yellowish-brown minerals which add color to sedimentary rocks
  • Mineral groups important as ores of metals
    • oxides - ores of iron
    • sulfides - ores of lead, zinc

Rocks are made of minerals (composition) put together in certain ways (texture). Rocks are interrelated to one another in the rock cycle, which describes the way in which both internal and external processes on Earth operate to create and modify earth materials.

Igneous rocks

These rocks form by the solidification of molten rock which is called magma beneath the surface and lava above the surface. The magma is created by heat sources, either at spreading centers or in subduction zones. Hence, igneous rocks are often formed by processes operating at plate boundaries. Exceptions to this are volcanoes such as those in the Hawaiian Islands which erupt in plate interiors where hot spots have developed.

Classification of igneous rocks is based upon composition and texture.

• Composition of igneous rocks - governed in part by type of magma - see classification chart for compositional families of igneous rocks
  • Ultramafic - extremely rich in iron and magnesium-bearing silicate minerals. Typical of rocks in mantle
  • Mafic - rich in iron and magnesium-bearing silicate minerals together with feldspars. Typical of rocks in oceanic crust
  • Sialic or felsic - rich in non-ferromagnesian silicate minerals including feldspar and often quartz. Typical of rocks in continental crust. The granites in Minnesota, such as those in the St. Cloud area, are sialic or felsic rocks.
• Texture of igneous rocks - governed in part by rate of cooling
  • slow cooling deep in crust results in larger crystals and coarsely crystalline texture
  • rapid cooling near or at the surface results in smaller crystals and finely crystalline textures. Glassy textures which have no mineral crystals present are formed by extremely rapid cooling.
• Study of texture enables us to tell igneous rocks formed at the earth's surface (extrusive or volcanic rocks) from igneous rocks formed deeper within the crust (intrusive or plutonic rocks)
• Plutons - shapes of igneous bodies and their relationship to surrounding (country) rocks
  • batholith - large discordant (cuts across) igneous body with an area greater than 100 square km
  • dike - tabular (sheet-like) discordant igneous body - example from St. Cloud
  • sill - tabular (sheet-like) concordant igneous body, intruded parallel to layering in surrounding rocks
• Volcanic features
  • Volcanic cones
    • shield volcanoes - very large, gently sloping cones built from innumerable mafic lava flows (basalt) associated with quiet Hawaiian type volcanic eruptions. Eruption is quiet because there is little gas in the magma, and what there is escapes quietly. Quiet volcanic events are most typical of mid-oceanic eruptions such as those in Iceland along the mid-ocean ridge, or in Hawaii at a hot spot within an oceanic plate.
    • stratovolcanoes or composite cones - very large, more steeply sloping symmetrical cones built from alternating lava flows and eruptions of volcanic solids such as blocks, cinder and ash. Eruption includes alternating quiet eruption of lava and explosive eruption of solids, because there is more gas in the magma. The gas builds up and escapes quickly in the more explosive events. Resulting rocks are more sialic or felsic than those of shield volcanoes. These volcanoes are typical of subduction zones.
  • Calderas - greatly enlarged craters, larger than 1 km in diameter, formed either by collapse of the summit of the volcano associated with previous eruption of large amounts of lava, or by violent explosion of the summit, associated with the extremely rapid escape of large amounts of gas.
  • Lava flows - may be composed of smooth ropy and billowy forms associated with fluid lavas that flow easily, or of rough clinkery forms associated with stiff or viscous lavas that flow slowly and with great difficulty
  • Pillow lavas - lava flow made up of pillow-like forms of meter-scale, formed when lava is erupted under water
• Distribution of Volcanic Activity - volcanoes are either found at plate margins, or at hot spots within plates, such as the Hawaiian hot spot

Sedimentary rocks

These rocks are formed in one of two ways and include surface processes of weathering, mass wasting, erosion and deposition, and the near-surface process of lithification following deposition and often involving compaction and cementation during burial

• Weathering at or near the earth's surface controls the derivation of sedimentary particles.
  • Mechanical weathering is a physical breakdown of rock material into smaller and smaller particles. The particles may either be fragments of the original bedrock or smaller grains of individual minerals broken down from the bedrock.
  • Chemical weathering is a chemical alteration of rock material by a chemically active water solution, usually carbonic acid (water reacted with carbon dioxide). Unstable silicate minerals such as feldspar will alter to clay + ions in solution, while soluble minerals such as calcite and dolomite may completely dissolve.
  • Whether mechanical or chemical weathering will be dominant depends on climate. Warm and moist climates promote chemical weathering, while semi-arid or arid climates promote mechanical weathering.
• Deposition of sedimentary particles derived from breakdown (weathering) of rocks on land with subsequent transport by some agent such as running water, and deposition in a basin, either on land or in the sea, results in terrigenous or siliciclastic sedimentary rocks which are classified largely on the basis of texture (particle size).
  • conglomerate - a coarse-grained rock containing pebbles or larger particles, often formed by rapid high-energy transport and deposition in settings such as channels of high-energy rivers
  • sandstone - a medium-grained rock containing grains of sand often dominated by quartz, and formed by transport and deposition in moderate energy settings such as sand bars in rivers, sand dunes, or sandy beaches or sand bars in coastal areas
  • shale - a fine-grained rock contining mostly clay minerals (very small, soft silicate minerals with a sheet-like structure), generally transported and deposited in low-energy settings such as floodplains, lakes, lagoons and deeper ocean settings.
• Precipitation of minerals within a sedimentary basin by chemical processes, often forming calcite, dolomite, or other salts. The resulting rocks are called chemical sedimentary rocks which are dominated by the carbonate rocks. These rocks are classified on the basis of composition, texture, and the types of particles present
  • Limestone is a rock composed of calcite formed largely by biochemical processes, often including the building of hard parts by marine invertebrate organisms such as corals, clams, oysters. Many limestones are fossil-bearing.
  • Dolomite is a rock composed of dolomite, formed mainly by chemical replacement of calcite

Three features of sedimentary rocks are important, not only for classification, but also for interpretation.

• Composition of terrigenous sedimentary rocks mainly reflects the source of the sediment (bedrock and the climate which influences the type of weathering).
• Texture, mainly described by particle size, reflects the energy of the transporting medium and the environment of deposition.
• Sedimentary structures mainly include the type of bedding which reflects the agent of transport. Cross-bedding can be used to interpret the agent's energy, direction, and environment of deposition. For example, cross-beds inclined in opposite directions one on top of another may indicate deposition by tidal currents.
• (Note that for chemical sedimentary rocks, composition and texture reflects the environment (water depth, water temperature, water chemistry) of the basin of deposition which influences the organic and inorganic chemical processes.)

Sediments are deposited in one of three major groups of sedimentary environments: continental, marginal marine and open marine. Interpreting the environment of deposition is an important goal in the study of sedimentary rocks, because the environment of deposition is an important aspect of the geologic history of an area.

Throughout all of this discussion, three factors stand out that control the development of sedimentary rocks:

• Source
• Transport and depositional history
• Burial history

Source controls the compostion of siliciclastic, terrigenous sedimentary rocks. Transport and depositional history control the textures and sedimentary structures of siliciclastic rocks. Carbonate rocks are mainly influenced by factors in the environment of deposition, because particles of carbonate sediment are created chemically within the actual environment in which deposition occurs.

Metamorphic rocks

Metamorphic rocks form by some sort of change in a parent rock, brought on by various combinations of elevated temperature, pressure, and the presence of hot chemical fluids. Because these agents of metamorphism are generally present at depth in the earth's crust, metamorphism usually occurs in deeper crustal settings.

• Regional metamorphism requires elevated temperature together with pressure.
  • Pressure results in the formation of layering or foliation in the newly formed metamorphic rock.
  • The layering may either be defined by micas arranged in closely spaced planes, or defined by light and dark color bands of non-ferromagnesian and ferromagnesian minerals respectively.
  • Slate is a metamorphic rock with closely-spaced planes witn micas invisible to the naked eye and represents a low grade of metamorphism
  • Schist is a metamorphic rock with closely-spaced planes with micas visible to the naked eye and represents an intermediate grade of metamorphism
  • Gneiss is a metamorphic rock with color banding. Gneisses represent the highest grade of metamorphism (greatest temperatures and pressures and therefore greates depth of burial) - Montevideo Gneiss is one of oldest rocks in crust
  • Regional metamorphism occurs at convergent plate margins where plates are colliding with one another. Because mountains are uplifted on continents or island arcs at convergent margins, regionally metamorphosed rocks are present in the cores of these mountain ranges
• Contact metamorphism occurs at the contact between magma and the surrounding rock (country rock). Heat and the transfer of hot chemical fluids from the magma into the surrounding rocks metamorphoses the country rocks and brings about textural and/or compositional changes, but no foliation is produced because no directed pressure is involved.
• Non-foliated metamorphic rocks include quartzite (parent rock=quartz sandstone), marble (parent rock = limestone or dolomite) and hornfels (parent rock=shale). These rocks may be produced by regional metamorphism, but no foliation is developed because platy or sheet-like minerals do not form, neither do alternating light and dark bands of silicate minerals. These rocks may also be produced by contact metamorphism.

Metamorphic rocks are classified on the basis of the presence or absence of foliation, together with the size of the crystals. Composition is important in the classification of the non-foliated metamorphic rocks.