9 - Understanding Landscapes#
9.1 - What Materials Compose Landscapes?#
A mineral is a naturally occurring, inorganic solid with an ordered (crystalline) internal structure and a relatively consistent chemical composition.
The most common family of minerals is silicate minerals, which account for over 90% of the mineral’s in Earth’s crust. Most of these minerals contain (SiO\(_4\))\(^{-4}\). Common silicate minerals include:
Quartz (SiO\(_2\)) is a relatively hard mineral that is generally transparent to white.
Feldspar is a very common mineral that comes in two varieties: potassium feldspar and plagioclase feldspar.
Mica is a family of minerals that break into sheets and flakes.
Mafic minerals contain magnesium and iron in addition to silicon and oxygen.
Minerals that do not contain silicon are called nonsilicate minerals, and and this group includes salt. Common nonsilicate minerals include:
Carbonate minerals contain carbon and oxygen bonded with other elements.
Oxide minerals contain oxygen bonded with a metallic element.
Salt minerals include several families, including halite (NaCl) and gypsum.
Sulfide minerals contain sulfur bonded with a metallic element.
9.2 - How Do Rocks Form?#
Loose material formed by the breaking and wearing away of other rocks in the landscape is called sediment. These may be formed around glaciers, steep mountain fronts, sand dunes, beaches, stream channels, and more.
Changing a rock by heat, pressure, or deformation is the process of metamorphism.
9.3 - What Can Happen to a Rock?#
A rock on the surface interacts with sunlight, rain, wind, plants, and animals. As a result, it may be mechanically broken apart or altered by chemical reactions via the process of weathering.
Rock pieces loosened or dissolved by weathering can be stripped away by erosion and moved away from their source.
When the kinetic energy associated with transportation stops, water, wind, and ice deposit their sediment.
In the process of lithification, sediment becomes sedimentary rock by being compacted.
After a rock forms, strong forces can squeeze the rock and fold its layers in a process called deformation.
A rock exposed to high temperatures may melt and form magma.
The process of magma cooling to become rock is called solidification. If crystals form during solidification, it is called crystallization.
A rock may be uplifted to the surface where it is exposed to weathering.
9.4 - What Are Some Common Sedimentary Rocks?#
Clastic rocks are sedimentary rocks that are composed of rock and mineral pieces, called clasts. We describe and classify clastic sedimentary rocks based primarily on the size of clasts, along with other aspects such as clast roundness.
Gravel-sized clasts:
Conglomerate has large, rounded clasts (pebbles, cobbles, or boulders) with sand and other fine sediment between the large clasts.
Breccia is similar to conglomerate except that the clasts are angular and the clasts range in size.
Sand-sized clasts:
Sandstone consists of sand-sized grains. It can contain some larger and smaller clasts, but it is mostly composed of sand.
Mud-sized clasts:
Shale consists mostly of fine-grained clasts. The minerals are aligned, so the rock breaks in sheetlike pieces or chips.
Nonclastic rocks are sedimentary rocks not composed of clasts.
Rock salt refers to rocks mostly composed of the mineral halite (NaCl).
Gypsum refers to both a mineral and rocks composed mostly of the mineral.
Limestone is mostly made of calcium carbonate, which forms when shells and coral skeletons are cemented together. Many limestones contain fossils.
Chalk is a soft, very fine-grained limestone that forms from the accumulation of dead microscopic organisms.
Chert is a silica-rich rock that can form in a variety of ways.
Iron formation is a rock composed of thick layers of iron oxide. Most iron formations precipitated from seawater early in Earth’s history, and it is the main source of iron used in steel.
Coal is formed from wood and other plant parts that have been buried, compacted, and heated enough to drive off most of their water and oxygen.
9.6 - What Are Metamorphic Processes and Rocks?#
For a rock to be metamorphosed, it must be subjected to conditions of temperature, pressure, and fluid chemistry that make it unstable.
Contact metamorphism is caused by local heating by magma, typically without deformation.
Regional metamorphism involves deformation along with heating over a broader region.
During metamorphism, movement of chemical elements can cause existing minerals to grow larger or form new minerals. If minerals are arranged in lines, the rock has lineation. If they are arranged in planes, the rock has foliation.
When shale is metamorphosed at a low to moderate temperature, it can develop cleavage and become slate.
At slightly higer temperatures, it becomes phyllite.
At higher temperatures, it becomes schist.
At even higher temperatures, it becomes gneiss.
When sandstone is metamorphosed, it becomes quartzite.
At higher temperatures, it can become a coarser-grained quartzite.
Limestone becomes marble at low temperatures.
At moderate temperatures, impurities in limestone may produce impurities.
9.7 - How Are Different Rock Types Expressed in Landscapes?#
The unifying feature of most sedimentary rocks is the presence of visible layers.
Some sandstone layers are resistant to erosion and appear massive from a distance because they have little variation of grain size. Sandstone layers may differ in color, grain size, and composition of grains.
Shale and other fine-grained clastic sedimentary rocks are somewhat easily eroded. Where exposed, these rocks typically form soft slopes covered by small, loose chips. The resulting formations are known as badlands.
Limestone is very soluble, so in wet climates it dissolves and weathers quickly, forming caves, sinkholes, pits, and depressions. The resulting terrain is known as karst terrain.
Igneous rocks form from magma, either from magma that solidifies below the surface, forming intrusive (plutonic) rocks, or magma that erupts onto the surface, forming extrusive (volcanic) rocks.
Most granite is fractured, and becomes rounded when weathered in a process called spheroidal weathering.
Volcanic rocks commonly have some sort of layers. Basaltic lava flows are the most common type of volcanic rock in most landscapes.
Many volcanic units have distinctive columnar fractures called columnar joints.
Rocks that have been metamorphosed by increased temperature and pressure are metamorphic rocks.
Metamorphic rocks commonly have mineral-filled fractures, called veins.
Metamorphic rock generally forms platy, jagged outcrops in the terrain.
9.8 - What Controls the Appearance of Landscapes?#
Geomorphologists are physical geographers and geologists who study the spatial pattern, form, and evolution of landforms.
9.11 - How Does Wind Help Shape Landscapes?#
Moderately strong winds can roll or slide grains of sand and other loose materials across the ground.
Strong winds can lift grains, carry them short distances, and drop them. This process, called saltation, is akin to bouncing a grain along the surface.
Wind can pick up and carry finer material, such as dust, silt, and salt, in a process called suspension.
9.12 - What Features Record Transport and Deposition of Material by the Wind?#
There are different types of sand dunes, each of which forms with a shape specific to the wind directions:
A crescent dune or barchan dune is a common dune type with a crescent shape. The tails point in the direction of the prevailing wind.
Transverse dunes are linear or gently curved, and can be many kilometers long and hundreds of meters high. They are perpendicular to the prevailing wind direction.
Longitudinal dunes are a type of linear dune that form parallel to the overall wind direction.
Composite dunes are a combination of several different types.
Star dunes have variably trending sand ridges radiating out from a central peak. They form where wind directions are highly variable.
Parabolic dunes have a curved or parabolic shape, but they form with tails facing into the wind. They form when wind removes an area of vegetation.