What makes rocks resistant to weathering

Water dissolves calcite more readily than it does feldspar, so calcite is considered to be more soluble than feldspar. Massive rocks like granite generally to not contain planes of weakness whereas layered sedimentary rocks have bedding planes that can be easily pulled apart and infiltrated by water. Weathering therefore occurs more slowly in granite than in layered sedimentary rocks. Rainfall and temperature can affect the rate in which rocks weather.

High temperatures and greater rainfall increase the rate of chemical weathering. Rocks in tropical regions exposed to abundant rainfall and hot temperatures weather much faster than similar rocks residing in cold, dry regions. Soils affect the rate in which a rock weathers. Soils retain rainwater so that rocks covered by soil are subjected to chemical reactions with water much longer than rocks not covered by soil.

Soils are also host to a variety of vegetation, bacteria and organisms that produce an acidic environment which also promotes chemical weathering. Minerals in a rock buried in soil will therefore break down more rapidly than minerals in a rock that is exposed to air. The longer a rock is exposed to the agents of weathering, the greater the degree of alteration, dissolution and physical breakup.

Lava flows that are quickly buried by subsequent lava flows are less likely to be weathered than a flow which remains exposed to the elements for long periods of time.

Chemical weathering is a process where minerals in a rock may be converted into clays, oxidized or simply dissolved. Silicates comprise almost all minerals in igneous rocks and are also important components in metamorphic rocks. Not all silicates, however, survive weathering processes to become incorporated into sedimentary rocks. Figure 6. For example, interlocking silicate grains in fresh granite gradually decay along crystal boundaries due to conversion to clays.

Eventually cracks open around the boundaries, the rock weakens and easily disintegrates. In certain places, such as Yellowstone National Park, the magma is as close as 40 miles below the surface.

On average, every feet you dig down into the earth, the temperature will increase about 1. Sometimes magma forces its way up to the surface through a vent such as a volcano and spills onto the surface.

This happened near Flagstaff, Arizona at Sunset Crater less than 1, years ago. Once magma comes out onto the surface of the earth it is called lava, and it cools rapidly at the surface. Extrusive igneous rocks can be distinguished by their small crystal sizes. In all extrusive igneous rock, it is nearly impossible to detect crystals without the aid of a microscope. The faster the rock cools, the smaller the crystals. Some extrusive igneous rocks cool so quickly that they have a glassy texture.

Common extrusive igneous rocks are: andesite, basalt, dacite, pumice, rhyolite, and obsidian. Intrusive igneous rocks solidify over a period of thousands of years. This slow cooling rate allows better development of mineral crystals.

Intrusive igneous rocks will have eye visible crystals and will appear coarse-grained. Igneous intrusive rocks eventually will become exposed at the surface of the earth by erosion of the overlying material. Common intrusive igneous rocks are: granite, diorite, gabbro, and peridotite.

The La Sal Mountains were formed by widespread igneous activity that began about 40 million years ago. Caldera explosions erupted thousands of cubic miles of volcanic rocks from several locations. Volcanoes spewed ash and lava. For 20 million years these extrusive volcanic rocks smoothed the landscape, filling depressions with accumulations of ash, flows, and debris literally miles thick.

These mostly pastel-colored extrusive rocks still blanket much of the high areas of central and southwestern Utah. Not all of the molten rising igneous material erupted as volcanic rocks; some material, along with its mineral-bearing fluids, congealed in the earth's crust. Several of these intruded masses having been exposed by erosion or encountered out by exploration drilling became great mining districts, such as at Alta, Brighton, Bingham, Park City, and Cedar City.

In the Colorado Plateau, bodies of intrusive rocks domed the overlying sedimentary rocks to form the Abajo and Henry Mountains as well as the La Sal Mountains. NOTE: This paragraph on laccoliths borrowed from here. Rocks which have undergone these sorts of changes are called metamorphic rocks. Agents of Matamorphosis. The three primary agents which metamorphose rock are temperature, pressure, and fluids. High temperatures can change rock by changing the structure of the minerals which make up the rocks; changing the structure of the minerals changes them into new minerals remember the definition of a mineral.

This increase in temperature with increase in depth is called the geotherm. Intrusions Another source of high temperatures inside the Earth is magma intruding cooler rock. These temperature increases are localized near the intrusion, but also metamorphose rock this is called contact metamorphism. Pressure or more properly, stress can also change rock. There are two main kinds I want you to know about: Confining Pressure Pressure due to the weight of overlying rock. This kind of pressure is roughly the same in all directions this is like water pressure when scuba diving , and is the kind which compacts rock during diagenesis.

Confining pressure changes rock by compaction and by changing the crystal structure of minerals from relatively open forms to more densely-packed forms. One mineral which does this is olivine, which changes from olivine isolated silica tetrahedra to spinel a much more tightly-bonded structure to perovskite a still more highly compressed structure. This kind of pressure is usually due to tectonic forces. It changes rocks by changing the structure of minerals and by changing the orientation of mineral grains, particularly platy minerals like mica or clay.

Fluids which metamorphose rock are not pore fluids remaining from when sedimentary rocks were deposited. Instead, they come from two main sources: hydrothermal fluids from magmatic intrusions and dehydration of minerals, like clay, which contain water in their structures hydrous minerals.

Whatever the source, fluids contain ions dissolved from other rock or from their original source. As fluids percolate through rocks, they can exchange ions with the existing minerals and thus change the chemical makeup of those minerals. The other way fluids change minerals is by hydrating minerals which previously did not contain water. Either way, fluids change the chemical makeup of minerals, turning them into new minerals, which changes the rocks which were made of the previous minerals.

This process of change by fluids is called metasomatism. Types of Rock Metamorphism. Some kinds of metamorphism: Burial Bury rocks deeply enough and they will warm up and change. This form of metamorphism is found anywhere where sediments and rocks are buried deeply, and should strike you as being pretty similar to diagenesis, which we discussed last time.

The line between diagenesis and burial metamorphism is fuzzy. Regional Caused by widespread moderate-to-high temperatures and pressures, as opposed to localized changes along faults or near magmatic intrusions. You find this type of metamorphism in mountain building regions and near subduction zone volcanism. Rocks that resist weathering remain at the surface and form ridges or hills. As the surrounding less resistant rocks were worn away, the resistant center of the volcano remained behind.

Different minerals also weather at different rates. Some minerals in a rock might completely dissolve in water, but the more resistant minerals remain. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock.

Climate is determined by the temperature of a region plus the amount of precipitation it receives. Climate is weather averaged over a long period of time.

Loess may also form in desert regions see Chapter Glacial sedimentation is very diverse, and generally consists of the most poorly-sorted sediment deposits found in nature. The main clast type is called diamictite , which literally means two sizes, referring to the unsorted mix of large and small rock fragments found in glacial deposits.

Many glacial till General term for very poorly sorted sediment that is of glacial origin. The surfaces of larger clasts typically have striations from the rubbing, scraping, and polishing of surfaces by abrasion during the movement of glacial ice. In addition to mineral composition and lithification process, geologists also classify sedimentary rock by its depositional characteristics, collectively called facies or lithofacies.

Geologists analyze sedimentary rock facies to interpret the original deposition environment, as well as disruptive geological events that may have occurred after the rock layers were established. It boggles the imagination to think of all the sedimentary deposition environments working next to each other, at the same time, in any particular region on Earth. The resulting sediment bed A specific layer of rock with identifiable properties. For example in the Grand Canyon, rock strata of the same geologic age includes many different depositional environments : beach sand, tidal flat silt, offshore mud, and farther offshore limestone.

In other words, each sedimentary or stratigraphic facies presents recognizable characteristics that reflect specific, and different, depositional environments that were present at the same time.

Facies may also reflect depositional changes in the same location over time. During periods of rising sea level, called marine transgression , the shoreline moves inland as seawater covers what was originally dry land and creates new offshore depositional environments. When these sediment bed A specific layer of rock with identifiable properties. Biological facies are remnants coal Former swamp-derived plant material that is part of the rock record.

The horizontal assemblage and vertical distribution of fossils are particularly useful for studying species evolution because transgression , deposition , burial, and compaction processes happen over a considerable geologic time range. During the Middle Cambrian period see Chapter 7, Geologic Time , regions around the Grand Canyon experienced marine transgression in a southeasterly direction relative to current maps.

This shift of the shoreline is reflected in the Tapeats Sandstone beach facies , Bright Angle Shale near- offshore mud facies , and Muav Limestone far- offshore facies. Marine organisms had plenty of time to evolve and adapt to their slowly changing environment; these changes are reflected in the biological facies , which show older life forms in the western regions of the canyon and younger life forms in the east.

Facies ties the classification of sedimentary rocks to the environment in which they formed. Which of these is the typical facies distribution along a shoreline? The typical facies distribution along a shoreline is sand on the beach, mud along the nearshore , and limes forming in the far offshore. Which of the following depositional environments is most likely to be at the lowest elevation? Paludal swamp deposition is commonly coastal or in other low elevation locations. On the dynamic Earth, shifting environments over time cause facies distributions that allow interpretation of ancient shifting shorelines.

In the Middle Cambrian strata of the Grand Canyon and surrounding regions, facies interpretation shows that, relative to modern directions, the ancient Cambrian ocean transgressed how?

Study of facies in the Cambrian Tapeats Sandstone , Bright Angel Shale , and Muav Limestone and equivalent rock strata in the region show a transgression of the Middle Cambrian ocean from northwest to southeast across the region during that ancient time. A stratum with a mix of sandstone and conglomerate sedimentary rocks with ripple marks, cross bed A specific layer of rock with identifiable properties.

Which of the following depositional environments is least likely to form fine-grained mud? Glacial environments typically produce sediments with a wide range of grain sizes. Which of the following depositional environments has the least water? Aeolian is transport and deposition by wind, without water involved. Sedimentary rocks are grouped into two main categories: clastic detrital and chemical.

Clastic detrital rocks are made of mineral clasts or sediment that lithifies into solid material. Sediment is produced by the mechanical or chemical weathering of bedrock and transported away from the source via erosion. Sediment that is deposited, buried, compacted, and sometimes cemented becomes clastic rock. Clastic rocks are classified by grain size ; for example sandstone is made of sand-sized particles. Chemical sedimentary rocks comes from minerals precipitated out an aqueous solution and is classified according to mineral composition.

The chemical sedimentary rock limestone is made of calcium carbonate. Sedimentary structures have textures and shapes that give insight on depositional histories.

Depositional environments depend mainly on fluid transport systems and encompass a wide variety of underwater and above ground conditions. Geologists analyze depositional conditions, sedimentary structures, and rock records to interpret the paleogeographic history of a region.

Use this quiz to check your comprehension of this chapter. What makes a shale different than a mudstone? Shale is fissile , meaning it has many thin layers. While there may be some detailed differences in environmental factors, the fundamental difference is layering. Which of the following is NOT part of the process of diagenesis and lithification of sediment into sedimentary rock? Melting belongs in another part of the rock cycle.

Imagine a new and previously unknown sedimentary structure is discovered in a deposit of sedimentary rock. What would be the best approach to try and understand it? Sedimentary structures are compared to their modern counterparts e. What is the most important thing that all sedimentary rocks can tell you, specifically regarding depositional environment? Study of sedimentary rocks including the environments in which they formed provides information about the ancient geography and landscapes on the Earth at that time.

Which is the correct order of grain sizes from smallest to largest for clastic detrital sedimentary rocks? Clay is the smallest, boulder is largest. What do chemical and detrital sedimentary rocks have in common? Think about it. Wind may have been the transporting medium for some detrital sediments , but water was involved in the weathering and lithification of all sedimentary materials. Good thinking. Soils make which essential element accessible to life? Nitrogen is an essential element for life, but is mainly found and supplied to life via soil A type of non-eroded sediment mixed with organic matter, used by plants.

What story does a sedimentary rock tell you? Study of sedimentary rocks and depositional environments can reconstruct past landscapes. What mainly causes agents of transport, like wind or water, to deposit sediment?

Typically, a slower speed of flow causes material to deposit. When the speed of flow of the medium drops below the settling velocity of a particle, it will settle out. Which property of water is most involved in chemical weathering? Because water is a universal solvent , it can chemically break down any rock, given enough time.

The House Range contains early Paleozoic marine rocks, highlighted by the Wheeler Formation, home to some of the best Cambrian fossils in Utah. Notch Peak contains one of the largest pure-vertical drops in North America at over feet. The water molecule consists of two hydrogen atoms covalently bond. Specific heat. Positive and negative side. Found more toward the poles. Dissolves better than normal water. More pure than normal water. Found more toward the equator.

What property of water causes it to form droplets as it rains? Hydrogen bonds. Specific gravity. A type of non-eroded sediment mixed with organic matter, used by plants. Valuable material in the Earth, typically used for metallic mineral resources.

Weathering describes the physical removal of sediments from one place to another, while erosion is the chemical breakdown that forms those sediments. Weathering is when oxygen attacks the rock, while erosion is when water freezes in the cracks causing the rocks to break apart into smaller pieces.

Weathering describes how rocks breakdown into smaller pieces, while erosion is the physical removal of those pieces to another location. Weathering describes how weather such as rain and temperature affects the rocks, while erosion is the physical deposition of sediment into a river. Weathering describes the physical removal of sediments from one place to another, while erosion is the physical breakdown that forms those sediments. Chemical weathering adds strength to mechanical weathering.

Chemical weathering creates surfaces for mechanical weathering to take place. Mechanical weathering creates surfaces for chemical weathering to take place. Chemical weathering adds speed to mechanical weathering. Mechanical weathering adds strength to chemical weathering. Which of these is NOT a component of soil.

Weathered rock. Eroded rock. Organic material humus. Oxidation rusting , dissolution , hydrolysis , and formation of soil. Empty space in a geologic material, either within sediments, or within rocks.

An atom or molecule that has a charge positive or negative due to the loss or gain of electrons. Porous variety of carbonate that form in relatively unheated water, sometimes as towers and spires. Spheres of calcite that form in saline waters with slight wave agitation. A topographic high found away from the beach in deeper water, but still on the continental shelf. Stronger bedrock. Lack of water in environment. Higher temperatures in environment. Longer distance transported. Shorter time since weathering.

Fossiliferous sandstone. Banded iron formation. Shale and sandstone are detrital even if they include fossils , banded iron formation is chemical, and coal. A large pile of sediment, deposited perpendicular to flow. Material filling in a cavity left by a organism that has dissolved away. Symmetrical ripple marks. Graded bedding. Asymmetrical ripple marks. Fissile shale. Which of these forms from a slower speed of flow?

Chutes and pools. Upper plane bed. Next fastest are cross bed. Which of these indicate changing water conditions, from wet to dry? Geopetal structures. Raindrop impressions.

Ripple marks. Sole marks. Place where rivers enter a large body of water, forming a triangular shape as the river deposits sediment and switches course. A rock layer that has been bent in a ductile way instead of breaking as with faulting. Movements of water rising and falling due to the gravity of the moon and sun.

Nearshore mud , Beach sand , Offshore limes. Beach sand , Paludal organic deposits , Nearshore mud. Offshore limes , Beach sand , Nearshore mud.

Paludal organic deposits , Nearshore mud , Offshore limes. Beach sand , Nearshore mud , Offshore limes. East to west. Northwest to southeast. North to south. Southeast to northwest. A stratum with a mix of sandstone and conglomerate sedimentary rocks with ripple marks, cross bed.

Lower shoreface. Upper shoreface. Chapter 5 Review Use this quiz to check your comprehension of this chapter. Different minerals. Darker colored. Depositional environment. Thinly bedded fissile. Examine the thickness of the layer. Look at the rocks above and below it. Look for microfossils within the layer. Find a similar structure in a modern environment. Examine the minerals found within the layer.

Types of organisms present. Ideas about temperature in the past. Speed of the river that made them. Ideas about ancient landscapes. Number of organisms present. Both involved water in their formation. Both were made by erosion. Both made by organisms. Both come from older bedrock. Both have rounded grains. Nitrogen is an essential element for life, but is mainly found and supplied to life via soil. Volume of the river that made them.

Types of volcanoes present. Increased carbon dioxide. Decreased transport speed. Decreased temperature. Increased weathering. Decreased weathering. Universal solvent. High boiling point. Less dense when solid. Affolter, M. Ayrton, H. Bagnold, R. Flow Regime part. Plane bed. Lower plane bed , flat laminations.

Small with respect to flow inclined layers dipping downflow. Larger inclined cross bed A specific layer of rock with identifiable properties. Flat layers, can include lined-up grains parting lineations. Hard to preserve reverse dune A large pile of sediment, deposited perpendicular to flow.

Erosional, not really a bedform ; rarely found preserved. Common Rock Types. Typical Fossils. Sedimentary Structures. Submarine fan. Continental slope.