waethring and ersion Flashcards
Mechanical weathering breaks down rocks without changing their composition.
Mechanical weathering breaks down rocks without changing their composition.
Water, ice, air, and living things can break rocks into tiny pieceswmechanically weather themwin several ways:
ExpansionwMaterials seep into the cracks, expand, and force the rocks apart, thereby breaking them.
Pressure releasewWhen overlying rock layers are removed, pressure on the underlying rock is relieved.
The outer layers of the underlying rock expand more than the inner layers. This expansion separates the rock into onionlike layers, which is a process called sheeting.
Chemical weathering changes the composition of the rocks, thereby breaking them apart.
Chemical weathering changes the composition of the rocks, thereby breaking them apart.
If you place an iron nail in a glass of water and let it sit for a while, what will happen? It will rust, won’t it? Similarly, when iron-containing rocks are exposed to the air and get wet, oxygen in the air combines with the iron to make iron oxide (rust), which is different from the iron.
Acids will also change the composition of rocks. For example, carbonic acid (formed from carbon dioxide and water vapor in the atmosphere) will combine with calcium in limestone (calcium carbonate) to make calcium bicarbonate. In the atmosphere, water vapor also combines with sulfur dioxide to make sulfuric acid, and nitrogen can form nitrous acids. These acids are all part of acid rain, which weathers rocks.
Living things can weather rocks by both mechanical and chemical means.
Living things can weather rocks by both mechanical and chemical means.
Living things grow on and burrow into rocks. In doing so, they can break down the rocks mechanically. For example, tree roots can grow into cracks in rocks and force them apart. Burrowing animals can dig and remove rocky material. Snails can scrape over rock surfaces to eat algae growing on the rocks. The snail’s hard groovelike teeth, called radula, can scour rock surfaces like sandpaper.
Living things can also chemically weather rocks. For example, tree roots and some mollusks secrete acids. The acids chemically break down the minerals in the rocks.
Erosion moves weathered rock pieces to other places.
Erosion moves weathered rock pieces to other places.
Once weathering loosens rock pieces, several agents can erode them, or move them from one place to another. These agents of erosion include water, gravity, ice, and air.
rivers and streams erode narrow valleys into broad floodplains.
rivers and streams erode narrow valleys into broad floodplains.
With stream erosion, the stream valley deepens and widens and the stream begins to meander, or bend and turn. In a bend, the water flows faster on the outside than on the inside, so the stream erodes the outer banks of each turn and deposits sediment on the inner banks of each turn. The meandering stream eventually cuts a wide valley with a near-level floodplain.
Water can erode underground rocks to form caves.
Water can erode underground rocks to form caves.
When we think of flowing water, we usually think of rivers and streams. But groundwater flows between spaces in the soil and rocks underground and can erode the rocks there. Groundwater often chemically weathers and erodes underground limestone to form caves and caverns. Within these caves and caverns, the dissolved limestone and rocks deposit to form beautiful stalactites (icicle-like rock formations hanging from the ceiling) and stalagmites (similar formations rising from the floor).
Glaciers can cut great valleys.
Glaciers can cut great valleys.
When mountaintop snows from previous winters do not completely melt before new snowfalls arrive, a glacier forms. Eventually large sheets of ice develop as the top layers compress the snow beneath. The glacier grows and gravity pulls its growing edges down the mountainside. When the climate warms, the glacier melts and retreats. The edges of the melting ice deposit the debris that was held in the frozen glacier. An area of accumulated deposits from a glacier is called a moraine.
Wave action and storms continuously reshape shorelines.
Wave action and storms continuously reshape shorelines.
Waves constantly pound the shorelines on beaches. Wave action mechanically weathers rocks to make sand, and currents of flowing water transport the sand from one place on the coastline to another. This movement along the shoreline erodes beaches in one place and nourishes them in another.
The shifting sands create problems for developers who build on barrier islands on the Atlantic coast. The storm surges from severe storms and hurricanes can also erode vast portions of these islands in a short time.
Agents of erosion deposit their weathered fragments when they weaken.
Agents of erosion deposit their weathered fragments when they weaken.
Water, ice, and air carry weathered rock fragments in suspension. When water slows down, it releases fragments out of suspension that get deposited, usually in sediments. When winds run into trees, high dunes, or mountains, they deposit the dust they carry. When glaciers melt and retreat, they leave behind the rocks carried within them. In all of these examples, the deposits build up to form new featureswsediments, dunes, and deltas.
As streams slow down, different environments of deposition form.
As streams slow down, different environments of deposition form.
There are different environments of deposition in areas where a river begins in high mountains and runs its course to the sea. Bangladesh, a country occupying the region between the Himalayas and the Indian Ocean, is one example of just such an environment.
Rivers that flow from the high mountains have a lot of energy to erode sediment, but they lose energy as they approach the sea and different types of sediment are deposited.
Weathering and erosion continuously change the earth’s surface.
Weathering and erosion continuously change the earth’s surface.
External forces such as water, ice, air, and living things can weather rocks through mechanical and chemical means. Mechanical weathering does not change the composition of the rocks, but chemical weathering does.
Erosion transports weathered fragments over great distances. When the forces of erosionwwater, wind, and icewweaken, they deposit the fragments that build new geologic formations such as river deltas, glacial valleys, and new barrier islands.
