Day 5: Erosion (Glaciers and Gravity) by 3/17 (A) 3/16 (B)
Introduction (Glaciers)
Glaciers cover about 10% of the land surface near Earth’s poles and they are also found in high mountains. During the Ice Ages, glaciers covered as much as 30% of Earth. Around 600 to 800 million years ago, geologists think that almost all of the Earth was covered in snow and ice. Scientists use the evidence of erosion and deposition left by glaciers to do a kind of detective work to figure out where the ice once was.
Formation and Movement of Glaciers
Glaciers are solid ice that move extremely slowly along the land surface (Figure below). Glacial ice erodes and shapes the underlying rocks. Glaciers also deposit sediments in characteristic landforms. The two types of glaciers are:
Glaciers cover about 10% of the land surface near Earth’s poles and they are also found in high mountains. During the Ice Ages, glaciers covered as much as 30% of Earth. Around 600 to 800 million years ago, geologists think that almost all of the Earth was covered in snow and ice. Scientists use the evidence of erosion and deposition left by glaciers to do a kind of detective work to figure out where the ice once was.
Formation and Movement of Glaciers
Glaciers are solid ice that move extremely slowly along the land surface (Figure below). Glacial ice erodes and shapes the underlying rocks. Glaciers also deposit sediments in characteristic landforms. The two types of glaciers are:
- Continental glaciers are large ice sheets that cover relatively flat ground. These glaciers flow outward from where the greatest amount of snow and ice accumulate.
- Alpine or valley glaciers flow downhill through mountains along existing valleys.
Glacial Erosion
Glaciers erode the underlying rock by abrasion and plucking. Glacial meltwater seeps into cracks of the underlying rock, the water freezes and pushes pieces of rock outward. The rock is then plucked out and carried away by the flowing ice of the moving glacier (Figure below). With the weight of the ice over them, these rocks can scratch deeply into the underlying bedrock making long, parallel grooves in the bedrock, called glacial striations.
Glaciers erode the underlying rock by abrasion and plucking. Glacial meltwater seeps into cracks of the underlying rock, the water freezes and pushes pieces of rock outward. The rock is then plucked out and carried away by the flowing ice of the moving glacier (Figure below). With the weight of the ice over them, these rocks can scratch deeply into the underlying bedrock making long, parallel grooves in the bedrock, called glacial striations.
Mountain glaciers leave behind unique erosional features. When a glacier cuts through a ‘V’ shaped river valley, the glacier pucks rocks from the sides and bottom. This widens the valley and steepens the walls, making a ‘U’ shaped valley (Figure below).
Smaller tributary glaciers, like tributary streams, flow into the main glacier in their own shallower ‘U’ shaped valleys. A hanging valley forms where the main glacier cuts off a tributary glacier and creates a cliff. Streams plunge over the cliff to create waterfalls (Figure below).
Up high on a mountain, where a glacier originates, rocks are pulled away from valley walls. Some of the resulting erosional features are shown: Figure below.
Depositional Features of Glaciers
As glaciers flow, mechanical weathering loosens rock on the valley walls, which falls as debris on the glacier. Glaciers can carry rock of any size, from giant boulders to silt (Figure below). These rocks can be carried for many kilometers for many years. These rocks with a different rock type or origin from the surrounding bedrock are glacial erratics. Melting glaciers deposit all the big and small bits of rocky material they are carrying in a pile. These unsorted deposits of rock are called glacial till.
As glaciers flow, mechanical weathering loosens rock on the valley walls, which falls as debris on the glacier. Glaciers can carry rock of any size, from giant boulders to silt (Figure below). These rocks can be carried for many kilometers for many years. These rocks with a different rock type or origin from the surrounding bedrock are glacial erratics. Melting glaciers deposit all the big and small bits of rocky material they are carrying in a pile. These unsorted deposits of rock are called glacial till.
Glacial till is found in different types of deposits. Linear rock deposits are called moraines. Geologists study moraines to figure out how far glaciers extended and how long it took them to melt away. Moraines are named by their location relative to the glacier:
- Lateral moraines form at the edges of the glacier as material drops onto the glacier from erosion of the valley walls.
- Medial moraines form where the lateral moraines of two tributary glaciers join together in the middle of a larger glacier (Figure below).
- Sediment from underneath the glacier becomes a ground moraine after the glacier melts. Ground moraine contributes to the fertile transported soils in many regions.
- Terminal moraines are long ridges of till left at the furthest point the glacier reached.
- End moraines are deposited where the glacier stopped for a long enough period to create a rocky ridge as it retreated. Long Island in New York is formed by two end moraines.
While glaciers dump unsorted sediments, glacial meltwater can sort and re-transport the sediments (Figure above). As water moves through unsorted glacial till, it leaves behind the larger particles and takes away the smaller bits of sand and silt (Figure below).
Introduction (Gravity)
Gravity shapes the Earth’s surface by moving weathered material from a higher place to a lower one. This occurs in a variety of ways and at a variety of rates including sudden, dramatic events as well as slow steady movements that happen over long periods of time. The force of gravity is constant and it is changing the Earth’s surface right now.
Types of Movement Caused by Gravity
Movements caused by gravity are together referred to as mass wasting or mass movement. Weathered material may fall away from a cliff because there is nothing to keep it in place. Rocks that fall to the base of a cliff make a talus slope (Figure below). Sometimes as one rock falls, it hits another rock, which hits another rock, and begins a landslide.
Gravity shapes the Earth’s surface by moving weathered material from a higher place to a lower one. This occurs in a variety of ways and at a variety of rates including sudden, dramatic events as well as slow steady movements that happen over long periods of time. The force of gravity is constant and it is changing the Earth’s surface right now.
Types of Movement Caused by Gravity
Movements caused by gravity are together referred to as mass wasting or mass movement. Weathered material may fall away from a cliff because there is nothing to keep it in place. Rocks that fall to the base of a cliff make a talus slope (Figure below). Sometimes as one rock falls, it hits another rock, which hits another rock, and begins a landslide.
Landslides and Avalanches
Landslides and avalanches are the most dramatic, sudden, and dangerous examples of earth materials moved by gravity. Landslides are sudden falls of rock, whereas avalanches are sudden falls of snow.
When large amounts of rock suddenly break loose from a cliff or mountainside, they move quickly and with tremendous force (Figure below). Air trapped under the falling rocks acts as a cushion that keeps the rock from slowing down. Landslides and avalanches can move as fast as 200 to 300 km/hour.
Landslides and avalanches are the most dramatic, sudden, and dangerous examples of earth materials moved by gravity. Landslides are sudden falls of rock, whereas avalanches are sudden falls of snow.
When large amounts of rock suddenly break loose from a cliff or mountainside, they move quickly and with tremendous force (Figure below). Air trapped under the falling rocks acts as a cushion that keeps the rock from slowing down. Landslides and avalanches can move as fast as 200 to 300 km/hour.
Landslides are exceptionally destructive. Homes may be destroyed as hillsides collapse. Landslides can even bury entire villages. Landslides may create lakes when the rocky material dams a stream. If a landslide flows into a lake or bay, they can trigger a tsunami (Figure below).
Landslides often occur on steep slopes in dry or semi-arid climates. The California coastline, with its steep cliffs and years of drought punctuated by seasons of abundant rainfall, is prone to landslides. At-risk communities have developed landslide warning systems. Around San Francisco Bay, the National Weather Service and the U.S. Geological Survey use rain gauges to monitor soil moisture. If soil becomes saturated, the weather service issues a warning. Earthquakes, which may occur on California’s abundant faults, can also trigger landslides.
Mudflows and Lahars
Added water creates natural hazards produced by gravity (Figure below). On hillsides with soils rich in clay, little rain, and not much vegetation to hold the soil in place, a time of high precipitation will create a mudflow. Mudflows follow river channels, washing out bridges, trees, and homes that are in their path.
Mudflows and Lahars
Added water creates natural hazards produced by gravity (Figure below). On hillsides with soils rich in clay, little rain, and not much vegetation to hold the soil in place, a time of high precipitation will create a mudflow. Mudflows follow river channels, washing out bridges, trees, and homes that are in their path.
A lahar is mudflow that flows down a composite volcano (Figure below). Ash mixes with snow and ice melted by the eruption to produce hot, fast-moving flows. The lahar caused by the eruption of Nevado del Ruiz in Columbia in 1985 killed more than 23,000 people.
Slump and Creep
Less dramatic types of downslope movement move earth materials slowly down a hillside. Slump moves materials as a large block along a curved surface (Figure below). Slumps often happen when a slope is undercut, with no support for the overlying materials, or when too much weight is added to an unstable slope.
Less dramatic types of downslope movement move earth materials slowly down a hillside. Slump moves materials as a large block along a curved surface (Figure below). Slumps often happen when a slope is undercut, with no support for the overlying materials, or when too much weight is added to an unstable slope.
Creep is the extremely gradual movement of soil downhill. Curves in tree trunks indicate creep because the base of the tree is moving downslope while the top is trying to grow straight up (Figure below). Tilted telephone or power company poles are also signs of creep.
Contributing Factors
There are several factors that increase the chance that a landslide will occur. Some of these we can prevent and some we cannot.
Water
A little bit of water helps to hold grains of sand or soil together. For example, you can build a larger sand castle with slightly wet sand than with dry sand. However too much water causes the sand to flow quickly away. Rapid snow melt or rainfall adds extra water to the soil, which increases the weight of the slope and makes sediment grains lose contact with each other, allowing flow.
Rock Type
Layers of weak rock, such as clay, also allow more landslides. Wet clay is very slippery, which provides an easy surface for materials to slide over.
Undercutting
If people dig into the base of a slope to create a road or a homesite, the slope may become unstable and move downhill. This is particularly dangerous when the underlying rock layers slope towards the area (Figure below).
There are several factors that increase the chance that a landslide will occur. Some of these we can prevent and some we cannot.
Water
A little bit of water helps to hold grains of sand or soil together. For example, you can build a larger sand castle with slightly wet sand than with dry sand. However too much water causes the sand to flow quickly away. Rapid snow melt or rainfall adds extra water to the soil, which increases the weight of the slope and makes sediment grains lose contact with each other, allowing flow.
Rock Type
Layers of weak rock, such as clay, also allow more landslides. Wet clay is very slippery, which provides an easy surface for materials to slide over.
Undercutting
If people dig into the base of a slope to create a road or a homesite, the slope may become unstable and move downhill. This is particularly dangerous when the underlying rock layers slope towards the area (Figure below).
When construction workers cut into slopes for homes or roads, they must stabilize the slope to help prevent a landslide (Figure below). Trees roots or even grasses can bind soil together. It is also a good idea to provide drainage so that the slope does not become saturated with water.
Ground shaking
An earthquake, volcanic eruption, or even just a truck going by can shake unstable ground loose and cause a slide. Skiers and hikers may disturb the snow they travel over and set off an avalanche.
Questions
An earthquake, volcanic eruption, or even just a truck going by can shake unstable ground loose and cause a slide. Skiers and hikers may disturb the snow they travel over and set off an avalanche.
Questions
- How much of the Earth’s land surface is covered by glaciers today? Where are they found?
- What is the shape of a valley that has been eroded by rivers? How does a glacier change that shape and what does it become?
- How do glaciers erode the surrounding rocks?
- Describe three ways that gravity moves materials.
- What natural events and human actions can trigger a landslide or avalanche?
- What can people do to help prevent landslides or mudflows?