🏝️Earth Science Unit 3 Review

Cirque glaciers form in bowl-shaped depressions on mountainsides. These are the smallest type of alpine glacier.
Valley glaciers flow down pre-existing river valleys, often extending for many kilometers.
Piedmont glaciers form when valley glaciers spill out onto flatter plains at the base of mountains, spreading into broad lobes.

Continental glaciers (also called ice sheets) are far larger and can cover entire landmasses. The two present-day ice sheets are in Antarctica and Greenland. The Antarctic ice sheet alone covers about 14 million square kilometers.

Characteristics and Formation of Glaciers

Glaciers are massive bodies of ice that persist year-round and flow under their own weight due to gravity. They form through a multi-step process:

Snow accumulates in an area where more falls each year than melts.
Over time, the weight of new snow compresses the layers beneath, squeezing out air.
The compressed snow gradually transforms into dense, crystalline glacial ice.

Once thick enough, glaciers begin to move downslope or outward from their source. This movement is slow, typically a few centimeters to a few meters per day. Speed depends on the thickness of the ice, the steepness of the slope, and the temperature at the base of the glacier (warmer bases allow faster sliding).

Glacial Erosion and Deposition

Processes of Glacial Erosion

Glaciers reshape the landscape through three main erosion processes:

Plucking happens when the glacier freezes onto bedrock and rips out chunks of rock as it moves.

Water seeps into cracks in the bedrock and freezes, expanding and loosening the rock.
As the glacier advances, it pulls out those loosened fragments, leaving behind jagged, irregular surfaces.

Abrasion occurs when rocks and debris embedded in the base of the glacier scrape against the bedrock beneath, like sandpaper on wood.

This creates smooth, polished surfaces and parallel scratches called striations.
Geologists use striations to determine the direction a glacier was moving.

Freeze-thaw action (also called frost wedging) happens when water seeps into cracks in rock, freezes, and expands, breaking the rock apart. Repeated cycles of freezing and thawing progressively weaken the bedrock, making it easier for the glacier to erode.

Types of glaciers, 16.3 Glacial Erosion – Physical Geology

Glacial Deposition and Landforms

When a glacier melts, it drops the sediment it has been carrying. This process creates several distinctive landforms.

Moraines are ridges of deposited sediment, classified by their position relative to the glacier:
- Lateral moraines form along the sides of the glacier.
- Medial moraines form where two glaciers merge, combining their lateral moraines.
- Terminal moraines mark the farthest point the glacier reached.
Drumlins are elongated, teardrop-shaped hills made of glacial till (unsorted sediment deposited directly by ice). They form beneath the glacier and point in the direction of ice flow, making them useful indicators of past glacier movement.
Eskers are long, winding ridges of sand and gravel deposited by meltwater streams that flowed beneath or within the glacier. They can stretch for kilometers and are sometimes used as natural roadbeds.

Landforms of Glacial Activity

Erosional Landforms

Glaciers carve a recognizable set of landforms into mountainous terrain. These features are often connected to each other.

Cirques are bowl-shaped depressions carved into mountainsides. They have steep headwalls and flat or overdeepened floors. After the glacier melts, a small lake called a tarn often fills the depression. Cirques are where alpine glaciers begin.
Arêtes are sharp, knife-edged ridges that form when two cirques erode back-to-back into the same mountain. The Garden Wall in Glacier National Park, Montana, is a well-known example.
Horns are steep, pyramid-shaped peaks that form when three or more cirques erode into the sides of a single mountain. The Matterhorn in the Swiss Alps is the classic example.
U-shaped valleys are carved by valley glaciers that widen and deepen pre-existing river valleys. They have steep, parallel walls and a broad, flat floor. Compare this to the narrow, V-shaped profile of a river valley. Yosemite Valley in California is a famous U-shaped valley.
Hanging valleys are smaller tributary valleys left elevated above the main glacial valley. The main glacier eroded its valley much deeper than the smaller tributary glaciers could. Streams in hanging valleys often cascade down as waterfalls, like Bridal Veil Falls in Yosemite.

Depositional Landforms

The same landforms described in the deposition section above also appear across glaciated landscapes at a regional scale. A few real-world examples help show the scale of these features:

The Outer Lands of Cape Cod, Massachusetts, are a terminal moraine complex left behind by the last ice age.
The Drumlin Field in Wisconsin contains thousands of drumlins formed during the most recent glaciation.
The Denali Highway in Alaska follows the path of a large esker deposited by glacial meltwater.

These features are some of the strongest evidence geologists use to map the extent of past ice sheets.

Causes and Effects of Ice Ages

Causes of Ice Ages

Ice ages result from a combination of factors that reduce the amount of heat Earth retains.

Milankovitch cycles are variations in Earth's orbit that change how much solar energy different parts of the planet receive. There are three components:

Eccentricity: Earth's orbit shifts from nearly circular to more elliptical over a ~100,000-year cycle.
Obliquity: The tilt of Earth's axis varies between 22.1° and 24.5° over a ~41,000-year cycle. Greater tilt means more extreme seasons.
Precession: Earth's axis wobbles like a spinning top, affecting which hemisphere is tilted toward the sun during summer. This cycle takes about 26,000 years.

Changes in atmospheric composition also play a role. Lower concentrations of greenhouse gases like $CO_2$ and $CH_4$ allow more heat to escape into space, contributing to global cooling.

Variations in solar output can influence climate as well. For example, decreased solar activity during the Maunder Minimum (1645–1715) coincided with an unusually cold period in Europe.

Effects of Ice Ages

Ice ages transform Earth's surface, climate, and life in dramatic ways.

Sea levels drop because enormous amounts of water become locked in ice sheets. During the last ice age, sea levels were roughly 120 meters lower than today. This exposed land bridges like the Bering Land Bridge, which connected Asia and North America and allowed humans and other species to migrate between continents.
Ecosystems shift as colder temperatures and reduced precipitation cause grasslands and tundra to expand while forests shrink. Some species adapt; others go extinct due to habitat loss.
Positive feedback loops amplify the cooling. The most important is the ice-albedo feedback: as ice sheets expand, they reflect more sunlight back into space (higher albedo), which cools the planet further, which allows more ice to form. This cycle continues until other factors, like rising greenhouse gas levels, eventually reverse the trend.

Studying Past Ice Ages

The Pleistocene Epoch (about 2.6 million to 11,700 years ago) saw multiple glacial-interglacial cycles. The most recent glacial maximum occurred roughly 26,500 to 19,000 years ago, when ice sheets covered much of North America and northern Europe.

Scientists reconstruct past ice ages using several methods:

Ice cores drilled from glaciers in Greenland and Antarctica trap tiny bubbles of ancient atmosphere, providing records of past temperature and $CO_2$ levels going back hundreds of thousands of years.
Glacial landforms and sediments reveal the extent and movement patterns of past ice sheets.
Fossil records show how ecosystems and species responded to changing climate conditions.

By studying the causes and cyclical nature of past glaciations, researchers can improve climate models and better anticipate how current changes in greenhouse gas concentrations may affect Earth's climate in the future.

🏝️Earth Science Unit 3 Review