Water movement on Earth's surface

Water moves across Earth's surface in ways that shape the landscape. This movement starts with runoff, which is water that flows over the ground after rain or snowmelt instead of soaking in. Runoff collects and forms streams and rivers, carrying water downhill due to gravity.

How runoff creates streams and rivers

• Collection of water - Rain or melted snow flows over surfaces that can't absorb it, like rocky or saturated ground
• Channel formation - As runoff gathers, it creates small channels that grow into streams
• River development - Streams join together, forming larger rivers that flow toward oceans or lakes
• Landscape impact - This flowing water constantly reshapes the land by picking up and moving soil and rocks

These surface movements are key to understanding how water interacts with the ground to cause changes over time.

Water movement underground

Not all water stays on the surface—some soaks into the ground and becomes groundwater. Groundwater is water that fills spaces in soil and rocks below Earth's surface. It moves slowly through these spaces, driven by gravity and the structure of the underground layers.

Groundwater flow and cave formation

Groundwater flows through porous (full of small holes) soil and rock, sometimes dissolving materials along the way. This process can create underground features.

How caves form:

1. Infiltration - Water from rain or rivers seeps into the ground through cracks and pores in soil and rock
2. Flow through spaces - It moves downward and sideways, filling empty spaces in underground layers
3. Chemical dissolution - In rocks like limestone, groundwater mixes with carbon dioxide to form a weak acid that dissolves the rock over time
4. Cave formation - Dissolved rock creates empty spaces that enlarge into caves as more material is removed

Caves form slowly through this chemical process, often taking thousands of years.

Features inside caves

Inside caves, groundwater creates special structures as it drips and deposits minerals.

Stalactites and stalagmites:

• Stalactites - These hang from cave ceilings like icicles. They form when mineral-rich water drips down and leaves behind tiny deposits that build up over time
• Stalagmites - These grow upward from cave floors. They develop where water drips hit the ground and deposit minerals, slowly building a column

Both features result from the same process of water evaporating and leaving minerals behind, but their positions differ based on where the deposits occur.

Weathering processes that break down rocks

Weathering is the process that breaks down rocks into smaller pieces without moving them. It happens through physical and chemical means, often involving water. This prepares rocks for later erosion.

Physical weathering: freeze-thaw cycles

Freeze-thaw weathering occurs in cold areas where water enters rock cracks and expands when it freezes.

How freeze-thaw weathering works:

1. Water entry - Rain or melted snow seeps into small cracks in rocks
2. Freezing - When temperatures drop, the water freezes and expands by about 9%, pushing against the rock walls
3. Crack widening - The expansion creates pressure that widens the cracks
4. Thawing and repetition - When it thaws, water flows deeper into the larger cracks, and the cycle repeats until pieces break off

This process gradually weakens and breaks apart rocks over many cycles.

Chemical weathering: acidic rainwater dissolving minerals

Chemical weathering changes the makeup of rocks through reactions, often with water.

How chemical weathering works:

1. Acid formation - Rainwater absorbs carbon dioxide from the air, forming a weak acid called carbonic acid
2. Contact with rocks - This acidic water touches rocks, especially those with minerals like calcium carbonate
3. Dissolution - The acid reacts with and dissolves the minerals, breaking down the rock structure
4. Weakening over time - Repeated exposure causes the rock to crumble as minerals are removed

This type of weathering is common in wet climates and affects rocks like limestone.

Erosion and the transport of weathered material

Erosion differs from weathering because it involves the removal and transport of broken-down material. Flowing water is a main agent of erosion, picking up weathered bits and carrying them away.

How erosion works

• Pickup of material - Flowing water in streams and rivers loosens and lifts weathered rock fragments, soil, and sediment
• Transport - The water carries this material downstream, with faster flows moving larger pieces
• Deposition - When water slows, it drops the material in new places, reshaping the landscape
• Comparison to weathering - While weathering breaks rocks in place, erosion moves the pieces, often leading to further breakdown during transport

Erosion constantly changes Earth's surface by wearing down high areas and building up low ones.

Formation of landforms through weathering and erosion

Weathering and erosion work together to create various landforms, reshaping landscapes over time. Flowing water plays a key role in carving and depositing materials.

Key landforms created by these processes

• Valleys - Rivers erode downward and sideways, cutting deep channels with V-shaped sides through weathering and material removal
• Canyons - Similar to valleys but deeper and steeper, formed by powerful rivers eroding resistant rock over millions of years
• Deltas - At river mouths, slowing water deposits sediment, building fan-shaped land where rivers meet larger bodies of water
• Floodplains - During floods, rivers overflow and deposit fertile soil across flat areas beside them, creating wide, level plains

Continuous landscape reshaping

These processes happen continuously, with weathering preparing material and erosion moving it. Over time, mountains wear down, rivers change course, and new landforms emerge, all driven by water's persistent action.