Changes on Earth's surface. Earth is a dynamic planet with topography that is constantly changing. Landforms are continually built up or torn down.

Earth's topography is built up by mountain building and other tectonic processes. Mountains are created at where two continental plates converge. Volcanoes grow upward at subduction zones or at hotspots.

Earth's topography is reshaped by the weathering of rock and soil and by the transportation and deposition of sediment.

Both building up and tearing down landforms can be so slow they are not easily observable. These processes can also be so fast they are dangerous.

Stratovolcano. A stratovolcano builds as eruptions of ash and lava pile up on top of each other.

Weathering. Weathering processes break down rocks into smaller sediments like pebbles, sand, silt and mud.

Weathering processes can be physical (mechanical) or chemical. Physical weathering does not alter the chemical makeup of a material; it just breaks the material down into smaller pieces. Chemical weathering changes the chemical composition of the material.

An example of physical weathering is the freezing and thawing of water in cracks in a rock to break it apart. The breakdown of rocks by plant roots is another.

In chemical weathering, gases in the atmosphere interact with the rock material. The change of iron to iron oxide (rust) due to the addition of oxygen is a well known example of chemical weathering. Water and plant materials can also contribute to chemical weathering.

Salt crystal weathering. In arid environments, salty water seeps into cracks in a rock. The water evaporates, leaving behind the salt crystals. When the salt crystals warm on a hot day, they expand and exert pressure on the rock surrounding them. Over time, this process can wedge chunks of rock apart.

Erosion. Rocks, minerals, soil and other particles that have been made small enough are then removed by erosion. As erosion removes material, it alters the shape of landforms.

Running water, ocean waves, wind, and ice are the agents of erosion. Gravity is also an agent of erosion since it causes landslides. All of these erosional agents are active throughout California and the rest of the world in shaping landscapes.

Some erosion is extremely rapid, like landslides and explosive volcanic eruptions.

Most erosion takes place very slowly. Some erosion is so slow that it can only be “seen” with instruments; it is not observable. Soil sliding imperceptible down a hill is an example of s slow erosional process known as creep.

Slow processes may have a greater effect on the surface over the long term than rapid and dramatic processes.

Creep. The bend in these tree trunks show that the soil is very slowly creeping downhill.

Stream erosion. Water running downhill is the dominant agent of erosion.

Running water has enough energy that it can pick up and carry sediment. The energy of a stream depends on its slope and the volume-flow of water. Steeper and fuller streams have more energy for erosion. The amount of water in a stream usually varies seasonally with larger volumes in spring and snow melts and rains fall. The faster a stream moves, the larger the sediment it can carry.

Water removes the material from landforms and reshapes them. Erosion can lower mountains and cut valleys.

Sediment carried by a stream can abrade a rock at the bottom or sides of the stream. Similarly, a rock at the base of a glacier will gouge a trail into the rock beneath the ice to show the direction the glacier is moving.

Grand Canyon. The Grand Canyon is a good example of erosion by a stream, in this case the Colorado River.

Glacial erosion. Glaciers are not as common an erosional force as streams, but they are immensely powerful and leave behind tremendous reminders of their handiwork.

In cold times, such as during an ice age, mountain glaciers may travel down valleys. These glaciers carve away enormous amounts of rock. Some of the features they leave behind as evidence are U-shaped valleys, and steep cliffs with waterfalls where rivers are left hanging.

Yosemite Valley. The Sierra Nevada Mountains of California have tremendous evidence of erosion by glaciers. Yosemite Valley is famous worldwide for the erosional features left by ice age glaciers.

Transport. After material is picked up by erosion it is transported. Material can be transported short or long distances.

Just like erosion, waves, wind, water and ice are transport agents. Water flowing in streams is energetic enough to pick up and carry silt, sand, pebbles, mud, or at flood stage even boulders.

Transport can take place at different rates. Streams can run very quickly and ice moves quite slowly.

Transport by glaciers. A glacier transports rocks at its tops, along its sides and at its base.

Deposition. Material that is transported is eventually deposited.

Waves, wind, water and ice all deposit the sediments that they eroded and transported. This happens when the transporting agent slows and no longer has the energy to carry the material. In glaciers material is deposited when ice melts back.

When sediments are deposited, they create landforms. The type of landform left behind is a record of the transporting agents that left the material behind.

Glaciers dump the rocks they carry into piles known as moraines. The presence of a glacial moraine in a region indicates that a glacier was once there.

Most rivers carry sediments to the sea and deposit them in deltas. Wind deposits sand in dunes.

Fun fact: Scientists say that it took more than 10 million years for the Colorado River to carve the Grand Canyon, maybe more than 20 million.

Dunes. Wind transports sand in arid regions. When the wind slows, the sand is deposited in dunes. These dunes are in the Mojave Desert of California.