# Where are the oldest rocks found in an undisturbed rock profile?

As a result, the youngest rocks are found at the top, while the oldest rocks are located at the bottom. Oldest rocks can be up to 4 billion years old, while the most recent rocks formed during this time are less than 10 million years old.

Rocks form from the solidification of molten material. As a result, the oldest rocks on Earth are basaltic lava flows that date back 4,500 million years. Other older rocks include gneiss (2,000 million years old), schists (1,000 million years old), and quartzite (450 million years old).

Because all rocks decay over time, the location of their origin is usually determined by how they are currently being used. For example, the oldest rocks in North America are exposed at high altitudes as a result of post-glacial rebound—the land has risen since being released after being locked up for thousands of years.

Rocks deep within the mantle that make their way to the surface often occur in large bodies called orogens (pronounced /ɔːrəjən/). These rocks shape the landscape around us, including the Alps, Himalaya, and Rockies.

## How can you determine the relative age of undisturbed rock layers?

Calculating the Relative Age of Rocks According to the Law of Superposition, in undisturbed sedimentary rock strata, younger rocks are on top and older rocks are on the bottom. Forces like as bending, faulting, and rising, on the other hand, can rearrange the rock layers such that the youngest is not necessarily found on top. In this case, geologists use other information about the rock layer to help determine their relative age.

Younger layers are typically less weathered than older ones, with the exception of wind-blown sand deposits which often have an equal distribution of young and old particles. Layers containing fossils or minerals unique to younger rocks also indicate that they are relatively new. Older layers tend to be more weathered, with finer particles distributed more evenly across the landscape unless a destructive force has recently moved them around (i.e., a flood).

Layers composed of sand or silt spread by rivers or ocean currents can be used to estimate how long ago the underlying rock was exposed to air and water. For example, if a layer of sand overlies a layer of clay, the clay layer will usually be thicker than the sand one. This means that the rock must have been exposed for some time before being covered by soil. If no further changes occur, the sand layer would remain unchanged for as much as 100 years depending on its thickness.

## When studying rock strata, are the oldest rocks usually found at the bottom or in the top layers?

According to the concept of superposition, each layer of sedimentary rock in an undamaged sequence is older than the one above it and younger than the one below it (Figures 1 and 2). As a result, the oldest rocks in a series are at the bottom of the sequence, while the youngest rocks are at the top. However, this relationship between position and age does not hold for all sequences of sedimentary rocks. For example, in some cases the youngest rocks are found at the bottom of the sequence and the oldest ones at the top.

Within a formation, certain areas may be more highly eroded than others. These "unstable" areas will collapse under their own weight or be undermined as deeper erosion removes the protective force of overlying rock. The most common example of this type of instability is the valley where two sides of a mountain come together. One side may be heavily worn by running water, while the other is relatively smooth. This is because water has more energy when it flows over a large surface area and thus can wear away the rock more quickly.

Instability can also arise due to tectonic activity. Areas that slip past one another often cause fractures in the surrounding rock that become filled with debris from the fractured rock above and below them. Over time, these fillings harden into rock formations in their own right - known as fault blocks or block folds.

Fault blocks can range in size from just a few feet across to hundreds of miles long.

##### Catherine Wilson

Catherine Wilson is a respected teacher and scientist. She holds a PhD in chemistry, but her true passion lies with teaching children about the wonders of science. Catherine has an endless love for learning and is able to share this love with others through her lessons. One thing that Catherine loves about being a chemist is how it allows her to see the world differently by looking at everyday objects in new ways.