Stratum facts for kids

Man o' War Bay on the Dorset coast (Jurassic Coast). The strata here are nearly vertical, as a result of ancient orogeny.

This aerial photo shows the sloping strata in the cliff quite well

A stratum (pl.: strata) is a layer of rock or soil that forms over time. These layers are like the pages of a book, telling the story of Earth's history. Geologists study strata to learn about the past, including how the Earth looked, what the climate was like, and even what kinds of plants and animals lived long ago.

How are strata formed?

Strata are usually formed from sediments, like sand, mud, or tiny pieces of rock, that settle at the bottom of oceans, lakes, or rivers. Over millions of years, these sediments get squished together by pressure and heat, turning into solid rock. This process is called lithification.

Sometimes, strata can also form from volcanic activity. For example, when lava flows and cools, it creates layers of igneous rock. These layers can be studied just like sedimentary strata.

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Why are strata important?

• Each stratum represents a specific time period. By studying the layers, geologists can figure out what happened on Earth millions of years ago. For example, chalk layers are made from the remains of tiny algae called coccoliths, which lived in the ocean during the Cretaceous period.
• Fossils are often found in strata. These are the preserved remains of plants and animals that lived long ago. By studying fossils in different layers, scientists can learn how life on Earth has changed over time.
• Strata can show patterns of climate change. For example, alternating layers of clay and sandstone might tell us about times when the sea level rose and fell due to ice ages.

How do geologists study strata?

Geologists use tools like hammers, magnifying glasses, and even satellites to study strata. They also follow important rules, like the Law of superposition, which says that the oldest layers are at the bottom and the youngest are at the top.

Another important idea is the Principle of Faunal Succession, which means that fossils in strata appear in a specific order. This helps geologists match layers from different parts of the world.

Examples of famous strata

• The Grand Canyon: This massive canyon in the U.S. shows layers of rock that are millions of years old. Each layer tells a different part of Earth’s story.
• The Jurassic Coast: Located in England, this area has strata from the time of the dinosaurs. Some layers are even twisted and folded, showing how the Earth’s crust has moved.
• The Burgess Shale: Found in Canada, this stratum contains some of the oldest fossils of complex life forms.

Fun facts about strata

These are annual varves in limestone. Pleistocene strata, Scarboro Cliffs, Toronto, Ontario, Canada

• Some strata show repeating patterns, like stripes. These are called rhythmites. If the patterns are caused by yearly changes, like seasons, they’re called varves.
• Sometimes, layers are missing because of erosion or other forces. These gaps are called unconformities, and they help geologists understand how the Earth’s surface has changed.
• In some places, like mountains, strata can be bent or folded. This happens because of huge forces inside the Earth, like when tectonic plates collide.

Images for kids

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  Stair Hole from the East. This hole on the Jurassic Coast behind the cliff shows contorted strata.

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  Strata in Salta (Argentina).

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  Goldenville strata in quarry in Bedford, Canada. These are Middle Cambrian marine sediments. This formation covers over half of Nova Scotia and is recorded as being 29,000 feet thick in some areas.

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  The Permian through Jurassic strata in the Colorado Plateau area of southeastern Utah demonstrates the principles of stratigraphy. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.

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  Strata on a mountain face in the French Alps

See also

In Spanish: Estrato para niños