NGS Earth's History Sec. 2: Determining Ages of Rocks

What is the law of superposition?

• The principle that rock layers are always horizontal

• The principle that the oldest rock layer is at the top

• The principle that the oldest rock layer is at the bottom

• The principle that all rock layers are the same age

Tags

NGSS.MS-ESS1-4

Why are index fossils important?

• They are the oldest fossils found

• They are found only in one location

• They help match rock layers of the same age in different locations

• They help determine the absolute age of rocks

Answer explanation

Index fossils are the "superstars" of the prehistoric world because they act like time-stamps for the Earth. To be an index fossil, a species had to live for a very short time but be found all over the globe, making them the perfect tools for stratigraphy, or the science of studying Earth's layers to figure out the it's past.

When scientists find the same index fossil in two different states or even different continents, they know those rock layers were formed at the exact same time! It’s like finding the same limited-edition sticker in two different notebooks; you immediately know both pages come from the same era.

What is radioactive decay?

• The process of an element releasing particles and energy

• The process of rocks breaking down into smaller pieces

• The process of fossils forming in rocks

• The process of magma cooling into rock

Answer explanation

Radioactive decay is like a tiny, natural countdown happening inside unstable atoms. Some elements are "restless" because they have too much energy or the wrong balance of parts, so they shoot out particles and energy to try and calm down.

As these atoms "decay," they actually transform into a completely different, stable element over time. Scientists use the steady pace of this process, known as a half-life, like a super-accurate atomic clock to figure out exactly how many millions of years old a rock is!

Tags

NGSS.HS-PS1-8

What is an extrusion in geology?

• Lava that hardens beneath the Earth's surface

• Lava that hardens on the Earth's surface

• A break in the Earth's crust

• A gap in the geological record

Answer explanation

An extrusion is like Earth’s way of frosting a cake; it happens when molten rock (lava) bursts through the crust and hardens right on the surface. Because the air or water outside is much cooler than the underground, this lava cools down fast, often creating rocks with tiny crystals like basalt.

In a rock layer "timeline," an extrusion is always younger than the rocks directly beneath it. It’s a clear marker for geologists, showing exactly when a volcanic eruption occurred in that specific area!

What is an intrusion?

• Lava that hardens on the Earth's surface

• Magma that cools and hardens beneath the Earth's surface

• A break in the Earth's crust

• A gap in the geological record

Tags

NGSS.MS-ESS2-2

What is a fault?

• A type of radioactive element

• A type of fossil

• A break in the Earth's crust

• A type of rock layer

What is an unconformity?

• A type of radioactive element

• A type of fossil

• A gap in the geological record

• A continuous sequence of rock layers

Tags

NGSS.MS-ESS1-4

What is the absolute age of a rock?

• The age compared to other rocks

• The age of the rock's surface

• The number of years since the rock formed

• The age of the fossils within the rock

What is the relative age of a rock?

• The age of the rock's surface

• The age of the fossils within the rock

• The age compared to other rocks

• The exact number of years since the rock formed

Tags

NGSS.MS-ESS1-4

What is a half-life?

• The time it takes for a rock to form

• The time it takes for a fossil to form

• The time it takes for half of the radioactive atoms to decay

• The time it takes for all radioactive atoms to decay

Tags

NGSS.HS-PS1-8