Rocket holds up the rolled timeline and the week 9 rock bag together. "The rocks are pages. The roll is the book."
Raven lays a sorted rock on the Sky Deck. "This sandstone took hundreds of meters of burial. The roll says there was time for that."
"What do you notice about the slow things?" she asks. "A millimeter of sediment a year. A mineral grain growing a millimeter in a million years."
"And still we get mountains and canyons," Rocket says. "Because the roll is so long."
Nova hovers between the roll and the rocks. "Deep time is the engine behind every week so far," she says.
Rocket grins. "Next week we build the solar system." Nova hums. "First, finish this chapter."
The textbook says most geological processes are extremely slow, but very large things happen if they continue long enough.
Sediments typically accumulate at less than a millimeter per year. Yet they grow thick enough to be lifted into mountains and carved into canyons.
New minerals growing in a rock during metamorphism have been estimated at about 1 millimeter per million years.
Each step around the rock cycle takes roughly 20 million years by a conservative estimate. On your roll that is one fifth of a sheet.
Week 9: rocks change from one kind to another, powered by inner heat and the Sun. Deep time gives them room to do it.
Week 10: layers and fossils put events in relative order, older and younger. The geologic time scale is that order written as chapters.
Week 11: the chapters have proportions. Complex life fills only the last sheets, and humans a sliver.
Written records of human history go back only about 6,000 years. Everything before that is read from rocks, layers and fossils.
| Slow process from the textbook | Rate | What it builds with enough time |
|---|---|---|
| sediment piling up | less than a millimeter per year | layers thick enough to become mountains and canyons |
| minerals growing during metamorphism | about 1 millimeter per million years | banded metamorphic rock |
| the Atlantic Ocean widening | about 2.5 centimeters per year | an ocean over 5,000 kilometers wide in 200 million years |
| one step of the rock cycle | roughly 20 million years | a new kind of rock |
| Week review | True or false? |
|---|---|
| Earth is about 4.6 billion years old. | ? |
| The time scale is divided into eons, eras, periods and epochs. | ? |
| The Paleozoic, Mesozoic and Cenozoic eras cover most of Earth's history. | ? |
| Humans have existed for about 0.06 percent of geological time. | ? |
| Slow processes cannot build large features. | ? |
Strong week, time traveler. Tomorrow is Sky Night: a family walk down the timeline and a Moon check.