Mark's father parked his car. "There's a crowd forming around Old Faithful," he said.
"The geyser is probably about to blow. Come on, Mark!
If we hurry, we might get to see the eruption."
Mark had never seen a geyser erupt, so he and his father had come to Yellowstone National Park for the day.
The park was famous for its geysers and hot springs, especially the geyser known as Old Faithful.
Mark's dad had told him that geysers are one of the most amazing things found in nature.
A crack in the ground is filled with hot water, with bubbles and steam rising to the surface.
Then—often with little warning—it erupts.
A giant burst of steam and water shoots high into the air.
Mark and his father made it to the geyser just in time.
"Old Faithful is about to blow!" a park ranger shouted.
Mark felt a vibration coming from deep beneath the earth.
There was a strange gurgling sound, and then, from a hole in the ground, steam and water shot up over forty meters into the air!
"Whoa!" Mark cried.
Soon the water flowed back down into the hole.
To Mark, it sounded just like water gurgling down the drain of a sink.
Mark had a question for the ranger. "Why is this geyser named Old Faithful?" he asked.
"Most geysers are unpredictable," the ranger explained.
"But Old Faithful's eruptions follow a predictable pattern.
If it erupts for two minutes, then it erupts again in sixty minutes.
If it erupts for five minutes, it erupts again in one hundred minutes."
"I get it," said Mark. "Old Faithful always does the same thing!"
"Exactly." The ranger grinned at Mark.
Soon Mark and his father followed the park ranger to an exhibit.
There, Mark learned that Yellowstone Park contains more than half of all the geysers and hot springs on earth.
"What are hot springs?" Mark asked the ranger. "Are they different from geysers?"
"Yes, they are different," the ranger replied.
"They form because of earth's water table.
When rain falls, some water sinks deep into the earth until it reaches a layer of rock.
The water sits on top of the rock and forms what's called the water table—a layer of water.
"In some places, such as Yellowstone Park, the hot core of the earth is near this water table.
So the water gets heated to boiling hot temperatures.
If there is a crack in the earth above the water, the water expands and forces its way up. Water then pools around the opening."
"I see." Mark nodded. "Hot springs don't erupt. But then what makes geysers erupt?"
"Well," the park ranger said. "A hot spring's crack, or fissure, is shaped like a pipe.
But a geyser's fissure is wider, forming a large cavern underground.
Hot water and steam rise and fill the cavern while the water at the top of the cavern cools off quickly.
This cool water acts like a heavy lid on a pot of boiling water.
Hot pressure builds up in the cavern.
Suddenly all the pressure is released at once, shooting water and steam into the air."
Mark and his father stayed a while longer, exploring the exhibit.
They found out that Iceland, a small country in Europe, also has a large number of geysers and hot springs.
The people living there can even use the energy from geysers and hot springs to heat their homes and generate electricity!
"Look, Dad!" Mark pointed to a picture of a giant mud bath.
"Sometimes the hot spring mixes with soil to form mud baths like this one.
I bet it feels good to lay in that!"
"Many people find these mud baths soothing," replied Mark's dad.
"Japan is famous for its relaxing hot springs.
Some people even believe that mud baths and hot springs can cure diseases—or at least take away some aches and pains!"
The ranger heard them talking. "In Yellowstone Park the hot springs' water is usually too hot to touch," he said.
"So unfortunately, people can't bathe in the hot springs here.
But there is something special about our hot springs.
Some forms of bacteria and algae live in them.
These life forms are different colors and give the hot springs an unusual and beautiful appearance."
Later Mark and his father thanked the park ranger and headed back toward their car. It was time to go home.
"Dad, I wish we could go to one of those hot springs where you can swim and relax," Mark said. "That sounds like fun!"
"I've heard there are hot springs like that in Canada," replied Mark's father.
"You can sit in a hot spring high in the mountains.
Would you like to go there on our next vacation?" he asked.
"Yes!" Mark said, grinning. "That sounds cool . . . er, I mean, hot!"