The Fuss, if many tabs open, you must close one by one. Moreover, closing unnecessary tabs may reduce the memory RAM is used.
For iPhone users, three (3) ways you can follow to close all tabs in Safari and Chrome browsers, as reported by CNET, Thursday (12/08/2016) the following :
Option to close a tab in Safari browser is somewhat hidden. You can click the tab at the bottom right window to display a row of open tabs.
Click and hold the "Done" button to get a window option "New Tab" and "Close Tabs X" (X refers to the number of tabs you want to close). Then, click on the last option.
In the Chrome browser, do a little more than Safari. First, click the tab button in the upper right corner to display the number of open tabs. Afterwards, click one of the tabs and click the menu with triple dot icon in the upper right corner. Select "Close All Tabs".
To tab in the Google Search app, click the button to open the window tab row of tabs open. Google Search app will not show the option "Close All Tabs" to you shortly swipe to get rid of one of the open tabs. Once you swipe, the "Clear All Buttons" will appear in the upper right corner.